A more technical example to illustrate genotype is the  single-nucleotide polymorphism  or SNP.  A SNP occurs when corresponding sequences of  DNA  from different individuals differ at one DNA base, for example where the sequence AAGCCTA changes to AAGCTTA. [ 6 ]  This contains two alleles : C and T. SNPs typically have three genotypes, denoted generically AA Aa and aa. In the example above, the three genotypes would be CC, CT and TT. Other types of  genetic marker , such as  microsatellites , can have more than two alleles, and thus many different genotypes.


Penetrance is the proportion of individuals showing a specified genotype in their phenotype under a given set of environmental conditions. [ 7 ]


Traits that are determined exclusively by genotype are typically inherited in a  Mendelian  pattern. These laws of inheritance were described extensively by  Gregor Mendel , who performed experiments with pea plants to determine how traits were passed on from generation to generation. [ 8 ]  He studied phenotypes that were easily observed, such as plant height, petal color, or seed shape. [ 8 ]  He was able to observe that if he crossed two true-breeding plants with distinct phenotypes, all the offspring would have the same phenotype. For example, when he crossed a tall plant with a short plant, all the resulting plants would be tall. However, when he self-fertilized the plants that resulted, about 1/4 of the second generation would be short. He concluded that some traits were  dominant , such as tall height, and others were recessive, like short height. Though Mendel was not aware at the time, each phenotype he studied was controlled by a single gene with two alleles. In the case of plant height, one allele caused the plants to be tall, and the other caused plants to be short. When the tall allele was present, the plant would be tall, even if the plant was heterozygous. In order for the plant to be short, it had to be homozygous for the recessive allele. [ 8 ] [ 9 ]


One way this can be illustrated is using a  Punnett square . In a Punnett square, the genotypes of the parents are placed on the outside. An uppercase letter is typically used to represent the dominant allele, and a lowercase letter is used to represent the recessive allele. The possible genotypes of the offspring can then be determined by combining the parent genotypes. [ 10 ]  In the example on the right, both parents are heterozygous, with a genotype of Bb. The offspring can inherit a dominant allele from each parent, making them homozygous with a genotype of BB. The offspring can inherit a dominant allele from one parent and a recessive allele from the other parent, making them heterozygous with a genotype of Bb. Finally, the offspring could inherit a recessive allele from each parent, making them homozygous with a genotype of bb. Plants with the BB and Bb genotypes will look the same, since the B allele is dominant. The plant with the bb genotype will have the recessive trait.


These inheritance patterns can also be applied to  hereditary diseases  or conditions in humans or animals. [ 11 ] [ 12 ] [ 13 ]  Some conditions are inherited in an  autosomal  dominant pattern, meaning individuals with the condition typically have an affected parent as well. A classic pedigree for an autosomal dominant condition shows affected individuals in every generation. [ 11 ] [ 12 ] [ 13 ]


Other conditions are inherited in an autosomal recessive pattern, where affected individuals do not typically have an affected parent. Since each parent must have a copy of the recessive allele in order to have an affected offspring, the parents are referred to as carriers of the condition. [ 11 ] [ 12 ] [ 13 ]
In autosomal conditions, the sex of the offspring does not play a role in their risk of being affected. In sex-linked conditions, the sex of the offspring affects their chances of having the condition. In humans, females inherit two  X chromosomes , one from each parent, while males inherit an X chromosome from their mother and a Y chromosome from their father. X-linked dominant conditions can be distinguished from autosomal dominant conditions in pedigrees by the lack of transmission from fathers to sons, since affected fathers only pass their X chromosome to their daughters. [ 13 ] [ 11 ] [ 14 ]  In X-linked recessive conditions, males are typically affected more commonly because they are hemizygous, with only one X chromosome. In females, the presence of a second X chromosome will prevent the condition from appearing. Females are therefore carriers of the condition and can pass the trait on to their sons. [ 13 ] [ 11 ] [ 14 ]


Mendelian patterns of inheritance can be complicated by additional factors. Some diseases show incomplete  penetrance , meaning not all individuals with the disease-causing allele develop signs or symptoms of the disease. [ 13 ] [ 15 ] [ 16 ]  Penetrance can also be age-dependent, meaning signs or symptoms of disease are not visible until later in life. For example,  Huntington disease  is an autosomal dominant condition, but up to 25% of individuals with the affected genotype will not develop symptoms until after age 50. [ 17 ]  Another factor that can complicate Mendelian inheritance patterns is variable  expressivity , in which individuals with the same genotype show different signs or symptoms of disease. [ 13 ] [ 15 ] [ 16 ]  For example, individuals with  polydactyly  can have a variable number of extra digits. [ 15 ] [ 16 ]


Many traits are not inherited in a Mendelian fashion, but have more complex patterns of inheritance.


For some traits, neither allele is completely dominant. Heterozygotes often have an appearance somewhere in between those of homozygotes. [ 18 ] [ 19 ]  For example, a cross between true-breeding red and white  Mirabilis jalapa  results in pink flowers. [ 19 ]


Codominance refers to traits in which both alleles are expressed in the offspring in approximately equal amounts. [ 20 ]  A classic example is the  ABO blood group system  in humans, where both the A and B alleles are expressed when they are present. Individuals with the AB genotype have both A and B proteins expressed on their red blood cells. [ 20 ] [ 18 ]


Epistasis is when the phenotype of one gene is affected by one or more other genes. [ 21 ]  This is often through some sort of masking effect of one gene on the other. [ 22 ]  For example, the "A" gene codes for hair color, a dominant "A" allele codes for brown hair, and a recessive "a" allele codes for blonde hair, but a separate "B" gene controls hair growth, and a recessive "b" allele causes baldness. If the individual has the BB or Bb genotype, then they produce hair and the hair color phenotype can be observed, but if the individual has a bb genotype, then the person is bald which masks the A gene entirely.


A polygenic trait is one whose phenotype is dependent on the additive effects of multiple genes. The contributions of each of these genes are typically small and add up to a final phenotype with a large amount of variation. A well studied example of this is the number of sensory bristles on a fly. [ 23 ]  These types of additive effects is also the explanation for the amount of variation in human eye color.


Genotyping refers to the method used to determine an individual's genotype. There are a variety of techniques that can be used to assess genotype. The genotyping method typically depends on what information is being sought. Many techniques initially require amplification of the DNA sample, which is commonly done using  PCR .


Some techniques are designed to investigate specific SNPs or alleles in a particular gene or set of genes, such as whether an individual is a carrier for a particular condition. This can be done via a variety of techniques, including  allele specific oligonucleotide  (ASO) probes or  DNA sequencing . [ 24 ] [ 25 ]  Tools such as  multiplex ligation-dependent probe amplification  can also be used to look for duplications or deletions of genes or gene sections. [ 25 ]  Other techniques are meant to assess a large number of SNPs across the genome, such as  SNP arrays . [ 24 ] [ 25 ]  This type of technology is commonly used for  genome-wide association studies .


Large-scale techniques to assess the entire genome are also available. This includes  karyotyping  to determine the number of chromosomes an individual has and  chromosomal microarrays  to assess for large duplications or deletions in the chromosome. [ 24 ] [ 25 ]  More detailed information can be determined using  exome sequencing , which provides the specific sequence of all DNA in the coding region of the genome, or  whole genome sequencing , which sequences the entire genome including non-coding regions. [ 24 ] [ 25 ]


In linear models, the genotypes can be encoded in different manners. Let us consider a biallelic locus with two possible alleles, encoded by

 A

 {\textstyle A}

  and

 a

 {\displaystyle a}

 . We consider

 a

 {\displaystyle a}

  to correspond to the dominant allele to the reference allele

 A

 {\textstyle A}

 . The following table details the different encoding. [ 26 ]


In  biology ,  polymorphism  is the occurrence of two or more clearly different forms or  phenotypes  in a  population  of a species. Different types of polymorphism have been identified and are listed separately.


In 1973,  M. J. D. White , then at the end of a long career investigating  karyotypes , gave an interesting summary of the distribution of chromosome polymorphism.


This suggests, once again, that polymorphism is a common and important aspect of adaptive evolution in natural populations.


All the common  blood types , such as the  ABO blood group system , are genetic polymorphisms. Here we see a system where there are more than two morphs: the  phenotypes  A, B, AB and O are present in all human populations, but vary in proportion in different parts of the world. The phenotypes are controlled by multiple  alleles  at one  locus . These polymorphisms are seemingly never eliminated by natural selection; the reason came from a study of disease statistics.


Statistical research has shown that an individual of a given phenotype will generally be, compared to an individual of a differing phenotype, more resistant to certain diseases while less resistant to others. For example, an individual's susceptibility to  cholera  (and other diarrheal infections) is correlated with their blood type: those with type O blood are the most susceptible, while those with type AB are the most resistant. Between these two extremes are the A and B blood types, with type A being more resistant than type B. This suggests that the  pleiotropic  effects of the genes set up opposing selective forces, thus maintaining a balance. [ 2 ] [ 3 ] [ 4 ]  Geographical distribution of blood groups (the differences in gene frequency between populations) is broadly consistent with the classification of " races " developed by early anthropologists on the basis of visible features. [ 5 ] : 283–291


Sickle-cell anaemia  is found mostly in tropical populations in  Africa  and  India . An individual  homozygous  for the  recessive  sickle  hemoglobin , HgbS, has a short expectancy of life, whereas the life expectancy of the standard hemoglobin (HgbA) homozygote as well as  heterozygote  is normal (though heterozygote individuals will suffer periodic problems). The sickle-cell variant survives in the population because the heterozygote is resistant to malaria and the  malarial parasite  kills a huge number of people each year. This is  balancing selection  or genetic polymorphism, balanced between fierce selection against homozygous sickle-cell sufferers, and selection against the standard HgbA homozygotes by malaria. The heterozygote has a permanent advantage (a higher fitness) so long as malaria exists; and it has existed as a human  parasite  for a long time. Because the heterozygote survives, so does the HgbS  allele  survive at a rate much higher than the  mutation rate . [ 6 ] [ 7 ]


The  Duffy antigen  is a protein located on the surface of red blood cells, encoded by the  FY (DARC)   gene . [ 8 ]  The protein encoded by this gene is a non-specific  receptor  for several  chemokines , and is the known entry-point for the human malarial parasites  Plasmodium vivax  and  Plasmodium knowlesi . Polymorphisms in this gene are the basis of the Duffy blood group system. [ 9 ]


In humans, a mutant variant at a single site in the FY cis-regulatory region abolishes all expression of the gene in  erythrocyte  precursors. As a result,  homozygous  mutants are strongly protected from infection by  P. vivax , and a lower level of protection is conferred on  heterozygotes . The variant has apparently arisen twice in geographically distinct human populations, in Africa and  Papua New Guinea . It has been driven to high frequencies on at least two  haplotypic  backgrounds within Africa. Recent work indicates a similar, but not identical, pattern exists in baboons ( Papio cynocephalus ), which suffer a mosquito-carried malaria-like pathogen,  Hepatocystis kochi . Researchers interpret this as a case of  convergent evolution . [ 10 ]


Glucose-6-phosphate dehydrogenase  human polymorphism is also implicated in malarial resistance. G6PD alleles with reduced activity are maintained at a high level in endemic malarial regions, despite reduced general viability. Variant A (with 85% activity) reaches 40% in sub-Saharan Africa, but is generally less than 1% outside Africa and the Middle East. [ 11 ] [ 12 ]


A famous puzzle in human genetics is the genetic ability to taste  phenylthiocarbamide  (phenylthiourea or PTC), a morphism which was discovered in 1931. This substance, which is bitter to some people and tasteless to others, is of no great significance in itself, yet it is a genetic dimorphism. Because of its high frequency (which varies in different ethnic groups) it must be connected to some function of selective value. The ability to taste PTC itself is correlated with the ability to taste other bitter substances, many of which are toxic. Indeed, PTC itself is toxic, though not at the level of tasting it on  litmus . Variation in PTC perception may reflect variation in dietary preferences throughout human evolution, and might correlate with susceptibility to diet-related diseases in modern populations. There is a statistical correlation between PTC tasting and liability to  thyroid disease .


Fisher, Ford, and Huxley tested  orangutans  and  chimpanzees  for PTC perception with positive results, thus demonstrating the long-standing existence of this dimorphism. [ 13 ]  The PTC gene, which accounts for 85% of the tasting variance, has now been analysed for sequence variation with results which suggest selection is maintaining the morphism. [ 14 ]


The genes of the  major histocompatibility complex  (MHC) are highly polymorphic, [ 15 ]  and this diversity plays a very important role in resistance to pathogens. This is true for other species as well.


DNA repair  gene polymorphisms are associated with increased risk of  colorectal cancer  development. [ 16 ] [ 17 ]   DNA repair gene polymorphisms also appear to influence the risk of  lung cancer . [ 18 ]


Some frog species display polymorphism by presence/absence of a light stripe going along the central part of their back. A light mid-dorsal stripe has been shown to be
determined by a simple dominant gene in  Rana limnocharis , [ 19 ]   Rana ridibunda , [ 20 ]   Rana sylvatica [ 21 ]  and  Rana arvalis ; [ 22 ]  that means the individuals both homozygtes by allele determining the presence of stripe and  heterozygotes  have the stripe, whereas only the individuals homozygotic by recessive allele are non-striped. The proportions of striped specimens in populations of some frogs show clinal variations. For example, the proportion of striped  Rana sylvatica  in North America generally increases towards the west and north. [ 23 ]  The variations in the proportion of different color may relate to either genetic-stochastic processes. [ 24 ]  or their adaptive importance. [ 25 ] [ 26 ]  For different colour morphs of  Acris crepitans , the hypothesis about the direct adaptive value of different colour morphs (for escaping predation) competes with the hypothesis that these morphs correlate with thermotolerance. [ 27 ]  Striped specimens  Rana sylvatica , striped specimens better perform in open areas. Differences in the proportion of striped frogs in  Rana arvalis  are explained with physiological differences between the morphs. [ 26 ]  Striped recently metamorphosed frogs have a relatively large liver, in comparison with unstriped ones, and their weight increases more rapidly. Tadpoles of striped  Rana arvalis  need more time for completing metamorphosis but, after  metamorphosis , their growth is faster than that of unstriped froglets. [ 26 ]  In a frog widespread in  Turkey  and the  Caucasus ,  Rana macrocnemis , the proportion of frogs with the stripe increases with the altitude in mountains of the  Lesser Caucasus , but not in the  Greater Caucasus . [ 28 ]  Given the same altitude, non-striped frogs from the Greater Caucasus grow slower and maturate later than the striped frogs from the Lesser Caucasus, which provides them selective advantage in high mountains, but their tadpoles are likely to be less resistant to overheating than those of the non-striped frogs. [ 29 ] [ 30 ]


Over fifty species in this family of birds practice  brood parasitism ; the details are best seen in the common cuckoo ( Cuculus canorus ). In a season the female lays one egg in 15–20 other bird nests. She removes some or all of the host's clutch of eggs, and lays an egg which closely matches the host eggs. In Britain the cuckoo lays small eggs that match the size of the smaller host's. The eggs are thick-shelled as a defense to protect the egg if the host detects the fraud.


The intruded egg develops exceptionally quickly; when the newly hatched cuckoo is only ten hours old, and still blind, it exhibits an urge to eject the other eggs or nestlings. It rolls them into a special depression on its back and heaves them out of the nest. The cuckoo nestling is apparently able to pressure the host adults for feeding by mimicking the cries of the host nestlings. The diversity of the cuckoo's eggs is extraordinary, the forms resembling those of its most usual hosts. In Britain these are:


Each female cuckoo lays one type only; the same type laid by her mother. In this way female cuckoos are divided into groups (known as  gentes , singular  gens ), each parasitises the host to which it is adapted. The male cuckoo has its own territory, and mates with females from any gens; thus the population (all gentes) is interbreeding.


The standard explanation of how the inheritance of gens works is as follows. The egg colour is inherited by sex chromosome. In birds  sex determination  is ZZ/ZW, and unlike mammals, the heterogametic sex is the female. [ 31 ]  The determining gene (or super-gene) for the inheritance of egg colour is believed to be carried on the W chromosome, which is directly transmitted in the female line. The female behaviour in choosing the host species is set by  imprinting  after birth, a common mechanism in bird behaviour. [ 32 ] [ 33 ]


Ecologically, the system of multiple hosts protects host species from a critical reduction in numbers, and maximises the egg-laying capacity of the population of cuckoos. It also extends the range of habitats where the cuckoo eggs may be raised successfully. Detailed work on the cuckoo started with E. Chance in 1922, [ 34 ]  and continues to the present day; in particular, the inheritance of gens is still a live issue.


Whereas Darwin spent just five weeks in the  Galápagos , and  David Lack  spent three months,  Peter and Rosemary Grant  and their colleagues have made research trips to the Galápagos for about thirty years, particularly studying  Darwin's finches . The  Española cactus finch  ( Geospiza conirostris ) lives on Isla  Genovesa  (formerly Tower Island) which is formed from a shield volcano, and is home to a variety of birds. These birds, like all well-studied groups, [ 35 ]  show various kinds of morphism.


Males are dimorphic in song type: songs A and B are quite distinct. Also, males with song A have shorter bills than B males. This is also a clear difference. With these beaks males are able to feed differently on their favourite cactus, the  prickly pear   Opuntia . Those with long beaks are able to punch holes in the cactus fruit and eat the fleshy  aril  pulp which surrounds the seeds, whereas those with shorter beaks tear apart the cactus base and eat the pulp and any insect larvae and pupae (both groups eat flowers and buds). This dimorphism clearly maximises their feeding opportunities during the non-breeding season when food is scarce.


Territories of type A and type B males are random if not mated but alternate if mated: no two breeding males of the same song type shared a common boundary. This initially suggested the possibility of  assortative mating  by  female choice . [ 36 ] [ 37 ]  However, further work showed that "the choice of a male by a female is independent of any conditioning influence of her father's song type and there is no evidence of assortative mating by bill type... Hence there is no direct evidence of reproductive subdivision in the population". [ 38 ]  In 1999 Peter Grant agreed that "sympatric speciation [in this example] is unlikely to occur". [ 39 ] : 428


If the population is  panmixic , then  Geospiza conirostris  exhibits a balanced genetic polymorphism and not, as originally supposed, a case of nascent  sympatric speciation . The selection maintaining the polymorphism maximises the species' niche by expanding its feeding opportunity. The genetics of this situation cannot be clarified in the absence of a detailed breeding program, but two loci with  linkage disequilibrium [ 40 ] : ch. 5   is a possibility.


Another interesting dimorphism is for the bills of young finches, which are either "pink" or "yellow". All species of Darwin's finches exhibit this morphism, which lasts for two months. No interpretation of this phenomenon is known. [ 39 ] : plate 10


The  white-throated sparrow  ( Zonotrichia albicollis ), a  passerine   bird  of the  American sparrow  family  Passerellidae , shows a clear dimorphism in both sexes throughout its large range.


Their heads are either white-striped or tan-striped. These differences in plumage result from a balanced  chromosomal inversion  polymorphism; in white-striped (WS) birds, one copy of chromosome 2 is partly inverted, while in tan-striped (TS) birds, both copies are uninverted. [ 41 ]


The plumage differences are paralleled by differences in behavior and breeding strategy. WS males sing more, are more aggressive and more frequently engage in extra-pair copulation than their TS counterparts. [ 42 ]  TS birds of both sexes provide more parental care than WS birds.


The polymorphism is maintained by negative  assortative mating —each morph mates with its opposite. [ 43 ]  Dimorphic pairs may have an advantageous balance between parental care and aggressive territorial defense. In addition, as in many other polymorphisms, heterozygote advantage seems to help maintain this one; the proportion of WS birds homozygotic for the inversion is even lower than would be expected from the low frequency (4%) of pairings of the same morph. [ 44 ]


In the underlying chromosomal polymorphism, the standard (ZAL2) and alternative (ZAL2m) arrangements differ by a pair of included pericentric inversions at least. ZAL2m suppresses  recombination  in the heterokaryotype and is evolving as a rare nonrecombining  autosomal  segment of the  genome . [ 45 ]


Arctic char  are notable for exhibiting numerous, seemingly distinct morphological variants or morphs throughout the range of the species. [ 46 ] [ 47 ] [ 48 ] [ 49 ] [ 50 ]  Consequently, Arctic char have been referred to as the 'most variable vertebrate on Earth'. [ 48 ]  These morphs are often  sympatric  within lakes or rivers. [ 46 ] [ 47 ] [ 49 ] [ 50 ]  Morphs often vary significantly in size, shape and colour. [ 46 ] [ 47 ] [ 49 ] [ 50 ]  Morphs often demonstrate differences in migratory behaviour, being resident or anadromous fish, and in feeding behaviour and niche placement. [ 47 ] [ 49 ] [ 50 ]  Morphs often interbreed, but they can also be reproductively isolated and represent genetically distinct populations, [ 50 ]  which have been cited as examples of  incipient speciation . [ 47 ]


Ants  exhibit a range of polymorphisms. First, there is their characteristic  haplodiploid sex determination system , whereby all males are haploid, and all females diploid. Second, there is differentiation between both the females and males based mostly on feeding of larvae, which determines, for example, whether the imago is capable of reproduction. Lastly, there is differentiation of size and 'duties' (particularly of females), which are usually controlled by feeding and/or age, but which may sometimes be genetically controlled. Thus the order exhibits both genetic polymorphism and extensive polyphenism. [ 51 ] [ 52 ]


In the 1930s Dobzhansky and his co-workers collected  Drosophila pseudoobscura  and  D. persimilis  from wild populations in  California  and neighbouring states. Using Painter's technique [ 53 ]  they studied the  polytene  chromosomes and discovered that the wild populations were polymorphic for  chromosomal inversions . All the flies look alike whatever inversions they carry: this is an example of a cryptic polymorphism. Accordingly, Dobzhansky favoured the idea that the morphs became fixed in the population by means of  Sewall Wright 's  drift . [ 54 ]  However, evidence rapidly accumulated to show that natural selection was responsible:


1. Values for heterozygote inversions of the third chromosome were often much higher than they should be under the null assumption: if no advantage for any form the number of heterozygotes should conform to N s  (number in sample) = p 2 +2pq+q 2  where 2pq is the number of heterozygotes (see  Hardy–Weinberg equilibrium ).


2. Using a method invented by l'Heretier and Teissier, Dobzhansky bred populations in  population cages , which enabled feeding, breeding and sampling whilst preventing escape. This had the benefit of eliminating  migration  as a possible explanation of the results. Stocks containing inversions at a known initial frequency can be maintained in controlled conditions. It was found that the various chromosome types do not fluctuate at random, as they would if selectively neutral, but adjust to certain frequencies at which they become stabilised. With  D. persimilis  he found that the caged population followed the values expected on the Hardy–Weinberg equilibrium when conditions were optimal (which disproved any idea of non-random mating), but with a restricted food supply heterozygotes had a distinct advantage.


3. Different proportions of chromosome morphs were found in different areas. There is, for example, a polymorph-ratio  cline  in  D. robusta  along an 18-mile (29 km) transect near  Gatlinburg ,  TN  passing from 1,000 feet (300 m) to 4,000 feet. [ 55 ]  Also, the same areas sampled at different times of year yielded significant differences in the proportions of forms. This indicates a regular cycle of changes which adjust the population to the seasonal conditions. For these results selection is by far the most likely explanation.


4. Lastly, morphs cannot be maintained at the high levels found simply by mutation, nor is drift a possible explanation when population numbers are high.


By the time Dobzhansky published the third edition of his book in 1951, he was persuaded that the chromosome morphs were being maintained in the population by the selective advantage of the heterozygotes, as with most polymorphisms. Later he made yet another interesting discovery. One of the inversions, known as PP, was quite rare up to 1946, but by 1958 its proportion had risen to 8%. Not only that, but the proportion was similar over an area of some 200,000 square miles (520,000 km 2 ) in California. This cannot have happened by migration of PP morphs from, say, Mexico (where the inversion is common) because the rate of dispersal (at less than 2 km/year) is of the wrong order. The change therefore reflected a change in prevailing selection whose basis was not yet known. [ 5 ] [ 32 ] [ 56 ]


Hoverfly mimics can be seen in almost any garden in the  temperate zone . The  Syrphidae  are a large (5600+ species) family of flies; their  imagoes  feed on nectar and pollen, and are well known for their mimicry of social  hymenoptera . The mimicry is  Batesian  in nature: hoverflies are palatable but hymenoptera are generally unpalatable and may also be protected by stingers and/or armour.


Many social wasp ( Vespidae ) species exhibit  Müllerian mimicry , where a group of unpalatable species benefit from sharing the same kind of warning ( aposematic ) colouration. Wasps are decidedly noxious: nasty-tasting and with a painful sting. They form a Mullerian 'ring' of similarly coloured models; the wasps are often accompanied by clusters of hover-fly mimics, who tend to arrive at the flowers at a similar time of day, and whose flight pattern is passably similar to wasp flight.


Observers in a garden can see for themselves that hoverfly mimics are quite common, usually many times more common than the models, and are (to our sight) relatively poor mimics, often easy to distinguish from real wasps. However, it has been established in other cases that imperfect mimicry can confer significant advantage to the mimic, especially if the model is really noxious. [ 57 ]  Also, not only is polymorphism absent from these mimics, it is absent in the wasps also: these facts are presumably connected. [ 58 ]


The situation with  bumblebees  ( Bombus ) is rather different. They too are unpalatable, in the sense of being difficult to eat: their body is covered with  setae  (like  carpet pile ) and is armoured; they are sometimes described as being 'non-food'. Mostler in 1935 carried out tests of their palatability: with the exception of specialist bee-eaters, adults of 19 species of birds ate only 2% of 646 bumblebees presented to them. After various trials, Mostler attributed their avoidance mainly to mechanical difficulties in handling: one young bird took 18 minutes to subdue, kill and eat a bumblebee. [ 59 ]


Bumblebees form Mullerian rings of species, and they do often exhibit polymorphism. The hoverfly species mimicking bumblebees are generally accurate mimics, and many of their species are polymorphic. Many of the polymorphisms are different between the sexes, for example by the mimicry being limited to one sex only.


The question is, how can the differences between social wasp mimics and bumblebee mimics be explained? Evidently if model species are common, and have overlapping distributions, they are less likely to be polymorphic. Their mimics are widespread and develop a kind of rough and ready jack-of-all-trades mimicry. But if model species are less common and have patchy distribution they develop polymorphism; and their mimics match them more exactly and are polymorphic also. The issues are currently being investigated. [ 60 ] [ 61 ] [ 62 ]


The peppered moth,  Biston betularia , is justly famous as an example of a population responding in a  heritable  way to a significant change in their ecological circumstances. E.B. Ford described  peppered moth evolution  as "one of the most striking, though not the most profound, evolutionary changes ever actually witnessed in nature". [ 63 ]


Although the moths are cryptically  camouflaged  and rest during the day in unexposed positions on trees, they are predated by birds hunting by sight. The original camouflage (or  crypsis ) seems near-perfect against a background of  lichen  growing on trees. The sudden growth of industrial pollution in the nineteenth century changed the effectiveness of the moths' camouflage: the trees became blackened by soot, and the lichen died off. In 1848 a dark version of this moth was found in the  Manchester  area. By 1895 98% of the peppered moths in this area were black. This was a rapid change for a species that has only one generation a year.


In Europe, there are three morphs: the typical white morph ( betularia  or  typica ), and  carbonaria , the melanic black morph. They are controlled by  alleles  at one  locus , with the carbonaria being  dominant . There is also an intermediate or semi-melanic morph  insularia , controlled by other alleles. [ 64 ] [ 65 ]


A key fact, not realised initially, is the advantage of the heterozygotes, which survive better than either of the homozygotes. This affects the caterpillars as well as the moths, in spite of the caterpillars being monomorphic in appearance (they are twig mimics). In practice  heterozygote advantage  puts a limit to the effect of selection, since neither homozygote can reach 100% of the population. For this reason, it is likely that the carbonaria allele was in the population originally, pre-industrialisation, at a low level. With the recent reduction in pollution, the balance between the forms has already shifted back significantly.


Another interesting feature is that the carbonaria had noticeably darkened after about a century. This was seen quite clearly when specimens collected about 1880 were compared with specimens collected more recently: clearly the dark morph has been adjusted by the strong selection acting on the gene complex. This might happen if a more extreme allele was available at the same locus; or genes at other loci might act as modifiers. We do not, of course, know anything about the genetics of the original melanics from the nineteenth century.


This type of industrial melanism has only affected such moths as obtain protection from insect-eating birds by resting on trees where they are concealed by an accurate resemblance to their background (over 100 species of moth in Britain with melanic forms were known by 1980). [ 33 ]  No species which hide during the day, for instance, among dead leaves, is affected, nor has the melanic change been observed among butterflies. [ 66 ] [ 64 ] [ 67 ]
This is, as shown in many textbooks, "evolution in action".


Much of the early work was done by  Bernard Kettlewell , whose methods came under scrutiny later on. The entomologist  Michael Majerus  discussed criticisms made of Kettlewell's experimental methods in his 1998 book  Melanism: Evolution in Action . [ 68 ]  This book was misrepresented in some reviews, and the story picked up by  creationist  campaigners.


Judith Hooper , in her controversial book  Of Moths and Men  (2002), implied that Kettlewell's work was fraudulent or incompetent. Careful studies of Kettlewell's surviving papers by Rudge (2005) and Young (2004) found that Hooper's accusation of fraud was unjustified, and that "Hooper does not provide one shred of evidence to support this serious allegation". [ 69 ] [ 70 ]  Majerus himself described  Of Moths and Men  as "littered with errors, misrepresentations, misinterpretations and falsehoods". [ 68 ]  A suitably restrained 2004 summary of opinion mostly favoured predation as the main selective force. [ 71 ]


Starting in 2000, Majerus conducted a detailed seven-year study of moths, experimenting to assess the various criticisms. He concluded that differential bird predation was a major factor responsible for the decline in  carbonaria  frequency compared to  typica  in Cambridge during the study period, [ 72 ]  and described his results as a complete vindication of the peppered moth story. He said, "If the rise and fall of the peppered moth is one of the most visually impacting and easily understood examples of Darwinian evolution in action, it should be taught. It provides after all the proof of evolution." [ 73 ]


Current interpretation of the available evidence is that the peppered moth is in fact a valid example of  natural selection  and adaptation. It illustrates a polymorphic species maintaining adaptation to a varied and sometimes changing environment.


Adalia bipunctata , the two-spotted ladybird, is highly polymorphic. Its basic form is red with two black spots, but it has many other forms, the most important being melanic, with black elytra and red spots. The curious fact about this morphism is that, although the melanic forms are more common in industrial areas, its maintenance has nothing to do with cryptic camouflage and predation. The  Coccinellidae  as a whole are highly noxious, and experiments with birds and other predators have found this species quite exceptionally distasteful. [ 74 ]  Therefore, their colour is warning ( aposematic ) colouration, and all the morphs are quite conspicuous against green vegetation. The field studies identify differing proportions of morphs at different times of year and in different places, which indicates a high level of selection. However, the basis of that selection is still not known for sure, though many theories have been proposed. [ 75 ] [ 76 ]  Since all the morphs are aposematically coloured, it seems unlikely that the difference between the colour of morphs is directly under selection. Perhaps  pleiotropic  effects of the genes acting on colour also affect the beetle's physiology, and hence its relative fitness. A similar polymorphic system is found in many other species in this family:  Harmonia axyridis  is a good example.


The  scarlet tiger moth   Callimorpha  ( Panaxia )  dominula  (family  Arctiidae ) occurs in continental Europe, western Asia and southern England. It is a day-flying moth, noxious-tasting, with brilliant warning colour in flight, but cryptic at rest. The moth is colonial in habit, and prefers marshy ground or hedgerows. The preferred food of the larvae is the herb  Comfrey  ( Symphytum officinale ). In England it has one generation per year.


The moth is known to be polymorphic in its colony at  Cothill , about five miles (8 km) from  Oxford , with three forms: the typical  homozygote ; the rare homozygote ( bimacula ) and the  heterozygote  ( medionigra ). It was studied there by Ford and later by Sheppard and their co-workers over many years. Data is available from 1939 to the present day, got by the usual field method of capture-mark-release-recapture and by genetic analysis from breeding in captivity. The records cover gene frequency and population-size for much of the twentieth century. [ 32 ] : ch. 7


In this instance the genetics appears to be simple: two  alleles  at a single  locus , producing the three  phenotypes . Total captures over 26 years 1939–64 came to 15,784 homozygous  dominula  (i.e.  typica ), 1,221 heterozygous  medionigra  and 28 homozygous  bimacula . Now, assuming equal viability of the genotypes 1,209 heterozygotes would be expected, so the field results do  not  suggest any heterozygous advantage. It was Sheppard who found that the polymorphism is maintained by selective mating: each genotype preferentially mates with other morphs. [ 77 ]  This is sufficient to maintain the system despite the fact that in this case the heterozygote has slightly lower viability. [ 78 ]


Genetic polymorphism of serum  transferrins  in  reindeer  is used in population and genetic studies. [ 79 ] [ 80 ]  Gene concentrations of  alleles  in populations of  reindeer  of the North-East of Siberia were compared with those in reindeer inhabiting Norway, the northern regions of the European part of the USSR and from North American  caribou . Researchers found that frequencies of Tf alleles of the Siberian reindeer differed from all the others. It is possible that resistance to necrobacteriosis is related to concentrations of alleles in certain reindeer populations. [ 80 ]


Cepaea nemoralis  and  Cepaea hortensis  are famous for the polymorphism of their shells in colour and banding pattern. The background colour of the shell is controlled by a series of  multiple alleles , with brown  dominant  to pink, which is dominant to yellow. Bands may be present or absent and if present vary from one to five in number. Unbanded is dominant, and the number of bands is controlled by modifier genes at unlinked loci (see  epistasis ). The polymorphism exists within most sites, though the proportions of morphs varies considerably between sites. [ 81 ]  Some of the loci controlling the polymorphism are closely linked, forming a  super-gene . It has been hypothesised that selection has brought the loci concerned together, since this avoids undesirable recombinants. [ 32 ]


In England, the snail is regularly predated by the  song thrush   Turdus philomelos , which breaks them open on  thrush anvils  (large stones). Here, fragments accumulate, permitting researchers to analyse the snails taken. The thrushes hunt by sight, and selectively capture those forms which stand out against the background, so predation may be one factor influencing the different proportion of phenotypes (morphs) found in woodland than in open habitats. [ 82 ]


In addition, there is evidence of apostatic selection, with the birds preferentially taking the most common morph. [ 83 ]  This is the 'search image' effect, where a visual predator persists in targeting the morph which it had most often found. Apostatic selection could explain the polymorphism within sites, but song thrushes are not present everywhere.


Dark-coloured shells are at greater risk of overheating in the sun. [ 84 ]  This "climatic selection" provides an alternative explanation for the observed pattern of paler shells in open habitats, [ 85 ]  and also explains the broad trend of paler shells at more southern latitudes (in  C. nemoralis , but not  C. hortensis ). [ 81 ] [ 86 ]


Different colour morphs occupy different microhabitats, which provides another explanation for why different morphs coexist within a site. [ 87 ]  However, non-adaptive explanations for this variation are also plausible, because it takes many generations for natural selection to weed out less fit morphs, and this may be disrupted by  fluctuating selection  and occasional dispersal from adjacent habitats. [ 88 ]  Likewise chance  founder effects  are now the preferred explanation for the  area effects  that puzzled early researchers: a consistent ratio of morphs may occur over a wide area, but then suddenly switch to another ratio in an adjacent area despite no obvious change in habitat. [ 89 ]


An example of a botanical genetic polymorphism is  heterostyly , in which flowers occur in different forms with different arrangements of the  pistils  and the  stamens . The system is called  heteromorphic self-incompatibility , and the general 'strategy' of stamens separated from pistils is known as  herkogamy .


Pin and thrum heterostyly occurs in dimorphic species of  Primula , such as  P. vulgaris . There are two types of flower. The  pin  flower has a long style bearing the stigma at the mouth and the stamens halfway down; and the  thrum  flower has a short style, so the stigma is halfway up the tube and the stamens are at the mouth. So when an insect in search of nectar inserts its proboscis into a long-style flower, the pollen from the stamens stick to the proboscis in exactly the part that will later touch the stigma of the short-styled flower, and vice versa. [ 90 ] [ 91 ]


Another most important property of the heterostyly system is physiological. If thrum pollen is placed on a thrum stigma, or pin pollen on a pin stigma, the reproductive cells are incompatible and relatively little seed is set. Effectively, this ensures out-crossing, as described by Darwin. Quite a lot is now known about the underlying genetics; the system is controlled by a set of closely linked genes which act as a single unit, a  super-gene . [ 32 ] : ch. 10  [ 92 ] [ 40 ] : 86   All sections of the genus  Primula  have heterostyle species, altogether 354 species out of 419. [ 93 ]  Since heterostyly is characteristic of nearly all races or species, the system is at least as old as the genus. [ 94 ]


Between 1861 and 1863, Darwin found the same kind of structure in other groups:  flax  (and other species of  Linum ); and in  purple loosestrife  and other species of  Lythrum . Some of the  Lythrum  species are  trimorphic , with one style and two stamens in each form. [ 95 ]


Heterostyly is known in at least 51 genera of 18 families of Angiosperms. [ 96 ]


Another example is heterocarpy, in which two or more different fruit morphs occur within an individual or population.


Heterospermy is the occurrence of two or more different types of seed within an individual (e.g. as in  Calendula officinalis ) or population.


Heterophylly is the presence of two or more different forms of leaves within an individual.  Heteroblasty  is the occurrence of distinct leaf types at different stages of the life cycle. Other forms are seasonal heterophylly, distinctions between basal and cauline foliage, and between the foliage of old wood and new wood.


Male  common side-blotched lizards  ( Uta stansburiana ) exhibit polymorphism in their throat pigmentation, and these different phenotypes are correlated with different mating strategies. Orange-throated males are the largest and most aggressive, defending large territories and keeping harems of females. Blue-throated males are of intermediate size, and guard smaller territories containing only a single female. Yellow-throated males are the smallest, and instead of holding territories they mimic females in order to sneak matings away from the other two morphs. The balance between these three morphs is maintained by frequency-dependent selection. [ 97 ] [ 98 ]


The  common wall lizard  ( Podarcis muralis ) displays polymorphism and has six distinct morphs which vary by the colour of their throat and underbelly (underbelly colouration seen predominantly in males). [ 99 ]  There are three "pure" morphs of colours: red, yellow and white and three "intermediate" morphs which are a combination of the colours: white-red, white-yellow and red-yellow. [ 99 ]


The  Ctenophorus decresii  lizard displays polymorphism with varying colors of their throats. The throat colors range from white and gray to bright colors of red, orange, or blue. The diversity in throat color is due to a combination of sexual selection and natural selection.


Male  Ctenophorus pictus  lizards display different colors. The most common are red and yellow, but colors can range from brown to orange to red/orange. These morphs are maintained in nature through a combination of selective factors: natural selection and sexual selection.


Viviparous lizards  display color polymorphism in three ventral colors: yellow, orange, and a mixture of the two. These color morphs respond to variation in density frequency-dependence within their environment.


In  evolutionary biology ,  mimicry  is an evolved resemblance between an  organism  and another object, often an organism of another species. Mimicry may evolve between different species, or between individuals of the same species. In the simplest case, as in  Batesian mimicry , a mimic resembles a model, so as to deceive a dupe, all three being of different species. A Batesian mimic, such as a  hoverfly , is harmless, while its model, such as a  wasp , is harmful, and is avoided by the dupe, such as an insect-eating bird. Birds hunt by sight, so the mimicry in that case is visual, but in other cases mimicry may make use of any of the senses. Most types of mimicry, including Batesian, are deceptive, as the mimics are not harmful, but  Müllerian mimicry , where different harmful species resemble each other, is  honest , as when species of wasps and of bees all have genuinely  aposematic  warning coloration. More complex types may be bipolar, involving only two species, such as when the model and the dupe are the same; this occurs for example in  aggressive mimicry , where a  predator  in  wolf-in-sheep's-clothing  style resembles its prey, allowing it to hunt undetected. Mimicry is not limited to animals; in  Pouyannian mimicry , an orchid flower is the mimic, resembling a female bee, its model; the dupe is the male bee of the same species, which tries to copulate with the flower, enabling it to transfer pollen, so the mimicry is again bipolar. In  automimicry , another bipolar system, model and mimic are the same, as when blue  lycaenid  butterflies have 'tails' or  eyespots  on their wings that mimic their own heads, misdirecting predator dupes to strike harmlessly. Many other types of mimicry exist.


Use of the word  mimicry  dates to 1637. It derives from the  Greek  term  mimetikos , "imitative", in turn from  mimetos , the verbal adjective of  mimeisthai , "to imitate". [ 1 ]  "Mimicry" was first used in zoology by the English  entomologists   William Kirby  and  William Spence  in 1823. [ 2 ] [ 3 ]  Originally used to describe people, "mimetic" was used in zoology from 1851. [ 1 ]


Aristotle  wrote in his  History of Animals  that  partridges  use a deceptive  distraction display  to lure predators away from their flightless young: [ 2 ] [ 4 ]


When a man comes by chance upon a young brood [of partridges], and tries to catch them, the hen-bird rolls in front of the hunter, pretending to be lame: the man every moment thinks he is on the point of catching her, and so she draws him on and on, until every one of her brood has had time to escape; hereupon she returns to the nest and calls the young back.


The behaviour is recognised as a form of mimicry by biologists. [ 2 ]


In 1823, Kirby and Spence, in their book  An Introduction to Entomology , used the term "mimicry" informally to depict the way that the structure and coloration of some insects resembled objects in their environments: [ 2 ] [ 3 ]


A jumping bug, very similar to the one figured by  Schellenberg , also much resembles the  lichens  of the oak on which I took it. The spectre tribe ( Phasma ) go still further in this mimicry, representing a small branch with its spray. [ 3 ]


The English naturalist  Henry Walter Bates  worked for several years on butterflies in the  Amazon rainforest . Returning home, he described multiple forms of mimicry in an 1862 paper at the  Linnean Society  in London, [ 6 ]  and then in his 1863 book  The Naturalist on the River Amazons . [ 2 ] [ 7 ]  The term "Batesian mimicry" has since been used in his honour, its usage becoming restricted to the situation in which a harmless mimic gains protection from its predators by resembling a distasteful model. [ 2 ]  Among the observations in Bates's 1862 paper is the statement:


I was never able to distinguish the  Leptalides  from the species they imitated, although they belong to a family totally different in structure and metamorphosis from the  Heliconidae , without examining them closely after capture. [ 6 ]


The German naturalist  Fritz Müller  also spent many years studying butterflies in the Amazon rainforest. He first published a journal article on mimicry in German in 1878, [ 8 ]  followed in 1879 by a paper to the  Entomological Society of London  (translated and presented by Ralph Meldola). [ 9 ]  He described a situation where different species were each unpalatable to predators, and shared similar, genuine, warning signals. Bates found it hard to explain why this should be so, asking why they should need to mimic each other if both were harmful and could warn off predators on their own. Müller put forward  the first mathematical model of mimicry  for this phenomenon: if a common predator confuses the two species, individuals in both those species are more likely to survive, as fewer individuals of either species are killed by the predator. The term  Müllerian mimicry , named in his honour, has since been used for this mutualistic form of mimicry. [ 5 ] [ 10 ]


Müller wrote that


The resemblance of the genera named [ Ituna  and  Thyridia ] is the more worthy of notice since it occurs between insects both belonging to the group of butterflies which are protected by distastefulness. The explanation which applies in ordinary cases of [Batesian] mimicry—and no other has, so far as I know, been offered—cannot obtain for this imitation among protected species. [ 9 ]


Mimicry is an  evolved  resemblance between an organism and another object, often an organism of another species. Mimicry may evolve between different species, or between individuals of the same species. Often, mimicry functions  to protect from predators . [ 11 ]  Mimicry systems have three basic roles: a mimic, a model, and a dupe. When these correspond to three separate species, the system is called disjunct; when the roles are taken by just two species, the system is called bipolar. [ 2 ] [ 12 ]  Mimicry evolves if a dupe (such as a predator) perceives a mimic (such as a palatable prey) as a model (the organism it resembles), and  is deceived to change its behaviour  to the mimic's selective advantage. [ 13 ]  The resemblances can be via any sensory modality, including any combination of visual, acoustic, chemical, tactile, or electric. [ 13 ] [ 14 ]  Mimicry may be to the advantage of both organisms that share a resemblance, in which case it is  mutualistic ; or it can be to the detriment of one, making it  parasitic  or  competitive . The evolutionary convergence between groups is driven by the  selective  action of a dupe. [ 2 ] [ 15 ]  Birds, for example, use sight to identify palatable insects, [ 16 ]  whilst avoiding noxious ones. Over time, palatable insects may evolve to resemble noxious ones, making them mimics and the noxious ones models. Models do not have to be more abundant than mimics. [ 17 ]  In the case of mutualism, each model is also a mimic; all such species can be called "co-mimics". [ 17 ]  Many harmless species such as hoverflies are  Batesian mimics  of strongly defended species such as wasps, while many such well-defended species form  Müllerian mimicry  rings of co-mimics. [ 2 ]  In the evolution of wasp-like appearance, it has been argued that insects evolve to masquerade wasps since predatory wasps do not attack each other, and that this mimetic resemblance has had the useful side-effect of deterring vertebrate predators. [ 18 ]


Mimicry can result in an  evolutionary arms race  if mimicry negatively affects the model, in which case the model can evolve a different appearance from the mimic. [ 17 ] p161  Mimics may have different models for different  life cycle  stages, or they may be  polymorphic , with different individuals imitating different models, as occurs in  Heliconius  butterflies. Models tend to be relatively closely related to their mimics, [ 19 ]  but mimicry can be of vastly different species, for example when spiders mimic ants. Most known mimics are insects, [ 14 ]  though many other examples including  vertebrates , plants, and fungi exist. [ 20 ] [ 21 ] [ 22 ]


It is widely accepted that mimicry evolves as a positive adaptation. The  lepidopterist  and novelist  Vladimir Nabokov  however argued that although natural selection might stabilize a "mimic" form, it would not be necessary to create it. [ 23 ]  The most widely accepted model used to explain the evolution of mimicry in butterflies is the two-step hypothesis. The first step involves  mutation  in  modifier genes  that regulate a complex cluster of linked genes that cause large changes in morphology. The second step consists of selections on genes with smaller  phenotypic  effects, creating an increasingly close resemblance. This model is supported by empirical evidence that suggests that a few single point mutations cause large phenotypic effects, while numerous others produce smaller effects. Some regulatory elements collaborate to form a  supergene  for the development of butterfly color patterns. The model is supported by computational simulations of  population genetics . [ 24 ]  The Batesian mimicry in  Papilio polytes  is controlled by the  doublesex  gene. [ 25 ]


Some mimicry is imperfect. Natural selection drives mimicry only far enough to deceive predators. For example, when predators avoid a mimic that imperfectly resembles a coral snake, the mimic is sufficiently protected. [ 26 ] [ 27 ] [ 28 ]


Convergent evolution  is an alternative explanation for why coral reef fish have come to resemble each other; [ 29 ] [ 30 ]  the same applies to benthic marine invertebrates such as  sponges  and  nudibranchs . [ 31 ]


In its broadest definition, mimicry can include non-living models. The specific terms  masquerade  and  mimesis  are sometimes used when the models are inanimate, and the mimicry's purpose is  crypsis . [ 32 ] [ 14 ] [ 2 ]  For example, animals such as  flower mantises ,  planthoppers ,  comma  and  geometer moth  caterpillars resemble twigs, bark, leaves, bird droppings or flowers. [ 14 ] [ 17 ] [ 33 ] [ 34 ]  In addition, predators may make use of resemblance to harmless objects in aggressive masquerade, to enable them to approach prey. [ 35 ]  This  wolf in sheep's clothing  strategy differs from the more specific resemblance to the prey in  aggressive mimicry , where the prey is both model and dupe. [ 35 ]


Many animals bear  eyespots , which are hypothesized to resemble the eyes of larger animals. They may not resemble any specific organism's eyes, and whether or not animals respond to them as eyes is also unclear. [ 36 ] [ 37 ] [ 38 ] [ 39 ]  The model is usually another species, except in  automimicry , where members of the species mimic other members, or other parts of their own bodies, and in inter-sexual mimicry, where members of one sex mimic members of the other. [ 17 ]


Many types of mimicry have been described. An overview of each follows, highlighting the similarities and differences between the various forms. Classification is often based on  function  with respect to the mimic (e.g., avoiding harm). Some cases may belong to more than one class, e.g., automimicry and aggressive mimicry are not mutually exclusive, as one describes the species relationship between model and mimic, while the other describes the function for the mimic (obtaining food). The terminology used has been debated, as classifications have differed or overlapped; attempts to clarify definitions have led to the partial replacement of old terms with new ones. [ 2 ] [ 40 ] [ 41 ]


Mimicry is defensive or protective when organisms are able to avoid harmful encounters by deceiving enemies into treating them as something else.


In Batesian mimicry, the mimic resembles the model, but does not have the attribute that makes it unprofitable to predators (e.g., unpalatability, or the ability to sting). In other words, a Batesian mimic is a  sheep in wolf's clothing . Mimics are less likely to be found out (for example by predators) when in low proportion to their model. Such  negative frequency-dependent selection  applies in most forms of mimicry. Specifically, Batesian mimicry can only be maintained if the harm caused to the predator by eating a model outweighs the benefit of eating a mimic. The nature of learning is weighted in favor of the mimics, for a predator that has a bad first experience with a model tends to avoid anything that looks like it for a long time, and does not re-sample soon to see whether the initial experience was a false negative. However, if mimics become more abundant than models, then the probability of a young predator having a first experience with a mimic increases. Batesian systems are therefore most likely to be stable where the model is more abundant than the mimic. [ 44 ]


There are many Batesian mimics among  butterflies and moths .  Consul fabius  and  Eresia eunice  imitate unpalatable  Heliconius  butterflies such as  H. ismenius . [ 45 ]   Limenitis arthemis  imitate the poisonous pipevine swallowtail ( Battus philenor ). Several palatable moths produce ultrasonic click calls to mimic unpalatable tiger moths. [ 46 ] [ 47 ] [ 48 ] [ 49 ]   Octopuses  of the genus  Thaumoctopus  (the  mimic octopus ) are able to intentionally alter their body shape and coloration to resemble dangerous  sea snakes  or  lionfish . [ 50 ]  In the Amazon, the  helmeted woodpecker  ( Dryocopus galeatus ), a rare species which lives in the Atlantic Forest of Brazil, Paraguay, and Argentina, has a similar red crest, black back, and barred underside to two larger woodpeckers:  Dryocopus lineatus  and  Campephilus robustus . This mimicry reduces attacks on  D. galeatus . [ 51 ]  Batesian mimicry occurs in the plant kingdom, where the  chameleon vine  adapts its leaf shape and colour to match that of the plant it is climbing. [ 52 ]


In Müllerian mimicry, two or more species have similar warning or  aposematic  signals and both share genuine  anti-predation  attributes (e.g. being unpalatable), as first described in  Heliconius  butterflies. [ 53 ]  This type of mimicry is unique in several respects. Firstly, both the mimic and the model benefit from the interaction, which could thus be classified as  mutualism . The signal receiver also benefits by this system, despite being deceived about species identity, as it is able to generalize the pattern to potentially harmful encounters. The distinction between mimic and model that is clear in  Batesian mimicry  is also blurred. Where one species is scarce and another abundant, the rare species can be said to be the mimic. When both are present in similar numbers, however, it makes more sense to speak of each as a  co-mimic  than of distinct 'mimic' and 'model' species, as their warning signals tend to converge. [ 43 ]  Also, the mimetic species may exist on a continuum from harmless to highly noxious, so Batesian mimicry grades smoothly into Müllerian convergence. [ 54 ] [ 55 ]


Emsleyan or Mertensian mimicry describes the unusual case where a deadly prey mimics a less dangerous species. [ 2 ]  It was first proposed by M. G. Emsley in 1966 as a possible explanation for how a predator can learn to avoid a very dangerous aposematic animal, such as a  coral snake , when the predator is very likely to die, making learning unlikely. [ 56 ]  The theory was developed by the German biologist  Wolfgang Wickler  who named it after the  German   herpetologist   Robert Mertens . [ 14 ] [ 57 ] [ 58 ] [ 59 ]  The scenario is unlike Müllerian mimicry, where the most harmful species is the model. But if a predator dies on its first encounter with a deadly snake, it has no occasion to  learn  to recognize the snake's warning signals. There would then be no advantage for an extremely deadly snake in being aposematic: any predator that attacked it would be killed before it could learn to avoid the deadly prey, so the snake would be better off being camouflaged to avoid attacks. But if the predator first learnt to avoid a less deadly warning-coloured snake, the deadly species could profit by mimicking the less dangerous snake. [ 58 ] [ 59 ]  Some harmless  milk snakes  ( Lampropeltis triangulum ), the moderately toxic false coral snakes ( Erythrolamprus aesculapii ), and the deadly  coral snakes  ( Micrurus ) all have a red background color with black and white/yellow rings. In this system, both the milk snakes and the deadly coral snakes are mimics, while the false coral snakes are the model. [ 56 ]


In  Wasmannian mimicry , the mimic resembles a model that it  lives along with  in a nest or colony. Most of the models here are  eusocial  insects, principally ants. [ 60 ] [ 61 ]


Gilbertian mimicry is bipolar, involving only two species. The potential host (or prey) drives away its parasite (or predator) by mimicking it, the reverse of host-parasite aggressive mimicry. It was coined by Pasteur as a phrase for such rare mimicry systems, [ 2 ]  and is named after the American  ecologist   Lawrence E. Gilbert  who described it in 1975. [ 62 ]  The classical instance of Gilbertian mimicry is in the plant genus  Passiflora , which is grazed by the  micropredator  larvae of some  Heliconius  butterflies. The host plants have evolved  stipules  that mimic mature  Heliconius  eggs near the point of hatching. The butterflies avoid laying eggs near existing ones, reducing  intraspecific competition  between caterpillars, which are also  cannibalistic , so those that lay on vacant leaves provide their offspring with a greater chance of survival. The stipules thus appear to have evolved as Gilbertian mimics of butterfly eggs, under selection pressure from these caterpillars. [ 2 ] [ 19 ]


Browerian mimicry, named after  Lincoln P. Brower  and Jane Van Zandt Brower who first described it in 1967, [ 63 ] [ 64 ]  is a postulated form of  automimicry ; where the model belongs to the same species as the mimic. This is the analogue of Batesian mimicry within a single species, and occurs when there is a palatability spectrum within a population. Examples include the  monarch  and the  queen  from the subfamily  Danainae , which feed on  milkweed  species of varying toxicity. These species store toxins from its host plant, which are maintained even in the adult. As levels of toxin vary depending on diet, some individuals are more toxic than the rest, which profit from the toxicity of those individuals, just as hoverflies benefit from mimicking well-defended wasps. [ 2 ]


One form of automimicry is where one part of an organism's body resembles another part. For example, the tails of some snakes resemble their heads; they move backwards when threatened and present the predator with the tail, improving their chances of escape without fatal harm. Some fishes have  eyespots  near their tails, and when mildly alarmed swim slowly backwards, presenting the tail as a head. Some insects such as some  lycaenid  butterflies have tail patterns and appendages of various degrees of sophistication that promote attacks at the rear rather than at the head. Several species of  pygmy owl  bear "false eyes" on the back of the head, misleading predators into reacting as though they were the subject of an aggressive stare. [ 65 ]  Many insects have filamentous "tails" at the ends of their wings and patterns of markings on the wings themselves. These combine to create a "false head". This misdirects predators such as birds and  jumping spiders . Spectacular examples occur in the  hairstreak  butterflies; when perching on a twig or flower, they commonly do so upside down and shift their rear wings repeatedly, causing antenna-like movements of the "tails" on their wings. Studies of rear-wing damage support the hypothesis that this strategy is effective in deflecting attacks from the insect's head. [ 66 ] [ 67 ]


Aggressive mimicry  is found in predators or  parasites  that share some of the characteristics of a harmless species, allowing them to avoid detection by their prey or  host ; the strategy resembles a  wolf in sheep's clothing , though no conscious deceptive intent is involved. The mimic may resemble the prey or host itself, or another organism that does not threaten the prey or host.  [ 68 ]


Several  spiders  use aggressive mimicry to lure prey. [ 69 ]  Species such as the silver argiope ( Argiope argentata ) employ prominent patterns in the middle of their webs, such as zigzags. These may reflect ultraviolet light, and mimic the pattern seen in many flowers known as  nectar guides . Spiders change their web day to day, which can be explained by the ability of bees to remember web patterns. [ 70 ]


Another case is where males are lured towards what seems to be a  sexually receptive  female. The model in this situation is the same species as the dupe. Female  fireflies  of the genus  Photuris  emit light signals that mimic the mating signals of females of the genus  Photinus . [ 71 ]  Male fireflies from several different  genera  are attracted to these " femmes fatales ", and are captured and eaten. Each female has a repertoire of signals matching the delay and duration of the flashes of the female of the corresponding species. [ 72 ]


Some  carnivorous plants  may be able to increase their rate of capturing insect prey through mimicry. [ 73 ]


A different aggressive strategy is to mimic a mutualistic  symbiont  of the prey.  Cleaner fish  eat parasites and dead skin from client fish. Some allow the cleaner to venture inside their body to hunt these parasites. However, the sabre-toothed blenny or  false cleanerfish  ( Aspidontus taeniatus ) mimics the  bluestreak cleaner wrasse  ( Labroides dimidiatus ), which is recognized by other fishes as a cleaner. The false cleanerfish resembles the cleaner, and mimics the cleaner's "dance". Once it is allowed close to the client, it attacks, biting off a piece of its fin before fleeing. Fish wounded in this fashion soon learn to distinguish mimic from model, but because the similarity is close they also become much more cautious of the model. [ 74 ]


A mechanism that does not involve any luring is seen in the  zone-tailed hawk , which resembles the  turkey vulture . It flies amongst the vultures, effectively camouflaged as a vulture which poses no threat to the hawk's prey. It hunts by suddenly breaking from the formation and ambushing its prey. [ 75 ]


Parasites can be aggressive mimics, though the situation is somewhat different from those outlined previously. They can mimic their hosts' natural prey, allowing themselves to be eaten as a pathway into their host.  Leucochloridium , a genus of  flatworm , matures in the digestive system of  songbirds , their eggs then passing out of the bird in the  faeces . They are then taken up by  Succinea , a terrestrial snail. The eggs develop in this  intermediate host , and must then find a suitable bird to mature in. Since the host birds do not eat snails, the  sporocyst  has another strategy to reach its host's intestine. They are brightly coloured and move in a pulsating fashion. A sporocyst-sac pulsates in the snail's eye stalks, [ 76 ] [ 77 ]  coming to resemble an irresistible meal for a songbird. In this way, it can bridge the gap between hosts, allowing it to complete its life cycle. [ 14 ]  A nematode ( Myrmeconema neotropicum ) changes the colour of the abdomen of workers of the canopy ant  Cephalotes atratus  to make it appear like the ripe fruits of  Hyeronima alchorneoides . It also changes the behaviour of the ant so that the  gaster  (rear part) is held raised. This presumably increases the chances of the ant being eaten by birds. [ 78 ]


Reproductive mimicry occurs when the actions of the dupe directly aid in the mimic's  reproduction . This is common in plants with deceptive flowers that do not provide the reward they seem to offer and it may occur in Papua New Guinea fireflies, in which the signal of  Pteroptyx effulgens  is used by  P. tarsalis  to form aggregations to attract females. [ 79 ]  Other forms of mimicry have a reproductive component, such as  Vavilovian mimicry  involving seeds, vocal mimicry in birds, [ 80 ] [ 81 ] [ 82 ]  and aggressive and Batesian mimicry in brood parasite-host systems. [ 83 ]


Bakerian mimicry, named after  Herbert G. Baker , [ 84 ]  is a form of automimicry where female  flowers  mimic male flowers of their own species, cheating pollinators out of a reward. This reproductive mimicry may not be readily apparent as members of the same species may still exhibit some degree of  sexual dimorphism . It is common in many species of  Caricaceae . [ 85 ]


In Dodsonian mimicry, named after  Calaway H. Dodson , the model belongs to a different species than the mimic. [ 86 ]  By resembling the model, a flower can lure its pollinators without offering nectar. The mechanism occurs in several orchids, including  Epidendrum ibaguense  which mimics flowers of  Lantana camara  and  Asclepias curassavica , and is pollinated by monarch butterflies and perhaps  hummingbirds . [ 87 ]


Brood parasitism  or Kirbyan mimicry is a two species system where a brood parasite mimics its host.  Cuckoos  are a canonical example; the female cuckoo has its offspring raised by a bird of a different species, cutting down the biological mother's  parental investment . The ability to lay eggs that mimic the host eggs is the key  adaptation . The adaptation to different hosts is inherited through the female line in so-called  gentes  (gens, singular).  Intraspecific  brood parasitism, where a female lays in a conspecific's nest, as illustrated by the  goldeneye  duck ( Bucephala clangula ), do not involve mimicry [ 88 ]  The parasitic butterfly  Phengaris rebeli  parasitizes the ant species  Myrmica schencki  by releasing chemicals that fool the worker ants to believe that the caterpillar larvae are ant larvae. This enables the larvae to be brought directly into the ant's nest. [ 89 ]


In Pouyannian mimicry, a flower mimics a  female  of a certain  insect  species, inducing the  males  of that species to try to copulate with the flower. This is much like aggressive mimicry in fireflies, but with a more benign outcome for the pollinator. The mechanism is named after  Maurice-Alexandre Pouyanne , who first described the phenomenon. [ 2 ] [ 92 ] [ 93 ]  It is most common in orchids, which mimic females of the order  Hymenoptera  (generally bees and wasps), and may account for around 60% of pollinations. Depending on the morphology of the flower, a pollen sac called a  pollinium  is attached to the head or abdomen of the male. This is then transferred to the  stigma  of the next flower the male tries to inseminate, resulting in pollination. The mimicry is a combination of visual,  by olfaction , and by touch. [ 94 ]


Vavilovian mimicry is found in  weeds  that come to share characteristics with a  domesticated plant  through  unintentional selection . [ 2 ]  It is named after Russian  botanist  and  geneticist   Nikolai Vavilov . [ 95 ]  Selection against the weed may occur either by manually killing the weed, or by separating its seeds from those of the crop by  winnowing . Vavilovian mimicry illustrates unintentional  selection by man . Weeders do not want to select weeds and their seeds that look increasingly like cultivated plants, yet there is no other option. For example, early barnyard grass,  Echinochloa oryzoides , is a weed in  rice  fields and looks similar to rice; its seeds are often mixed in rice and have become difficult to separate through Vavilovian mimicry. [ 96 ]  Vavilovian mimics may eventually be domesticated themselves, as in the case of rye in wheat; Vavilov called these weed-crops  secondary crops . [ 95 ]


Inter-sexual mimicry (a type of automimicry, as it is within a single species) occurs when individuals of one sex in a species mimic members of the opposite sex to facilitate  sneak mating . An example is the three male forms of the marine  isopod   Paracerceis sculpta . Alpha males are the largest and guard a  harem  of females. Beta males mimic females and manage to enter the harem of females without being detected by the alpha males allowing them to mate. Gamma males are the smallest males and mimic juveniles. This also allows them to mate with the females without the alpha males detecting them. [ 97 ]  Similarly, among  common side-blotched lizards , some males mimic the yellow throat coloration and even mating rejection behaviour of the other sex to sneak matings with guarded females. These males look and behave like unreceptive females. This  strategy  is effective against "usurper" males with orange throats, but ineffective against blue throated "guarder" males, which chase them away. [ 98 ] [ 99 ]  Female  spotted hyenas  have  pseudo-penises  that make them look like males. [ 100 ]


In  evolutionary biology ,  mimicry in plants  is where a plant evolves to resemble another organism physically or chemically. Mimicry in plants has been studied far less than  mimicry in animals . It may provide  protection against herbivory , or may deceptively encourage mutualists, like  pollinators , to provide a service without offering a reward in return. [ 2 ]


Types of plant mimicry include  Bakerian , where female flowers imitate males of the same species;  Dodsonian , where a plant mimics a rewarding flower, luring pollinators by mimicking another species of flower, or fruit where feeders of the other species are attracted to a fake fruit to distribute seeds;  Gilbertian , where a plant has structures like butterfly eggs, dissuading egg-laying;  Vavilovian , where a  weed  is  unintentionally selected  to resemble a  crop plant ;  Pouyannian , in which a flower imitates a female mate, deceiving a male pollinating insect into  pseudocopulation ;  Batesian , where a harmless species deter predators by mimicking the characteristics of a harmful species; and leaf mimicry, where a plant resembles a nearby plant to evade the attention of herbivores.


Mimicry is an  adaptation  by a species, called the mimic, making it resemble something else, called the model, with the effect of deceiving another species, the dupe. The three are not always all distinct, as mimicry can for example be within a species. [ 3 ]  The adaptation is to the  evolutionary  advantage of the mimic. As such it can be any mechanism that may evolve by  natural selection ; no conscious intention is involved. There is no essential difference between the evolution of mimicry in plants and in other organisms such as  in animals , though mimicry in animals is better known. Plant mimicry can broadly be divided into reproductive mimicry that deceives  pollinators  into providing their service without rewarding them in return, and protective mimicry that deceives herbivores into not eating the plant. [ 2 ]


Mimicry mechanisms, including the  Batesian [ 4 ]  and  Müllerian [ 5 ]  mimicry, were described in animals in the 19th century. [ 3 ]   Charles Darwin 's 1862 book  Fertilisation of Orchids  laid the foundations for research into plant  reproductive strategies co-evolved with insects . In the book, Darwin wonders why some orchids apparently mimic bees, and how orchids without nectar succeed in attracting pollinators. [ 6 ]  Several mimicry mechanisms in plants were described in the 20th century, starting with  Pouyannian  (1916), [ 7 ]  Vavilovian (1951), [ 8 ]  and  Gilbertian  (1975). [ 9 ]  Study of mimicry in plants broadened in the 21st century, with the discovery of Batesian mimicry in plants in 2003, [ 10 ]  and crypsis or leaf mimicry in 2014. [ 1 ]  Until then, mimicry in plants was studied infrequently, both because it is uncommon and because botanists considered mainly physical factors in plant ecology. It may be that since plants often grow in clusters, foraging herbivores are hard to deceive once they have started to feed in an area. [ 11 ]


Carrion flowers , including the enormous  Amorphophallus titanum , [ 11 ]  mimic the scent and appearance of rotting flesh to attract necrophagous ( carrion -feeding) insects like flesh flies ( Sarcophagidae ), blowflies ( Calliphoridae ), house flies ( Muscidae ) and some beetles (e.g.,  Dermestidae  and  Silphidae ) which search for dead animals to use as brood sites. [ 12 ] [ 13 ]  The decaying smell of the flower comes from  oligosulfides , compounds found in decayed proteins with the sulfur-containing amino acids  methionine  and  cysteine . While carrion flowers do produce a small amount of  nectar , this does not necessarily make their relationship to necrophagous insects  mutualistic . Insects lay eggs on the carrion flowers, meaning they mistake the flowers for  oviposition  sites. The nectar acts as a lure to bring the insects closer to the reproductive parts of the flower. [ 12 ]


Bakerian mimicry, named after English naturalist  Herbert Baker , [ 14 ]  is a form of  automimicry  or  intraspecific mimicry  that occurs within a single species.  In plants, the female  flowers  mimic male flowers of their own species, cheating pollinators out of a reward.  This reproductive mimicry may not be readily apparent as members of the same species may still exhibit some degree of  sexual dimorphism , i.e. the  phenotypic  difference between males and females of the same species.  It is common in many species of  Caricaceae , a  family  of  flowering plants  in the order  Brassicales , found primarily in  tropical  regions of  Central  and  South America , and  Africa . [ 15 ] [ 16 ]


Dodsonian mimicry, named after the American  botanist   Calaway H. Dodson , is a form of reproductive floral mimicry where the model belongs to a different species than the mimic. By providing similar sensory signals to the model's flowers, the mimic lures the model's pollinators. Like Bakerian mimics, no  nectar  is provided. [ 17 ]


Epidendrum ibaguense , a species of  epiphytic  orchid of the genus  Epidendrum  that occurs in the northern part of South America, resembles flowers of  Lantana camara  and  Asclepias curassavica .  Epidendrum ibaguense  is pollinated by  monarch butterfly  ( Danaus plexippus ) and perhaps  hummingbirds . [ 18 ]  Similar cases are seen in some other species of the same family.  The mimetic species may still have pollinators of its own; for example, a  Lamellicorn  beetle helps to pollinate  Ophrys  species mainly pollinated by  bees . [ 19 ]


In  Pouyannian mimicry , [ 20 ] [ 7 ]  named after the French lawyer and amateur botanist Maurice-Alexandre Pouyanne, [ 21 ]  flowers mimic a male  pollinator 's potential female mate, visually or with other stimuli. [ 22 ]  Many orchids, including the  Ophrys  bee orchids, deceive male insects into  pseudocopulation  in this way, using them to transfer pollen. [ 23 ]  For instance, the orchid  Epipactis helleborine  is physiologically and  morphologically  adapted to attract  social wasps  as their primary pollinators. Social wasps feed their larvae on insects such as caterpillars. To locate that prey, they use a combination of visual and  olfactory  cues. The flowers of  E. helleborine  and  E. purpurata  emit green-leaf  volatiles  attractive to foragers of the social wasps  Vespula germanica  and  V. vulgaris .  E. helleborine  emits several green-leaf volatiles that induce a response in wasp antennae. These same volatiles are also produced by cabbage leaves infested with caterpillars ( Pieris brassicae ) , which are common prey items for wasps. Despite a large nectar reward, the species is almost entirely overlooked by other pollinators. [ 24 ]


In Batesian mimicry, named after the English naturalist  Henry Walter Bates , a harmless species has evolved to imitate the warning signals of a harmful species directed at a  predator . Batesian mimicry of  thorns [ 10 ]  and possibly of spider webs [ 25 ]  has been observed in plants.


Thorn mimicry of two types has been observed in plants. The first, a special case of intra-organismic Batesian mimicry characteristic of  Aloe  species ( Liliaceae ),  Washingtonia filifera  ( Arecaceae ), and dozens of species of  Agave , including  A. applanta ,  A. salmiana , and  A. obscura . These plants develop thornlike imprints or colorations on the face of their leaves due to the teeth along the margins of that leaf (or another leaf) pressing sustained indentations into the flesh of the non-spiny parts. The second type of thorn mimicry, a more classic case of Batesian mimicry, involves the pointed, colorful organs like  buds , leaves and fruit of mimetic plant species that mimic warning-coloured  aposematic  colorful thorns not found anywhere else in the organism. [ 10 ]


Several plants from different parts of the world may be mimics of spider webs. Dense, white  trichomes  are produced on newly extended stems and leaves that deter  herbivory  due to predatory habit or toxicity. This may be a case of visual mimicry or perceptual exploitation. Case examples include the new buds of  Onopordum  from Israel,  Carthamus  species from Greece, flower heads of  Arctium tomentosum  from Estonia, a fledgling leaf of  Tussilago farfara  from Estonia, and new  fronds  of  Osmunda japonica  from Japan. [ 25 ]


Another plant leaf pattern has been suggested to be mimetic: irregular white blotches on leaves of plants such as  Pulmonaria officinalis  perhaps protect against large herbivores through their resemblance to bird droppings, which could be avoided as possible sources of disease. [ 2 ]


Gilbertian [ 26 ]  or colonisation mimicry [ 2 ]  is bipolar, involving only two species. The potential host (or prey) drives away its parasite (or predator) by mimicking it, the reverse of host–parasite aggressive mimicry. Georges Pasteur named it after the American  ecologist   Lawrence E. Gilbert , who described it in 1975. [ 26 ] [ 9 ]  The classical instance of Gilbertian mimicry is in the plant genus  Passiflora , which is grazed by the  micropredator  larvae of some  Heliconius  butterflies. The host plants have evolved  stipules , small outgrowths at the base of each leaf, that mimic mature  Heliconius  eggs near the point of hatching. The butterflies avoid laying eggs near existing ones, reducing  intraspecific competition  between caterpillars, which are also  cannibalistic , so those that lay on vacant leaves provide their offspring with a greater chance of survival. The stipules thus appear to have evolved as Gilbertian mimics of butterfly eggs, under selection pressure from these caterpillars. [ 26 ] [ 27 ]


In ecology,  crypsis  is an organism's ability to avoid detection by other organisms. In cryptic mimicry, a prey organism deceives a potential predator by providing false, usually visual, signals or a lack of signals, with the effect of  camouflaging  itself.  Boquila trifoliata , a South American member of the family  Lardizabalaceae , is a climbing vine with a highly variable appearance ( phenotype ). It is capable of mimicking the leaf features of plant species that it clings to, such as their coloration, size, and shape. By camouflaging its leaves,  Boquila  reduces damage from herbivorous animals. [ 1 ]  It has been speculated that such plants may make use of "some kind of vision" using ocelli, or "delicate chemical sensing", to account for the mimic's ability to cope with such a large number of variables in its model's appearance, [ 29 ]  including the ability to mimic the foliage of an artificial host plant made of plastic. [ 30 ]  Another plant that could well be a cryptic mimic of its host is the parasitic Australian mistletoe,  Amyema cambagei , which has an "uncanny resemblance" to the foliage of  Casuarina  trees. [ 2 ]


Some hundreds of species in the  Mesembryanthemaceae  (ice plants) of Southern Africa are  camouflaged  as small stones, especially  Lithops , which are known as pebble plants or living stones. By appearing non-living, they are less likely to be eaten by herbivores, and in dusty dry conditions among stones are extremely difficult to detect. [ 11 ]


Vavilovian mimicry (also known as crop mimicry or weed mimicry [ 31 ] ) is named after the Russian plant geneticist who identified the  centres of origin  of cultivated plants,  Nikolai Vavilov . [ 8 ]  It is a form of mimicry in plants where a  weed  comes to share one or more characteristics with a  domesticated plant  through generations of  unintentional selection . Selection against the weed may occur by killing a young or adult weed, separating its  seeds  from those of the crop ( winnowing ), or both. This has been done manually since  Neolithic  times, and in more recent years by  agricultural machinery . [ 32 ]


Darwin's open questions about orchid fertilisation stimulated popularizers such as  Grant Allen  to write on the topic, and led the  science fiction  writer  H. G. Wells  to tell a tale of killer orchids, his 1896 " The Flowering of the Strange Orchid ". The vampire-like plant attacks people using "tentacle-like aerial rootlets". Other authors followed with similar tales of their own. [ 6 ] [ 33 ]


In  genetics , the  phenotype  (from  Ancient Greek     φαίνω  ( phaínō )  'to appear, show' and    τύπος  ( túpos )  'mark, type') is the set of observable characteristics or  traits  of an  organism . [ 1 ] [ 2 ]  The term covers the organism's  morphology  (physical form and structure), its  developmental  processes, its biochemical and physiological properties, its  behavior , and the products of behavior. An organism's phenotype results from two basic factors: the  expression  of an organism's genetic code (its  genotype ) and the influence of environmental factors. Both factors may interact, further affecting the phenotype. When two or more clearly different phenotypes exist in the same population of a species, the species is called  polymorphic . A well-documented example of polymorphism is  Labrador Retriever coloring ; while the coat color depends on many genes, it is clearly seen in the environment as yellow, black, and brown.  Richard Dawkins  in 1978 [ 3 ]  and then again in his 1982 book  The Extended Phenotype  suggested that one can regard  bird nests  and other built structures such as  caddisfly  larva cases and  beaver dams  as "extended phenotypes".


Wilhelm Johannsen  proposed the  genotype–phenotype distinction  in 1911 to make clear the difference between an organism's  hereditary material  and what that hereditary material produces. [ 4 ] [ 5 ]  The distinction resembles that proposed by  August Weismann  (1834–1914), who distinguished between  germ plasm  (heredity) and  somatic cells  (the body). More recently, in  The Selfish Gene  (1976), Dawkins distinguished these concepts as replicators and vehicles.


Despite its seemingly straightforward definition, the concept of the phenotype has hidden subtleties. It may seem that anything dependent on the  genotype  is a phenotype, including  molecules  such as  RNA  and  proteins . Most molecules and structures coded by the genetic material are not visible in the appearance of an organism, yet they are observable (for example by  Western blotting ) and are thus part of the phenotype; human  blood groups  are an example. It may seem that this goes beyond the original intentions of the concept with its focus on the (living) organism in itself. Either way, the term phenotype includes inherent traits or characteristics that are observable or traits that can be made visible by some technical procedure. [ citation needed ]


The term "phenotype" has sometimes been incorrectly used as a shorthand for the phenotypic difference between a mutant and its  wild type , which would lead to the false statement that a
"mutation has no phenotype". [ 6 ]


Behaviors and their consequences are also phenotypes, since behaviors are observable characteristics.  Behavioral phenotypes  include cognitive, personality, and behavioral patterns. Some behavioral phenotypes may characterize psychiatric disorders [ 7 ]  or syndromes. [ 8 ] [ 9 ]


A  phenome  is the set of all traits expressed by a  cell ,  tissue ,  organ ,  organism , or  species . The term was first used by Davis in 1949, "We here propose the name  phenome  for the sum total of extragenic, non-autoreproductive portions of the cell, whether cytoplasmic or nuclear.  The phenome would be the material basis of the phenotype, just as the genome is the material basis of the  genotype ." [ 10 ]


Although phenome has been in use for many years, the distinction between the use of phenome and phenotype is problematic. A proposed definition for both terms as the "physical totality of all traits of an organism or of one of its subsystems" was put forth by Mahner and Kary in 1997, who argue that although scientists tend to intuitively use these and related terms in a manner that does not impede research, the terms are not well defined and usage of the terms is not consistent. [ 11 ]


Some usages of the term suggest that the phenome of a given organism is best understood as a kind of matrix of data representing physical manifestation of phenotype. For example, discussions led by A. Varki among those who had used the term up to 2003 suggested the following definition: "The body of information describing an organism's phenotypes, under the influences of genetic and environmental factors". [ 12 ]  Another team of researchers characterize "the human phenome [as] a multidimensional search space with several neurobiological levels, spanning the proteome, cellular systems (e.g., signaling pathways), neural systems and cognitive and behavioural phenotypes." [ 13 ]


Plant biologists have started to explore the phenome in the study of plant physiology. [ 14 ]


In 2009, a research team demonstrated the feasibility of identifying genotype–phenotype associations using  electronic health records  (EHRs) linked to DNA  biobanks . They called this method  phenome-wide association study  (PheWAS). [ 15 ]


Inspired by the evolution from genotype to genome to  pan-genome , a concept of exploring the relationship ultimately among pan-phenome,  pan-genome , and pan- envirome  was proposed in 2023. [ 16 ]


Phenotypic variation (due to underlying heritable  genetic variation ) is a fundamental prerequisite for  evolution  by  natural selection . It is the living organism as a whole that contributes (or not) to the next generation, so natural selection affects the genetic structure of a population indirectly via the contribution of phenotypes. Without phenotypic variation, there would be no evolution by natural selection. [ 17 ]


The interaction between genotype and phenotype has often been conceptualized by the following relationship:


A more nuanced version of the relationship is:


Genotypes often have much flexibility in the modification and expression of phenotypes; in many organisms these phenotypes are very different under varying environmental conditions. The plant  Hieracium umbellatum  is found growing in two different  habitats  in Sweden. One habitat is rocky, sea-side  cliffs , where the plants are bushy with broad leaves and expanded  inflorescences ; the other is among  sand dunes  where the plants grow prostrate with narrow leaves and compact inflorescences. These habitats alternate along the coast of Sweden and the habitat that the seeds of  Hieracium umbellatum  land in, determine the phenotype that grows. [ 18 ]


An example of random variation in  Drosophila  flies is the number of  ommatidia , which may vary (randomly) between left and right eyes in a single individual as much as they do between different genotypes overall, or between  clones  raised in different environments. [ citation needed ]


The concept of phenotype can be extended to variations below the level of the  gene  that affect an organism's fitness. For example,  silent mutations  that do not change the corresponding amino acid sequence of a gene may change the frequency of  guanine - cytosine  base pairs ( GC content ). These base pairs have a higher thermal stability ( melting point ) than  adenine - thymine , a property that might convey, among organisms living in high-temperature environments, a selective advantage on variants enriched in GC content. [ citation needed ]


Richard Dawkins  described a phenotype that included all effects that a gene has on its surroundings, including other organisms, as an extended phenotype, arguing that "An animal's behavior tends to maximize the survival of the genes 'for' that behavior, whether or not those genes happen to be in the body of the particular animal performing it." [ 3 ]  For instance, an organism such as a  beaver  modifies its environment by building a  beaver dam ; this can be considered an  expression of its genes , just as its  incisor  teeth are—which it uses to modify its environment. Similarly, when a bird feeds a  brood parasite  such as a  cuckoo , it is unwittingly extending its phenotype; and when genes in an  orchid  affect  orchid bee  behavior to increase pollination, or when genes in a  peacock  affect the copulatory decisions of peahens, again, the phenotype is being extended. Genes are, in Dawkins's view, selected by their phenotypic effects. [ 19 ]


Other biologists broadly agree that the extended phenotype concept is relevant, but consider that its role is largely explanatory, rather than assisting in the design of experimental tests. [ 20 ]


Phenotypes are determined by an interaction of genes and the environment, but the mechanism for each gene and phenotype is different. For instance, an  albino  phenotype may be caused by a mutation in the gene encoding  tyrosinase  which is a key enzyme in  melanin  formation. However, exposure to  UV radiation  can increase melanin production, hence the environment plays a role in this phenotype as well. For most complex phenotypes the precise genetic mechanism remains unknown. For instance, it is largely unclear how genes determine the shape of bones or the human ear. [ citation needed ]


Gene expression plays a crucial role in determining the phenotypes of organisms. The level of gene expression can affect the phenotype of an organism. For example, if a gene that codes for a particular  enzyme  is expressed at high levels, the organism may produce more of that enzyme and exhibit a particular trait as a result. On the other hand, if the gene is expressed at low levels, the organism may produce less of the enzyme and exhibit a different trait. [ 22 ]


Gene expression is regulated at various levels and thus each level can affect certain phenotypes, including  transcriptional  and post-transcriptional regulation. [ citation needed ]


Changes in the levels of gene expression can be influenced by a variety of factors, such as environmental conditions, genetic variations, and  epigenetic  modifications. These modifications can be influenced by environmental factors such as diet, stress, and exposure to toxins, and can have a significant impact on an individual's phenotype. Some phenotypes may be the result of changes in gene expression due to these factors, rather than changes in genotype. An experiment involving  machine learning  methods utilizing gene expressions measured from RNA sequencing found that they can contain enough signal to separate individuals in the context of phenotype prediction. [ 23 ]


Although a phenotype is the ensemble of observable characteristics displayed by an organism, the word  phenome  is sometimes used to refer to a collection of traits, while the simultaneous study of such a collection is referred to as  phenomics . [ 24 ] [ 25 ]  Phenomics is an important field of study because it can be used to figure out which genomic variants affect phenotypes which then can be used to explain things like health, disease, and evolutionary fitness. [ 26 ]  Phenomics forms a large part of the  Human Genome Project . [ 27 ]


Phenomics has applications in agriculture. For instance, genomic variations such as drought and heat resistance can be identified through phenomics to create more durable GMOs. [ 28 ] [ 14 ]


Phenomics may be a stepping stone towards  personalized medicine , particularly  drug therapy . [ 29 ]  Once the phenomic database has acquired enough data, a person's phenomic information can be used to select specific drugs tailored to the individual. [ 29 ]