Large-scale genetic screens can identify the genes or  mutations  that affect the phenotype of an organism. Analyzing the phenotypes of mutant genes can also aid in determining gene function. [ 30 ]  Most genetic screens have used microorganisms, in which genes can be easily deleted. For instance, nearly all genes have been deleted in  E. coli [ 31 ]  and many other  bacteria , but also in several eukaryotic model organisms such as  baker's yeast [ 32 ]  and  fission yeast . [ 33 ]  Among other discoveries, such studies have revealed lists of essential genes .


More recently, large-scale  phenotypic screens  have also been used in animals, e.g. to study lesser understood phenotypes such as  behavior . In one screen, the role of mutations in mice were studied in areas such as learning and  memory ,  circadian rhythmicity , vision, responses to stress and response to  psychostimulants .


This experiment involved the progeny of mice treated with  ENU , or N-ethyl-N-nitrosourea, which is a potent mutagen that causes  point mutations . The mice were phenotypically screened for alterations in the different behavioral domains in order to find the number of putative mutants (see table for details). Putative mutants are then tested for heritability in order to help determine the inheritance pattern as well as map out the mutations. Once they have been mapped out, cloned, and identified, it can be determined whether a mutation represents a new gene or not.


These experiments showed that mutations in the  rhodopsin  gene affected vision and can even cause  retinal degeneration  in mice. [ 34 ]  The same  amino acid  change causes  human familial blindness , showing how phenotyping in animals can inform medical diagnostics and possibly therapy.


The  RNA world  is the hypothesized pre-cellular stage in the evolutionary history of life on earth, in which self-replicating RNA molecules proliferated prior to the evolution of  DNA  and proteins. [ 35 ]  The folded  three-dimensional  physical structure of the first RNA molecule that possessed ribozyme activity promoting replication while avoiding destruction would have been the first phenotype, and the  nucleotide sequence  of the first self-replicating RNA molecule would have been the original genotype. [ 35 ]


The term  poecilogony  was coined by  Alfred Mathieu Giard  to describe a  polymorphism  in  larval  development in  marine   invertebrates . To date, this life history trait is only known in a small number of  polychaete  taxa, [ 1 ]  as well as some  sacoglossan   mollusks . In some cases, the variation in larval type is a ' plastic ' trait in that it is environmentally mediated. In other cases, the larval type is genetically determined - a good example is the  polychaete   Streblospio benedicti , where some mothers release small  planktotrophic  eggs and other mothers release large  lecithotrophic  eggs. [ 2 ]  In either case, the variation in larval type generally involves the production of larvae that differ in feeding mode and/or developmental time.


Many types of  polymorphism  can be seen in the  insect   order   Lepidoptera . Polymorphism is the appearance of forms or "morphs" differing in color and number of attributes within a single species. [ 1 ] : 163  [ 2 ]  In Lepidoptera, polymorphism can be seen not only between individuals in a population but also between the sexes as  sexual dimorphism , between geographically separated populations in  geographical polymorphism  and also between generations flying at different seasons of the year ( seasonal polymorphism ). It also includes the phenomenon of  mimicry  when mimetic morphs fly alongside non-mimetic morphs in a population of a particular species. Polymorphism occurs both at a specific level with heritable variation in the overall morphological design of individuals as well as in certain specific morphological or physiological traits within a species. [ 1 ]


Genetic polymorphism occurs when the morphs are a result of genetic determination only. The extreme case of genetic polymorphism is that of the  papilionid  great Mormon ( Papilio memnon ), where four male forms and many as twenty-six female forms are reported. This species, and others in its genus, have been extensively studied for understanding the genetic basis for  polymorphism  and  Batesian mimicry . [ 3 ] [ 4 ] [ 5 ]


In the case of the  scarlet tiger moth   Callimorpha  ( Panaxia )  dominula  (family  Arctiidae ), which is a diurnal moth that occurs in continental Europe, western Asia, and southern England, three forms occur in England: the typical  homozygote ; the rare homozygote ( bimacula ) and the  heterozygote  ( medionigra ). It was studied there by  E. B. Ford , and later by  P. M. Sheppard  and their co-workers over many years. Data is available from 1939 to the present day, acquired 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. [ 6 ]  In this instance, the genetics appears to be simple: two  alleles  at a single  locus , producing the three  phenotypes . Total captures over 26 years 1939–1964 came to 15,784 homozygous  dominula  (i.e.  typica ), 1,221 heterozygous  medionigra,  and 28 homozygous  bimacula . Assuming equal viability of the genotypes 1,209 heterozygotes would be expected, so the field results do not suggest any heterozygous advantage. Sheppard found that the polymorphism is maintained by selective mating: each genotype preferentially mates with other morphs. [ 7 ]  This is sufficient to maintain the system, although in this case the heterozygote has slightly lower viability. [ 8 ]  Another example is the genetic polymorphism of larval developmental rates seen in the  Phengaris rebeli , in which there exist slow-developing larvae (SDL) (75% of the total  P. rebeli  larval population) and fast-developing larvae (FDL) (25% of the total  P. rebeli  larvae population). [ 9 ]  The slow-developing larvae do not grow much during the first year, but grow rapidly during the early part of the second summer and remain a second winter within the ant colonies. On the other hand, the FDL complete their growth the following spring after they are taken into the ant nest. [ 10 ]


Sexual dimorphism  is the occurrence of differences between males and females in a species. In Lepidoptera, sexual dimorphism is widespread and almost completely determined by genetic determination. [ 11 ]  Sexual dimorphism is present in all families of the Papilionoidea and more prominent in the  Lycaenidae ,  Pieridae , and certain taxa of the  Nymphalidae . Apart from a color variation which may differ from slight to completely different color-pattern combinations, secondary sexual characteristics may also be present. [ 12 ] : 25   Different genotypes maintained by natural selection may also be expressed simultaneously. [ 11 ]  Polymorphic and/or mimetic females occur in the case of some taxa in the  Papilionidae  primarily to obtain a level of protection not available to the male of their species.


The most distinct case of sexual dimorphism is that of adult females of many  Psychidae  species who have only vestigial wings, legs, and mouthparts as compared to the adult males who are strong fliers with well-developed wings and feathery antennae. [ 13 ]


Geographical polymorphism is where geographical isolation causes a divergence of a species into different morphs. A good example is the Indian white admiral ( Limenitis procris ) [ clarification needed ]  which has five forms, each geographically separated from the other by large mountain ranges. [ 12 ] : 26   An even more dramatic showcase of geographical polymorphism is the  Apollo butterfly  ( Parnassius apollo ). Due to the Apollos living in small local populations and having no contact with each other, but because of the strong stenotopic species and weak migration ability interbreeding between populations of one species practically does not occur; they form over 600 different morphs, with the size of spots on the wings of which varies greatly. [ 14 ]


Environmental polymorphism, where genetic heritability plays no role, is often termed  polyphenism . Polyphenism in Lepidoptera is commonly seen in the form of seasonal morphs, especially in the butterfly families of  Nymphalidae  and  Pieridae . An Old World pierid butterfly, the common grass yellow ( Eurema hecabe ) has a darker summer adult morph, triggered by a long day exceeding 13 hours in duration, while the shorter diurnal period of 12 hours or less induces a paler morph in the post-monsoon period. [ 11 ]  Polyphenism also occurs in caterpillars, an example being the  peppered moth  ( Biston betularia ). [ 15 ]


Batesian  and  Müllerian   mimicry  complexes are commonly found in Lepidoptera. Genetic polymorphism and natural selection give rise to otherwise edible species (the mimic) gaining a survival advantage by resembling inedible species (the model). Such a mimicry complex is referred to as  Batesian  and is most commonly known by the mimicry by the  limenitidine   viceroy butterfly  of the inedible  danaine   monarch . Later research has discovered that the viceroy is, in fact more toxic than the monarch and this resemblance should be considered as a case of Müllerian mimicry. [ 16 ]


In Müllerian mimicry, inedible species, usually within a taxonomic order, find it advantageous to resemble each other so as to reduce the sampling rate by predators who need to learn about the insects' inedibility. Taxa from the toxic genus  Heliconius  form one of the most well known Müllerian complexes. [ 17 ]  The adults of the various species now resemble each other so well that the species cannot be distinguished without close morphological observation and, in some cases, dissection or genetic analysis.


A  polyphenic trait  is a trait for which multiple, discrete  phenotypes  can arise from a single  genotype  as a result of differing environmental conditions. It is therefore a special case of  phenotypic plasticity .


There are several types of polyphenism in animals, from having sex determined by the environment to the castes of honey bees and other social insects. Some polyphenisms are seasonal, as in some butterflies which have different patterns during the year, and some Arctic animals like the  snowshoe hare  and  Arctic fox , which are white in winter. Other animals have predator-induced or resource polyphenisms, allowing them to exploit variations in their environment. Some nematode worms can develop either into adults or into resting  dauer larvae  according to resource availability.


A  polyphenism  is the occurrence of several phenotypes in a population, the differences between which are not the result of genetic differences. [ 2 ]  For example,  crocodiles  possess a  temperature-dependent sex determining polyphenism , where sex is the trait influenced by variations in nest temperature. [ 3 ]


When polyphenic forms exist at the same time in the same  panmictic  (interbreeding) population they can be compared to  genetic polymorphism . [ 4 ]  With polyphenism, the switch between morphs is environmental, but with genetic polymorphism the determination of morph is genetic. These two cases have in common that more than one morph is part of the population at any one time. This is rather different from cases where one morph predictably follows another during, for instance, the course of a year. In essence the latter is normal  ontogeny  where young forms can and do have different forms, colours and habits to adults.


The discrete nature of polyphenic traits differentiates them from traits like weight and height, which are also dependent on environmental conditions but vary continuously across a spectrum. When a polyphenism is present, an environmental cue causes the organism to develop along a separate pathway, resulting in distinct morphologies; thus, the response to the environmental cue is “all or nothing.” The nature of these environmental conditions varies greatly, and includes seasonal cues like temperature and moisture,  pheromonal  cues,  kairomonal  cues (signals released from one species that can be recognized by another), and nutritional cues.


Sex-determining polyphenisms allow a species to benefit from  sexual reproduction  while permitting an unequal  gender  ratio. This can be beneficial to a species because a large female-to-male ratio maximizes reproductive capacity. However, temperature-dependent sex determination (as seen in crocodiles) limits the range in which a species can exist, and makes the species susceptible to endangerment by changes in weather pattern. [ 3 ]  Temperature-dependent sex determination has been proposed as an explanation for the extinction of the dinosaurs. [ 5 ]


Population-dependent and reversible sex determination, found in animals such as the  blue wrasse  fish, have less potential for failure. In the blue wrasse, only one male is found in a given territory:  larvae  within the territory develop into females, and adult males will not enter the same territory. If a male dies, one of the females in his territory becomes male, replacing him. [ 5 ]  While this system ensures that there will always be a mating couple when two animals of the same species are present, it could potentially decrease genetic variance in a population, for example if the females remain in a single male's territory.


The caste system of insects enables  eusociality , the division of labor between non-breeding and breeding individuals. A series of polyphenisms determines whether larvae develop into queens, workers, and, in some cases soldiers. In the case of the  ant ,  P. morrisi , an embryo must develop under certain temperature and photoperiod conditions in order to become a reproductively-active queen. [ 6 ]  This allows for control of the mating season but, like sex determination, limits the spread of the species into certain  climates .
In bees,  royal jelly  provided by  worker bees  causes a developing larva to become a  queen . Royal jelly is only produced when the queen is aging or has died. This system is less subject to influence by environmental conditions, yet prevents unnecessary production of queens.


Polyphenic  pigmentation  is adaptive for insect species that undergo multiple mating seasons each year. Different pigmentation patterns provide appropriate  camouflage  throughout the seasons, as well as alter heat retention as temperatures change. [ 7 ]  Because insects cease growth and development after  eclosion , their pigment pattern is invariable in adulthood: thus, a polyphenic pigment adaptation would be less valuable for species whose adult form survives longer than one year. [ 5 ]


Birds and mammals are capable of continued physiological changes in adulthood, and some display reversible seasonal polyphenisms, such as in the  Arctic fox , which becomes all white in winter as  snow camouflage . [ 5 ]


Predator-induced polyphenisms allow the species to develop in a more reproductively-successful way in a  predator 's absence, but to otherwise assume a more defensible morphology. However, this can fail if the predator evolves to stop producing the  kairomone  to which the prey responds. For example, the midge larvae ( Chaoborus ) that feed on  Daphnia cucullata  (a  water flea ) release a kairomone that  Daphnia  can detect. When the midge larvae are present,  Daphnia  grow large helmets that protect them from being eaten. However, when the predator is absent,  Daphnia  have smaller heads and are therefore more agile swimmers. [ 5 ]


Organisms with resource polyphenisms show alternative phenotypes that allow differential use of food or other resources. One example is the  western spadefoot toad , which maximizes its reproductive capacity in temporary  desert  ponds. While the water is at a safe level, the tadpoles develop slowly on a diet of other opportunistic pond inhabitants. However, when the water level is low and desiccation is imminent, the tadpoles develop a morphology (wide mouth, strong jaw) that permits them to cannibalize. Cannibalistic tadpoles receive better nutrition and thus metamorphose more quickly, avoiding death as the pond dries up. [ 8 ]


Among  invertebrates , the nematode  Pristionchus pacificus  has one morph that primarily feeds on bacteria and a second morph that produces large teeth, enabling it to feed on other nematodes, including competitors for bacterial food. In this species, cues of starvation and crowding by other nematodes, as sensed by pheromones, trigger a  hormonal  signal that ultimately activates a developmental switch gene that specifies formation of the predatory morph. [ 9 ]


Density-dependent polyphenism allows species to show a different phenotype based on the population density in which it was reared. In  Lepidoptera ,  African armyworm  larvae exhibit one of two appearances: the gregarious or solitary phase. Under crowded or "gregarious" conditions, the larvae have black bodies and yellow stripes along their bodies. However, under solitary conditions, they have green bodies with a brown stripe down their backs. The different phenotypes emerge during the third instar and remain until the last instar. [ 10 ]


Under conditions of stress such as crowding and high temperature, L2 larvae of some free living nematodes such as  Caenorhabditis elegans  can switch development to the so-called  dauer larva  state, instead of going the normal molts into a reproductive adult. These dauer larvae are a stress-resistant, non-feeding, long-lived stage, enabling the animals to survive harsh conditions. On return to favorable conditions, the animal resumes reproductive development from L3 stage onwards.


A mechanism has been proposed for the  evolutionary  development of polyphenisms: [ 7 ]


Evolution of novel polyphenisms through this mechanism has been demonstrated in the laboratory. Suzuki and Nijhout used an existing mutation ( black ) in a monophenic green hornworm ( Manduca sexta ) that causes a black phenotype. They found that if larvae from an existing population of  black  mutants were raised at 20˚C, then all the final  instar  larvae were black; but if the larvae were instead raised at 28˚C, the final instar larvae ranged in color from black to green. By selecting for larvae that were black if raised at 20˚C but green if raised at 28˚C, they produced a polyphenic strain after thirteen generations. [ 11 ]


This fits the model described above because a new mutation (black) was required to reveal pre-existing genetic variation and to permit selection. Furthermore, the production of a polyphenic strain was only possible because of background variation within the species: two alleles, one temperature-sensitive and one stable, were present for a single gene upstream of  black  (in the pigment production pathway) before selection occurred. The temperature-sensitive allele was not observable because at high temperatures, it caused an increase in green pigment in hornworms that were already bright green. However, introduction of the black mutant caused the temperature-dependent changes in pigment production to become obvious. The researchers could then select for larvae with the temperature-sensitive  allele , resulting in a polyphenism. [ citation needed ]


In population genetics, a protected polymorphism is a mechanism that maintains multiple alleles at a certain locus. In detail, any of the several alleles will follow certain dynamics; When a certain allele is high in frequency (p

 →

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  1), it will decrease in frequency in the future and by that avoid from being fixated in the population. On the contrary, when a given allele is low in frequency (p

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  0) it will increase in frequency in the future, avoiding its extinction and maintaining polymorphism at the locus.


Sexual dimorphism  is the condition where  sexes  of the same  species  exhibit different  morphological  characteristics, including characteristics not directly involved in  reproduction . [ 1 ]  The condition occurs in most  dioecious  species, which consist of most animals and some plants. Differences may include  secondary sex characteristics , size, weight, color, markings, or behavioral or cognitive traits. Male-male reproductive competition has evolved a diverse array of sexually dimorphic traits. Aggressive utility traits such as "battle" teeth and blunt heads reinforced as battering rams are used as weapons in aggressive interactions between rivals. Passive displays such as ornamental feathering or song-calling have also evolved mainly through sexual selection. [ 2 ]  These differences may be subtle or exaggerated and may be subjected to  sexual selection  and  natural selection . The opposite of dimorphism is  monomorphism , when both biological sexes are  phenotypically  indistinguishable from each other. [ 3 ]


Common and easily identified types of dimorphism consist of  ornamentation  and coloration, though not always apparent. A difference in the coloration of sexes within a given species is called sexual dichromatism, commonly seen in many species of birds and reptiles. [ 4 ]   Sexual selection  leads to exaggerated dimorphic traits that are used predominantly in competition over mates. [ 5 ]  The increased fitness resulting from ornamentation offsets its cost to produce or maintain, suggesting complex evolutionary implications, but the costs and evolutionary implications vary from species to species. [ 6 ]


The  peafowl  constitute conspicuous illustrations of the principle. The ornate  plumage  of peacocks, as used in the courting display, attracts  peahens . At first sight, one might mistake peacocks and peahens for completely different species because of the vibrant colours and the sheer size of the male's plumage; the peahen is of a subdued brown coloration. [ 7 ]  The plumage of the peacock increases its vulnerability to predators because it is a hindrance in flight, and it renders the bird conspicuous in general. [ 7 ]  Similar examples are manifold, such as in  birds of paradise [ 8 ]  and  argus pheasants . [ citation needed ]


Another example of sexual dichromatism is that of nestling  blue tits . Males are chromatically more yellow than females. It is believed that this is obtained by the ingestion of green  Lepidopteran  larvae, which contain large amounts of the  carotenoids   lutein  and  zeaxanthin . [ 9 ]  This diet also affects the sexually dimorphic colours in the human-invisible  ultraviolet  spectrum. [ 10 ]  Hence, the male birds, although appearing yellow to humans, actually have a violet-tinted plumage that is seen by females. This plumage is thought to be an indicator of male parental abilities. [ 11 ]  Perhaps this is a good indicator for females because it shows that they are good at obtaining a food supply from which the carotenoid is obtained. There is a positive correlation between the chromas of the tail and breast feathers and body condition. [ 12 ]  Carotenoids play an important role in  immune function  for many animals, so carotenoid dependent signals might indicate health. [ 13 ]


Frogs constitute another conspicuous illustration of the principle. There are two types of dichromatism for frog species: ontogenetic and dynamic. Ontogenetic frogs are more common and have permanent color changes in males or females.  Ranoidea lesueuri  is an example of a dynamic frog with temporary color changes in males during the breeding season. [ 14 ]   Hyperolius ocellatus  is an ontogenetic frog with dramatic differences in both color and pattern between the sexes. At sexual maturity, the males display a bright green with white dorsolateral lines. [ 15 ]  In contrast, the females are rusty red to silver with small spots. The bright coloration in the male population attracts females and is an  aposematic  sign to potential predators.


Females often show a preference for exaggerated male  secondary sexual characteristics  in mate selection. [ 16 ]  The  sexy son hypothesis  explains that females prefer more elaborate males and select against males that are dull in color, independent of the species' vision. [ 17 ]


Similar sexual dimorphism and mating choice are also observed in many fish species. For example, male  guppies  have colorful spots and ornamentations, while females are generally grey. Female guppies prefer brightly colored males to duller males. [ 18 ] [ page needed ]


In  redlip blennies , only the male fish develops an organ at the anal-urogenital region that produces antimicrobial substances. During parental care, males rub their anal-urogenital regions over their nests' internal surfaces, thereby protecting their eggs from microbial infections, one of the most common causes for mortality in young fish. [ 19 ]


Most  flowering plants  are  hermaphroditic  but approximately 6% of species have separate males and females ( dioecy ). [ 20 ]  Sexual dimorphism is common in dioecious plants [ 21 ] : 403   and  dioicous  species. [ 22 ] : 71


Males and females in  insect-pollinated  species generally look similar to one another because plants provide rewards (e.g.  nectar ) that encourage  pollinators  to visit another similar  flower , completing  pollination .  Catasetum  orchids are one interesting exception to this rule. Male  Catasetum   orchids  violently attach  pollinia  to  euglossine  bee pollinators. The bees will then avoid other male flowers but may visit the female, which looks different from the males. [ 23 ]


Various other dioecious exceptions, such as  Loxostylis alata  have visibly different sexes, with the effect of eliciting the most efficient behavior from pollinators, who then use the most efficient strategy in visiting each gender of flower instead of searching, say, for pollen in a nectar-bearing female flower. [ citation needed ]


Some plants, such as some species of  Geranium  have what amounts to serial sexual dimorphism. The flowers of such species might, for example, present their  anthers  on opening, then shed the exhausted anthers after a day or two and perhaps change their colours as well while the  pistil  matures; specialist pollinators are very much inclined to concentrate on the exact appearance of the flowers they serve, which saves their time and effort and serves the interests of the plant accordingly. Some such plants go even further and change their appearance once fertilized, thereby discouraging further visits from pollinators. This is advantageous to both parties because it avoids damaging the developing fruit and wasting the pollinator's effort on unrewarding visits. In effect, the strategy ensures that pollinators can expect a reward every time they visit an appropriately advertising flower. [ citation needed ]


Females of the aquatic plant  Vallisneria americana  have floating flowers attached by a long  flower stalk  that are fertilized if they contact one of the thousands of free-floating flowers released by a male. [ 24 ] [ better source needed ]  Sexual dimorphism is most often associated with  wind-pollination  in plants due to selection for efficient  pollen  dispersal in males vs pollen capture in females, e.g.  Leucadendron rubrum . [ 25 ]


Sexual dimorphism in plants can also be dependent on reproductive development. This can be seen in  Cannabis sativa , a type of hemp, which have higher photosynthesis rates in males while growing but higher rates in females once the plants become sexually mature. [ 26 ]


Every sexually reproducing extant species of the vascular plant has an alternation of generations; the plants we see about us generally are  diploid   sporophytes , but their offspring are not the seeds that people commonly recognize as the new generation. The seed actually is the offspring of the  haploid  generation of  microgametophytes  ( pollen ) and  megagametophytes  (the  embryo sacs  in the  ovules ). Each pollen grain accordingly may be seen as a male plant in its own right; it produces a sperm cell and is dramatically different from the female plant, the megagametophyte that produces the female gamete. [ citation needed ]


Insects display a wide variety of sexual dimorphism between taxa including size, ornamentation and coloration. [ 27 ]  The female-biased sexual size dimorphism observed in many taxa evolved despite intense male-male competition for mates. [ 28 ]  In  Osmia rufa , for example, the female is larger/broader than males, with males being 8–10 mm in size and females being 10–12 mm in size. [ 29 ]  In the  hackberry emperor  females are similarly larger than males. [ 30 ]  The reason for the sexual dimorphism is due to provision size mass, in which females consume more pollen than males. [ 31 ]


In some species, there is evidence of male dimorphism, but it appears to be for distinctions of roles. This is seen in the bee species  Macrotera portalis  in which there is a small-headed morph, capable of flight, and large-headed morph, incapable of flight, for males. [ 32 ]   Anthidium manicatum  also displays male-biased sexual dimorphism. The selection for larger size in males rather than females in this species may have resulted due to their aggressive territorial behavior and subsequent differential mating success. [ 33 ]  Another example is  Lasioglossum hemichalceum , which is a species of sweat bee that shows drastic physical dimorphisms between male offspring. [ 34 ]  Not all dimorphism has to have a drastic difference between the sexes.  Andrena agilissima  is a mining bee where the females only have a slightly larger head than the males. [ 35 ]


Weaponry leads to increased fitness by increasing success in male–male competition in many insect species. [ 36 ]  The beetle horns in  Onthophagus taurus  are enlarged growths of the head or thorax expressed only in the males.  Copris ochus  also has distinct sexual and male dimorphism in head horns. [ 37 ]  Another beetle with a distinct horn-related sexual dimorphism is  Allomyrina dichotoma,  also known as the  Japanese rhinoceros beetle . [ 38 ]  These structures are impressive because of the exaggerated sizes. [ 39 ]  There is a direct correlation between male horn lengths and body size and higher access to mates and fitness. [ 39 ]  In other beetle species, both males and females may have ornamentation such as horns. [ 37 ]
Generally, insect sexual size dimorphism (SSD) within species increases with body size. [ 40 ]


Sexual dimorphism within insects is also displayed by dichromatism. In butterfly genera  Bicyclus  and  Junonia , dimorphic wing patterns evolved due to sex-limited expression, which mediates the  intralocus sexual conflict  and leads to increased fitness in males. [ 41 ]  The sexual dichromatic nature of  Bicyclus anynana  is reflected by female selection on the basis of dorsal UV-reflective eyespot pupils. [ 42 ]  The  common brimstone  also displays sexual dichromatism; males have yellow and iridescent wings, while female wings are white and non-iridescent. [ 43 ]  Naturally selected deviation in protective female coloration is displayed in mimetic butterflies. [ 44 ]


Many  arachnid  groups exhibit sexual dimorphism, [ 45 ]  but it is most widely studied in the spiders. In the orb-weaving spider  Zygiella x-notata , for example, adult females have a larger body size than adult males. [ 46 ]  Size dimorphism shows a correlation with  sexual cannibalism , [ 47 ]  which is prominent in spiders (it is also found in insects such as  praying mantises ). In the size dimorphic  wolf spider   Tigrosa helluo , food-limited females cannibalize more frequently. [ 48 ]  Therefore, there is a high risk of low fitness for males due to pre-copulatory cannibalism, which led to male selection of larger females for two reasons: higher  fecundity  and lower rates of cannibalism. [ 48 ]  In addition, female fecundity is positively correlated with female body size and large female body size is selected for, which is seen in the family  Araneidae . All  Argiope  species, including  Argiope bruennichi , use this method. Some males evolved ornamentation [ vague ]  including binding the female with silk, having proportionally longer legs, modifying the female's web, mating while the female is feeding, or providing a nuptial gift in response to sexual cannibalism. [ 48 ]  Male body size is not under selection due to cannibalism in all spider species such as  Nephila pilipes , but is more prominently selected for in less dimorphic species of spiders, which often selects for larger male size. [ 49 ]  In the species  Maratus volans , the males are known for their characteristic colorful fan which attracts the females during mating. [ 50 ]


Ray-finned fish are an ancient and diverse class, with the widest degree of sexual dimorphism of any animal class. Fairbairn notes that "females are generally larger than males but males are often larger in species with male–male combat or male paternal care ... [sizes range] from dwarf males to males more than 12 times heavier than females." [ 51 ] [ page needed ]


There are cases where males are substantially larger than females. An example is  Lamprologus callipterus , a type of cichlid fish. In this fish, the males are characterized as being up to 60 times larger than the females. The male's increased size is believed to be advantageous because males collect and defend empty snail shells in each of which a female breeds. [ 52 ]  Males must be larger and more powerful in order to collect the largest shells. The female's body size must remain small because in order for her to breed, she must lay her eggs inside the empty shells. If she grows too large, she will not fit in the shells and will be unable to breed. The female's small body size is also likely beneficial to her chances of finding an unoccupied shell. Larger shells, although preferred by females, are often limited in availability. [ 53 ]  Hence, the female is limited to the growth of the size of the shell and may actually change her growth rate according to shell size availability. [ 54 ]  In other words, the male's ability to collect large shells depends on his size. The larger the male, the larger the shells he is able to collect. This then allows for females to be larger in his brooding nest which makes the difference between the sizes of the sexes less substantial. Male–male competition in this fish species also selects for large size in males. There is aggressive competition by males over territory and access to larger shells. Large males win fights and steal shells from competitors. Another example is the  dragonet , in which males are considerably larger than females and possess longer fins.


Sexual dimorphism also occurs in hermaphroditic fish. These species are known as  sequential hermaphrodites . In fish,  reproductive histories  often include the sex-change from female to male where there is a strong connection between growth, the sex of an individual, and the mating system within which it operates. [ 55 ]  In protogynous mating systems where males dominate mating with many females, size plays a significant role in male reproductive success. [ 56 ]  Males have a propensity to be larger than females of a comparable age but it is unclear whether the size increase is due to a growth spurt at the time of the sexual transition or due to the history of faster growth in sex changing individuals. [ 57 ]  Larger males are able to stifle the growth of females and control environmental resources. [ citation needed ]


Social organization plays a large role in the changing of sex by the fish. It is often seen that a fish will change its sex when there is a lack of a dominant male within the social hierarchy. The females that change sex are often those who attain and preserve an initial size advantage early in life. In either case, females which change sex to males are larger and often prove to be a good example of dimorphism.


In other cases with fish, males will go through noticeable changes in body size, and females will go through morphological changes that can only be seen inside of the body.  For example, in  sockeye salmon , males develop larger body size at maturity, including an increase in body depth, hump height, and snout length.  Females experience minor changes in snout length, but the most noticeable difference is the huge increase in  gonad  size, which accounts for about 25% of body mass. [ 58 ]


Sexual selection was observed for female ornamentation in  Gobiusculus flavescens , known as two-spotted gobies. [ 59 ]  Traditional hypotheses suggest that male–male competition drives selection. However, selection for ornamentation within this species suggests that showy female traits can be selected through either female–female competition or male mate choice. [ 59 ]  Since carotenoid-based ornamentation suggests mate quality, female two-spotted guppies that develop colorful orange bellies during breeding season are considered favorable to males. [ 60 ]  The males invest heavily in offspring during incubation, which leads to the sexual preference in colorful females due to higher egg quality. [ 60 ]


In amphibians and reptiles, the degree of sexual dimorphism varies widely among  taxonomic groups . The sexual dimorphism in amphibians and reptiles may be reflected in any of the following: anatomy; relative length of tail; relative size of head; overall size as in many species of  vipers  and  lizards ; coloration as in many  amphibians ,  snakes , and lizards, as well as in some  turtles ; an ornament as in many  newts  and lizards; the presence of specific sex-related behaviour is common to many lizards; and vocal qualities which are frequently observed in  frogs . [ citation needed ]


Anole  lizards show prominent size dimorphism with males typically being significantly larger than females. For instance, the average male  Anolis sagrei  was 53.4 mm vs. 40 mm in females. [ 61 ]  Different sizes of the heads in anoles have been explained by differences in the estrogen pathway. [ 62 ]  The sexual dimorphism in lizards is generally attributed to the effects of sexual selection, but other mechanisms including ecological divergence and fecundity selection provide alternative explanations. [ 63 ]  The development of color dimorphism in lizards is induced by hormonal changes at the onset of sexual maturity, as seen in  Psamodromus algirus ,  Sceloporus gadoviae , and  S. undulates erythrocheilus . [ 63 ]  Sexual dimorphism in size is also seen in frog species like  P. bibroni i .


Male painted dragon lizards,  Ctenophorus pictus . are brightly conspicuous in their breeding coloration, but male colour declines with  aging .  Male coloration appears to reflect innate anti-oxidation capacity that protects against  oxidative DNA damage . [ 64 ]  Male breeding coloration is likely an indicator to females of the underlying level of oxidative DNA damage (a significant component of aging) in potential mates. [ 64 ]


Possible mechanisms have been proposed to explain macroevolution of sexual size dimorphism in birds. These include sexual selection, selection for fecundity in females, niche divergence between the sexes, and allometry, but their relative importance is still not fully understood . [ 65 ] [ 66 ]  Sexual dimorphism in birds can be manifested in size or plumage differences between the sexes. Sexual size dimorphism varies among taxa, with males typically being larger, though this is not always the case, e.g.  birds of prey ,  hummingbirds , and some species of flightless birds. [ 67 ] [ 68 ]  Plumage dimorphism, in the form of ornamentation or coloration, also varies, though males are typically the more ornamented or brightly colored sex. [ 69 ]  Such differences have been attributed to the unequal reproductive contributions of the sexes. [ 70 ]  This difference produces a stronger female choice since they have more risk in producing offspring. In some species, the male's contribution to reproduction ends at copulation, while in other species the male becomes the main (or only) caregiver. Plumage polymorphisms have evolved to reflect these differences and other measures of reproductive fitness, such as body condition [ 71 ]  or survival. [ 72 ]  The male phenotype sends signals to females who then choose the 'fittest' available male.


Sexual dimorphism is a product of both genetics and environmental factors. An example of  sexual polymorphism  determined by environmental conditions exists in the  red-backed fairywren . Red-backed fairywren males can be classified into three categories during  breeding season : black breeders, brown breeders, and brown auxiliaries. [ 71 ]  These differences arise in response to the bird's body condition: if they are healthy they will produce more androgens thus becoming black breeders, while less healthy birds produce less androgens and become brown auxiliaries. [ 71 ]  The  reproductive success  of the male is thus determined by his success during each year's non-breeding season, causing reproductive success to vary with each year's environmental conditions.


Migratory patterns and behaviors also influence sexual dimorphisms. This aspect also stems back to size dimorphism in species. It has been shown that the larger males are better at coping with the difficulties of migration and thus are more successful in reproducing when reaching the breeding destination. [ 73 ]  When viewing this from an evolutionary standpoint, many theories and explanations come into consideration. If these are the result for every migration and breeding season, the expected results should be a shift towards a larger male population through sexual selection. Sexual selection is strong when the factor of environmental selection is also introduced. Environmental selection may support a smaller chick size if those chicks were born in an area that allowed them to grow to a larger size, even though under normal conditions they would not be able to reach this optimal size for migration. When the environment gives advantages and disadvantages of this sort, the strength of selection is weakened and the environmental forces are given greater morphological weight. The sexual dimorphism could also produce a change in timing of migration leading to differences in mating success within the bird population. [ 74 ]  When the dimorphism produces that large of a variation between the sexes and between the members of the sexes, multiple evolutionary effects can take place. This timing could even lead to a speciation phenomenon if the variation becomes strongly drastic and favorable towards two different outcomes. Sexual dimorphism is maintained by the counteracting pressures of natural selection and sexual selection. For example, sexual dimorphism in coloration increases the vulnerability of bird species to predation by European sparrowhawks in Denmark. [ 75 ]  Presumably, increased sexual dimorphism means males are brighter and more conspicuous, leading to increased predation. [ 75 ]  Moreover, the production of more exaggerated ornaments in males may come at the cost of suppressed immune function. [ 71 ]  So long as the reproductive benefits of the trait due to sexual selection are greater than the costs imposed by natural selection, then the trait will propagate throughout the population. Reproductive benefits arise in the form of a larger number of offspring, while natural selection imposes costs in the form of reduced survival. This means that even if the trait causes males to die earlier, the trait is still beneficial so long as males with the trait produce more offspring than males lacking the trait. This balance keeps dimorphism alive in these species and ensures that the next generation of successful males will also display these traits that are attractive to females.


Such differences in form and reproductive roles often cause differences in behavior. As previously stated, males and females often have different roles in reproduction. The courtship and mating behavior of males and females are regulated largely by hormones throughout a bird's lifetime. [ 76 ]  Activational hormones occur during puberty and adulthood and serve to 'activate' certain behaviors when appropriate, such as territoriality during breeding season. [ 76 ]  Organizational hormones occur only during a critical period early in development, either just before or just after hatching in most birds, and determine patterns of behavior for the rest of the bird's life. [ 76 ]  Such behavioral differences can cause disproportionate sensitivities to anthropogenic pressures. [ 77 ]  Females of the whinchat in Switzerland breed in intensely managed grasslands. [ 77 ]  Earlier harvesting of the grasses during the breeding season lead to more female deaths. [ 77 ]  Populations of many birds are often male-skewed and when sexual differences in behavior increase this ratio, populations decline at a more rapid rate. [ 77 ]  Also not all male dimorphic traits are due to hormones like testosterone, instead they are a naturally occurring part of development, for example plumage. [ 78 ]  In addition, the strong hormonal influence on phenotypic differences suggests that the genetic mechanism and genetic basis of these sexually dimorphic traits may involve transcription factors or cofactors rather than regulatory sequences. [ 79 ]


Sexual dimorphism may also influence differences in parental investment during times of food scarcity. For example, in the  blue-footed booby , the female chicks grow faster than the males, resulting in booby parents producing the smaller sex, the males, during times of food shortage. This then results in the maximization of parental lifetime reproductive success. [ 80 ]  In  Black-tailed Godwits   Limosa limosa limosa  females are also the larger sex, and the growth rates of female chicks are more susceptible to limited environmental conditions. [ 81 ]


Sexual dimorphism may also only appear during mating season; some species of birds only show dimorphic traits in seasonal variation. The males of these species will molt into a less bright or less exaggerated color during the off-breeding season. [ 79 ]  This occurs because the species is more focused on survival than on reproduction, causing a shift into a less ornate state.  [ dubious  –  discuss ]


Consequently, sexual dimorphism has important ramifications for conservation. However, sexual dimorphism is not only found in birds and is thus important to the conservation of many animals. Such differences in form and behavior can lead to  sexual segregation , defined as sex differences in space and resource use. [ 82 ]  Most sexual segregation research has been done on ungulates, [ 82 ]  but such research extends to  bats , [ 83 ]   kangaroos , [ 84 ]  and birds. [ 85 ]  Sex-specific conservation plans have even been suggested for species with pronounced sexual segregation. [ 83 ]


The term sesquimorphism (the Latin  numeral prefix   sesqui - means one-and-one-half, so halfway between  mono - (one) and  di - (two)) has been proposed for bird species in which "both sexes have basically the same plumage pattern, though the female is clearly distinguishable by reason of her paler or washed-out  colour". [ 86 ] : 14     Examples include  Cape sparrow  ( Passer melanurus ), [ 86 ] : 67    rufous sparrow  (subspecies  P. motinensis motinensis ), [ 86 ] : 80   and  saxaul sparrow  ( P. ammodendri ). [ 86 ] : 245


Examining fossils of  non-avian dinosaurs  in search of sexually dimorphic characteristics requires the supply of complete and articulated skeletal and tissue remains. As terrestrial organisms, dinosaur carcasses are subject to ecological and geographical influence that inevitably constitutes the degree of preservation. The availability of well-preserved remains is not a probable outcome as a consequence of  decomposition  and  fossilization . Some  paleontologists  have looked for sexual dimorphism among dinosaurs using statistics and comparison to ecologically or  phylogenetically  related modern animals.


Apatosaurus and Diplodocus


Female  Apatosaurus  and  Diplodocus  had interconnected  caudal vertebrae  that allowed them to keep their tails elevated to aid in copulation. Discovering that this fusion occurred in only 50% of  Apatosaurus  and  Diplodocus  skeletons and 25% of  Camarasaurus  skeletons indicated that this is a sexually dimorphic trait.


Theropoda


It has been hypothesized that male  theropods  possessed a retractable penis, a feature similar to modern day  crocodilians . Crocodilian skeletons were examined to determine whether there is a skeletal component that is distinctive between both sexes, to help provide an insight on the physical disparities between male and female theropods. Findings revealed the caudal  chevrons  of male crocodiles, used to anchor the penis muscles, were significantly larger than those of females. There have been criticisms of these findings, but it remains a subject of debate among advocates and adversaries. [ citation needed ]


Ornithopoda


Studies of sexual dimorphism in  hadrosaurs  have generally centered on the distinctive  cranial crests , which likely provided a function in sexual display. A biometric study of 36 skulls found sexual dimorphism was exhibited in the crest of 3 species of hadrosaurids. The crests could be categorized as full (male) or narrow (female) and may have given some advantage in intrasexual mating-competition.


Ceratopsians


According to Scott D. Sampson, if ceratopsids were to exhibit sexual dimorphism, modern ecological analogues suggest it would be found in display structures, such as horns and frills. No convincing evidence for sexual dimorphism in body size or mating signals is known in ceratopsids, although there is evidence that the more primitive ceratopsian  Protoceratops andrewsi  possessed sexes that were distinguishable based on frill and nasal prominence size. This is consistent with other known  tetrapod  groups where midsized animals tend to exhibit markedly more sexual dimorphism than larger ones. However, it has been proposed that these differences can be better explained by intraspecific and ontogenic variation rather than sexual dimorphism. [ 87 ]  In addition, many sexually dimorphic traits that may have existed in ceratopsians include soft tissue variations such as coloration or  dewlaps , which would be unlikely to have been preserved in the fossil record.


Stegosaurians


A 2015 study on specimens of  Hesperosaurus   mjosi  found evidence of sexual dimorphism in the shape of the dermal plates. Two plate morphs were described: one was short, wide, and oval-shaped, the other taller and narrower. [ 88 ] [ 89 ]


In 45% of mammal species, males are larger than females, in 39%, males and females are the same size and in 16% of mammals, females are larger than males. [ 90 ]  Both  genes  and  hormones  affect the formation of many animal brains before " birth " (or  hatching ), and also behaviour of adult individuals. Hormones significantly affect human brain formation, and also brain development at puberty. A 2004 review in  Nature Reviews Neuroscience  observed that "because it is easier to manipulate hormone levels than the expression of sex chromosome genes, the effects of hormones have been studied much more extensively, and are much better understood, than the direct actions in the brain of sex chromosome genes." It concluded that while "the differentiating effects of gonadal secretions seem to be dominant," the existing body of research "support the idea that sex differences in neural expression of X and Y genes significantly contribute to sex differences in brain functions and disease." [ 91 ]


Marine mammals  show some of the greatest sexual size differences of mammals, because of sexual selection and environmental factors like breeding location. [ 92 ]  The mating system of  pinnipeds  varies from polygamy to  serial monogamy . Pinnipeds are known for early differential growth and maternal investment since the only nutrients for newborn pups is the milk provided by the mother. [ 93 ]  For example, the males are significantly larger (about 10% heavier and 2% longer) than the females at birth in sea lion pups. [ 94 ]  The pattern of differential investment can be varied principally prenatally and post-natally. [ 95 ]   Mirounga leonina , the  southern elephant seal , is one of the most dimorphic mammals. [ 96 ]


Top:  Stylised illustration of humans on the  Pioneer plaque , showing both male (left) and female (right).
 Bottom:  Comparison between male (left) and female (right)  pelvises .


According to Clark Spencer Larsen, modern day  Homo sapiens  show a range of sexual dimorphism, with average body mass between the sexes differing by roughly 15%. [ 97 ]  Substantial discussion in academic literature considers potential evolutionary advantages associated with sexual competition (both intrasexual and intersexual), as well as short- and long-term sexual strategies. [ 98 ]  According to Daly and Wilson, "The sexes differ more in human beings than in monogamous mammals, but much less than in extremely polygamous mammals." [ 99 ]


The average  basal metabolic rate  is about 6 percent higher in adolescent males than females and increases to about 10 percent higher after puberty. Females tend to convert more food into  fat , while males convert more into  muscle  and expendable circulating energy reserves. According to Tim Hewett, director of research in the department of sports medicine at  Ohio State University Wexner Medical Center , females have, on average, 50–60% of the upper body strength of males, and 80–90% of the lower body strength of males, relative to body size. [ 100 ]


The difference in strength relative to body mass is less pronounced in trained individuals. In Olympic weightlifting, male records vary from 5.5× body mass in the lowest weight category to 4.2× in the highest weight category, while female records vary from 4.4× to 3.8×, a weight-adjusted difference of only 10–20%, and an absolute difference of about 30% (i.e., 492 kg vs 348 kg for unlimited weight classes; see  Olympic weightlifting records ). A study, carried out by analyzing annual world rankings from 1980 to 1996, found that males' running times were, on average, 10% faster than females', with wider hips being a disadvantage for running. [ 101 ]  However, females have higher  endurance  than males [ 102 ] [ 103 ]  and there is less than a second difference in the 100m race of  Usain Bolt  and  Florence Griffith Joyner .


In early adolescence, females are on average taller than males (as females tend to  go through puberty earlier ), but males, on average, surpass them in height in later adolescence and adulthood. In the United States, adult males are on average 9% taller [ 104 ]  and 16.5% heavier [ 105 ]  than adult females.


Males typically have larger  tracheae  and branching  bronchi , with about 30 percent greater  lung volume  per  body mass . On average, males have larger  hearts , slower  heart rates , 10 percent higher  red blood cell  count, higher  hemoglobin , hence greater oxygen-carrying capacity. They also have higher circulating  clotting factors  ( vitamin K , pro thrombin  and  platelets ). These differences lead to faster healing of  wounds  and lower sensitivity to nerve  pain  after injury. [ 106 ]  In males, pain-causing injury to the  peripheral nerve  occurs through the  microglia , while in females it occurs through the  T cells  (except in pregnant women, who follow a male pattern). [ 107 ]


Females typically have more  white blood cells  (stored and circulating), as well as more  granulocytes  and B and T  lymphocytes . Additionally, they produce more  antibodies  at a faster rate than males, hence they develop fewer  infectious  diseases and succumb for shorter periods. [ 106 ]   Ethologists  argue that females, interacting with other females and multiple offspring in social groups, have experienced such traits as a  selective  advantage. [ 108 ] [ 109 ] [ 110 ] [ 111 ] [ 112 ]  Females have a higher sensitivity to pain due to aforementioned nerve differences that increase the sensation, and females thus require higher levels of pain medication after injury. [ 107 ]  Hormonal changes in females affect pain sensitivity, and pregnant women have the same sensitivity as males. Acute pain tolerance is also more consistent over a lifetime in females than males, despite these hormonal changes. [ 113 ]  Despite differences in physical feeling, both sexes have similar psychological tolerance to (or ability to  cope  with and ignore) pain. [ 114 ]


In the  human brain , a difference between sexes was observed in the  transcription  of the  PCDH11X /Y gene pair unique to  Homo sapiens . [ 115 ]  Sexual differentiation in the human brain from the undifferentiated state is triggered by testosterone from the fetal testis. Testosterone is converted to estrogen in the brain through the action of the enzyme aromatase. Testosterone acts on many brain areas, including the  SDN-POA , to create the masculinized brain pattern. [ 116 ]  The brains of pregnant females carrying male fetuses may be shielded from the masculinizing effects of androgen through the action of  sex hormone-binding globulin . [ 117 ]


The relationship between sex differences in the brain and human behavior is a subject of controversy in psychology and society at large. [ 118 ] [ 119 ]  Many females tend to have a higher ratio of  gray matter  in the left hemisphere of the brain in comparison to males. [ 120 ] [ 121 ]  Males on average have larger brains than females; however, when adjusted for total brain volume, the gray matter differences between sexes are almost nonexistent. Thus, the percentage of gray matter appears to be more related to brain size than it is to sex. [ 122 ] [ 123 ]  Differences in brain physiology between sexes do not necessarily relate to differences in intellect. Haier  et al.  found in a 2004 study that "men and women apparently achieve similar IQ results with different brain regions, suggesting that there is no singular underlying neuroanatomical structure to general intelligence and that different types of brain designs may manifest equivalent intellectual performance". [ 124 ]  (See the  sex and intelligence  article for more on this subject.) Strict graph-theoretical analysis of the human brain connections revealed [ 125 ]  that in numerous graph-theoretical parameters (e.g., minimum bipartition width, edge number, the  expander graph  property, minimum  vertex cover ), the structural connectome of women are significantly "better" connected than the connectome of men. It was shown [ 126 ]  that the graph-theoretical differences are due to the sex and not to the differences in the cerebral volume, by analyzing the data of 36 females and 36 males, where the brain volume of each man in the group was smaller than the brain volume of each woman in the group.


Sexual dimorphism was also described in the gene level and shown to extend from the sex chromosomes. Overall, about 6500 genes have been found to have sex-differential expression in at least one tissue. Many of these genes are not directly associated with reproduction, but rather linked to more general biological features. In addition, it has been shown that genes with sex-specific expression undergo reduced selection efficiency, which leads to higher population frequencies of deleterious mutations and contributes to the prevalence of several human diseases. [ 127 ] [ 128 ]


Sexual dimorphism in immune function is a common pattern in vertebrates and also in a number of invertebrates. Most often, females are more 'immunocompetent' than males. This trait is not consistent among all animals, but differs depending on taxonomy, with the most female-biased immune systems being found in insects. [ 129 ]  In mammals this results in more frequent and severe infections in males and higher rates of autoimmune disorders in females. One potential cause may be differences in gene expression of immune cells between the sexes. [ 130 ]  Another explanation is that endocrinological differences between the sexes impact the immune system – for example, testosterone acts as an immunosuppressive agent. [ 131 ]


Phenotypic differences between sexes are evident even in  cultured cells  from tissues. [ 132 ]  For example, female muscle-derived  stem cells  have a better muscle regeneration efficiency than male ones. [ 133 ]  There are reports of several metabolic differences between male and female cells [ 134 ]  and they also respond to  stress  differently. [ 135 ]


In theory, larger females are favored by competition for mates, especially in polygamous species. Larger females offer an advantage in fertility, since the physiological demands of reproduction are limiting in females. Hence there is a theoretical expectation that females tend to be larger in species that are monogamous.
Females are larger in many species of  insects , many  spiders , many  fish , many reptiles,  owls , birds of prey and certain mammals such as the  spotted hyena , and baleen whales such as  blue whale . As an example, in some species, females are sedentary, and so males must search for them. Fritz Vollrath and  Geoff Parker  argue that this difference in behaviour leads to radically different selection pressures on the two sexes, evidently favouring smaller males. [ 136 ]  Cases where the male is larger than the female have been studied as well, [ 136 ]  and require alternative explanations.


One example of this type of sexual size dimorphism is the bat  Myotis nigricans , (black myotis bat) where females are substantially larger than males in terms of body weight, skull measurement, and forearm length. [ 137 ]  The interaction between the sexes and the energy needed to produce viable offspring makes it favorable for females to be larger in this species. Females bear the energetic cost of producing eggs, which is much greater than the cost of making sperm by the males. The fecundity advantage hypothesis states that a larger female is able to produce more offspring and give them more favorable conditions to ensure their survival; this is true for most ectotherms. A larger female can provide parental care for a longer time while the offspring matures. The gestation and lactation periods are fairly long in  M. nigricans , the females suckling their offspring until they reach nearly adult size. [ 138 ]  They would not be able to fly and catch prey if they did not compensate for the additional mass of the offspring during this time. Smaller male size may be an adaptation to increase maneuverability and agility, allowing males to compete better with females for food and other resources.


Some species of  anglerfish  also display extreme sexual dimorphism. Females are more typical in appearance to other fish, whereas males are tiny rudimentary creatures with stunted digestive systems. A male must find a female and fuse with her: he then lives parasitically, becoming little more than a sperm-producing body in what amounts to an effectively hermaphrodite composite organism. A similar situation is found in the Zeus water bug  Phoreticovelia disparata  where the female has a glandular area on her back that can serve to feed a male, which clings to her (although males can survive away from females, they generally are not free-living). [ 139 ]  This is taken to the logical extreme in the  Rhizocephala  crustaceans, like the  Sacculina , where the male injects itself into the female's body and becomes nothing more than sperm producing cells, to the point that the superorder used to be mistaken for hermaphroditic. [ 140 ]


Some plant species also exhibit dimorphism in which the females are significantly larger than the males, such as in the moss  Dicranum [ 141 ]  and the liverwort  Sphaerocarpos . [ 142 ]  There is some evidence that, in these genera, the dimorphism may be tied to a sex chromosome, [ 142 ] [ 143 ]  or to chemical signalling from females. [ 144 ]


Another complicated example of sexual dimorphism is in  Vespula squamosa , the southern yellowjacket.  In this wasp species, the female workers are the smallest, the male workers are slightly larger, and the female queens are significantly larger than her female workers and male counterparts. [ citation needed ]


In 1871,  Charles Darwin  advanced the  theory of sexual selection , which related sexual dimorphism to  sexual selection . [ 146 ]


The first step towards sexual dimorphism is the size differentiation of sperm and eggs ( anisogamy ). [ 147 ] [ 148 ] [ 149 ] [ 150 ] : 917    Anisogamy  and the usually large number of small male gametes relative to the larger female gametes usually lies in the development of strong  sperm competition , [ 151 ] [ 152 ]  because small sperm enable organisms to produce a large number of sperm, and make males (or male function of hermaphrodites [ 153 ] ) more redundant.


Volvocine algae have been useful in understanding the evolution of sexual dimorphism  [ 154 ]  and species like the beetle  C. maculatus , where the females are larger than the males, are used to study its underlying genetic mechanisms.  [ 155 ]


In many non-monogamous species, the benefit to a male's reproductive fitness of mating with multiple females is large, whereas the benefit to a female's reproductive fitness of mating with multiple males is small or nonexistent. [ 156 ]  In these species, there is a  selection pressure  for whatever traits enable a male to have more matings. The male may therefore come to have different traits from the female.


These traits could be ones that allow him to fight off other males for control of territory or a  harem , such as large size or weapons; [ 157 ]  or they could be traits that females, for whatever reason, prefer in mates. [ 158 ]   Male–male competition  poses no deep theoretical questions [ 159 ]  but  mate choice  does.


Females may choose males that appear strong and healthy, thus likely to possess "good  alleles " and give rise to healthy offspring. [ 160 ]  In some species, however, females seem to choose males with traits that do not improve offspring survival rates, and even traits that reduce it (potentially leading to traits like the peacock's tail). [ 159 ]  Two hypotheses for explaining this fact are the  sexy son hypothesis  and the  handicap principle .


The sexy son hypothesis states that females may initially choose a trait because it improves the survival of their young, but once this preference has become widespread, females must continue to choose the trait, even if it becomes harmful. Those that do not will have sons that are unattractive to most females (since the preference is widespread) and so receive few matings. [ 161 ]


The handicap principle states that a male who survives despite possessing some sort of handicap thus proves that the rest of his genes are "good alleles". If males with "bad alleles" could not survive the handicap, females may evolve to choose males with this sort of handicap; the trait is acting as a hard-to-fake signal of fitness. [ 162 ]


Simple Sequence Length Polymorphisms  ( SSLPs ) are used as  genetic markers  with  polymerase chain reaction (PCR) . An SSLP is a type of  polymorphism : a difference in  DNA  sequence amongst individuals. SSLPs are repeated sequences over varying base lengths in intergenic regions of deoxyribonucleic acid (DNA). Variance in the length of SSLPs can be used to understand  genetic variation  between two individuals in a certain species. [ 1 ]


An example of the usage of SSLPs ( microsatellites ) is seen in a study by Rosenberg et al., where SSLPs were used to cluster different continental populations of human beings. [ 2 ]  The study was critical to  Nicholas Wade 's New York Times Bestseller,  Before the Dawn: Recovering the Lost History of Our Ancestors . [ 3 ]


Rosenberg studied 377 SSLPs in 1000 people in 52 different regions of the world. By using  PCR  and  cluster analysis , Rosenberg was able to group individuals that had the same SSLPs . These SSLPs were extremely useful to the experiment because they do not affect the  phenotypes  of the individuals, thus being unaffected by  natural selection . [ 2 ]


Hereditarianism  is the research program according to which  heredity  plays a central role in determining  human nature  and character traits, such as  intelligence  and  personality . Hereditarians believe in the power of  genetic influences  to  explain human behavior  and  solve human social-political problems.  They stress the value of  evolutionary explanations  in all areas of the  human sciences .


Most prominently in  intelligence research , they purport that  genetic predisposition  determines  individual life outcomes  more than do either structured  environmental influences  (i.e.  nurture ) or  developmental noise  respectively.


Social scientist  Barry Mehler  defines hereditarianism as "the belief that a substantial part of both group and individual differences in human behavioral traits are caused by genetic differences". [ 1 ]  Hereditarianism is sometimes used as a  synonym  for  biological  or  genetic determinism , though some scholars distinguish the two terms. When distinguished, biological determinism is used to mean that heredity is the only factor. Supporters of hereditarianism reject this sense of biological determinism for most cases. However, in some cases genetic determinism is true; for example,  Matt Ridley  describes  Huntington's disease  as "pure fatalism, undiluted by environmental variability". [ 2 ]  In other cases, hereditarians would see no role for genes; for example, the condition of " not knowing a word of Chinese " has nothing to do (directly) with genes. [ 3 ]


Hereditarians point to the heritability of cognitive ability, and the outsized influence that cognitive ability has on life outcomes, as evidence in favor of the hereditarian viewpoint. [ 4 ]  According to Plomin and Van Stumm (2018), "Intelligence is highly heritable and predicts important educational, occupational and health outcomes better than any other trait." [ 5 ]  Estimates for the  heritability of intelligence  range from 20% in infancy to 80% in adulthood. [ 6 ] [ 7 ]


Francis Galton is generally considered the father of hereditarianism. [ 1 ]  In his book  Hereditary Genius  (1869), Galton pioneered research on the heredity of  intelligence . Galton continued research into the heredity of human behavior in his later works, including "The History of Twins" (1875) [ 8 ]  and  Inquiries into Human Faculty and Its Development  (1883).


The Bell Curve  (1994), by psychologist  Richard Herrnstein  and political scientist  Charles Murray , argued that the heritability of cognitive ability, combined with a modern American society in which cognitive ability is the leading determinant of success, was leading to an increasingly rich and segregated " cognitive elite ". [ 9 ] [ 10 ]  Herrnstein and Murray also examined how cognitive ability predicts socially desirable behavior. [ 9 ]  They also discussed the debate regarding  race and intelligence , concluding that the evidence to date didn't justify an estimate on the degree of influence of genetics versus environmental causes for average differences in IQ test performance between racial groups. [ 11 ]  Today the scientific consensus is that genetics does not explain such differences, and that they are rather environmental in origin. [ 12 ] [ 13 ] [ 14 ] [ 15 ] [ 16 ] [ 17 ]


Cognitive psychologist  Steven Pinker , in his book  The Blank Slate  (2002), argues that biology explains much more about human nature than people generally acknowledge. [ 18 ]


In 1949, Nicolas Pastore claimed that hereditarians were more likely to be  conservative , [ 19 ]  that they view social and economic inequality as a natural result of variation in talent and character. Consequently, they explain class and race differences as the result of partly genetic group differences. Pastore contrasted this with the claim that  behaviorists  were more likely to be  liberals  or  leftists , that they believe economic disadvantage and structural problems in the social order were to blame for group differences. [ 19 ]


However, the historical correspondence between hereditarianism and conservatism has broken down at least among proponents of hereditarianism.  Philosopher  Peter Singer  describes his vision of a new liberal political view that embraces hereditarianism in his 1999 book,  A Darwinian Left . [ 20 ]


Ronald C. Bailey argues that hereditarianism is based on five fallacious assumptions. In a 1997 paper, he also wrote that "...behavior geneticists will continue to be very limited in their ability to partition the effects of genes, the environment, and their covariance and interaction on human behavior and cognitive ability." [ 21 ]


The  biological basis of personality  is a collection of  brain  systems and mechanisms that underlie  human personality .  Human neurobiology , especially as it relates to  complex traits  and behaviors, is not well understood, but research into the  neuroanatomical  and functional underpinnings of personality are an active field of research.  Animal models of behavior , molecular biology, and  brain imaging  techniques have provided some insight into human personality, especially  trait theories .


Much of the current understanding of personality from a neurobiological perspective places an emphasis on the biochemistry of the behavioral systems of reward, motivation, and punishment. This has led to a few biologically based personality theories such as  Eysenck's three factor model of personality , Grey's  reinforcement sensitivity theory  (RST), and  Cloninger's model of personality . The  Big Five model of personality  is not biologically based; yet some research in the differences in brain structures provided biological support also for this model.


Personality  can be defined as a set of characteristics or traits that drive individual differences in human behavior. From a biological perspective, these traits can be traced back to brain structures and neural mechanisms. However, this definition and theory of biological basis is not universally accepted. There are many conflicting theories of personality in the fields of  psychology , psychiatry, philosophy, and  neuroscience . A few examples of this are the  nature vs. nurture  debate and how the idea of a 'soul' fits into biological theories of personality. [ 1 ]


Since the time of the ancient Greeks, humankind has attempted to explain personality through spiritual beliefs, philosophy, and psychology. Historically, studies of personality have traditionally come from the social sciences and humanities, but in the past two decades neuroscience has begun to be more influential in the understanding of human personality. [ 2 ]


However, the most cited and influential figures in publishing the first biology-based personality theories are  Hans Eysenck  and  Jeffrey Alan Gray . Eysenck used both behavioral and  psychophysiological  methodologies to test and develop his theories. [ 3 ]  He published a book in 1947 called  Dimensions of Personality , describing the personality dimensions of extraversion and neuroticism. Gray, a student of Eysenck, studied personality traits as individual differences in sensitivity to rewarding and punishing stimuli. [ 3 ]  The significance of Gray's work and theories was his use of biology to define behavior, which stimulated a lot of subsequent research. [ 4 ]


In 1951,  Hans Eysenck  and  Donald Prell  published an experiment in which  identical (monozygotic) and fraternal (dizygotic) twins , ages 11 and 12, were tested for neuroticism. It is described in detail in an article published in the  Journal of Mental Science  in which Eysenck and Prell concluded that "The factor of neuroticism is not a statistical artifact, but constitutes a biological unit which is inherited as a whole ... neurotic  predisposition is to a large extent hereditarily determined ." [ 5 ]  The study concluded that the neuroticism trait was a result of up to eighty percent of genetics. There was a stronger correlation among identical twins rather than fraternal twins. [ 6 ]


The idea of biology-based personality research is relatively new, but growing in interest and number of publications. [ 7 ]  In August 2004, there was a conference specifically on the topic, called  The Biological Basis of Personality and Individual Differences . [ 8 ]  This allowed for presenting and sharing of ideas between psychologists, psychiatrists, molecular geneticists, and neuroscientists, and eventually gave birth to the book under the same title. [ 8 ]  The book is a collection of current research (as of 2006) in the field contributed by many authors and edited by Turhan Canli. Recently, psychology professor Colin G. DeYoung has even named the idea as the field of "Personality Neuroscience." [ 9 ]  Furthermore, a journal devoted to cultivating research investigating the neurobiological basis of personality has recently been established and is called "Personality Neuroscience." [ 10 ]


There are many theories of personality that centre on the identification of a set of traits that encompass human personality. Few however, are biologically based. This section will describe some theories of personality that have a biological basis.


Eysenck's three-factor model of personality was a causal theory of personality based on activation of  reticular formation  and  limbic system . The reticular formation is a region in the  brainstem  that is involved in mediating arousal and consciousness. The limbic system is involved in mediating emotion, behavior, motivation, and long-term memory.


Gray's  reinforcement sensitivity theory  (RST) is based on the idea that there are three brain systems that all differently respond to rewarding and punishing stimuli. [ 3 ]


This model of personality is based on the idea that different responses to punishing, rewarding, and novel stimuli the main characteristics of the human mind is caused by an interaction of the three dimensions below:


The  five factor model  (also known as the Big Five) is a widely used personality assessment that describes five core traits that a person possesses:


There is large body of research relating the Big Five traits to individual differences in the brain's structure and function, as measured by MRI-based techniques. A selection of these findings are outlined in the "Brain imaging basis of personality" section below.


A higher-order factor structure can be derived from the Big Five traits, as these traits have often been found to be correlated. Agreeableness, Conscientiousness, and Neuroticism (reversed) can be distilled into a single factor α, or the Stability factor. On the other hand, Extraversion and Openness can be distilled into a single factor β, or the Plasticity factor. [ 11 ] [ 12 ]  These two meta-traits have been shown to be significantly heritable using behavior genetic analysis, [ 13 ]  which suggests a neurobiological basis that is unique and specific to these meta-traits. Indeed, a growing body of evidence demonstrates that serotonin is associated with Stability and dopamine is associated with Plasticity. [ 11 ] [ 12 ] [ 14 ]


There are many experimental techniques for measuring the biology of the brain, but there are five main methods used to investigate the biological basis of personality. [ 15 ]  The biological data from these methods are commonly correlated with personality traits. These personality traits are often determined by personality questionnaires. However, personality questionnaires may be biased because they are self-reported. As a result, scientists emphasize using several different measures of personality, [ 15 ] [ 16 ]  rather than solely self-reported measures of personality. For example, another measure of personality traits is observation of behavior. Both humans and animals have been observed to measure personality traits, but animals are particularly useful for studying the long-term behavioral-biological relationship of personality. [ 17 ]


Another interesting method that has become more sophisticated and affordable to researchers is the method of whole genome expression analysis. This method involves collecting data for a large number of genes simultaneously which provides many advantages in studying personality. In an article written by  Alison M. Bell  and Nadia Aubin-Horth, they describe the advantages very clearly by stating, "For one, it is probable that the genetic basis of personality is polygenic, so it makes sense to simultaneously study many genes. In addition, gene products rarely act alone. Instead, they perform their function by interacting together in pathways and networks. As a result, the molecular changes that characterize a phenotype are frequently not based on a single marker or gene, but rather on an entire pathway. Whole genome expression profiling therefore has the potential to reveal new candidates genes and pathways." [ 18 ]


The biology-based personality theories (discussed below) are based on correlating personality traits with behavioral systems related to motivation, reward, and punishment. On a broad level, this involves the autonomic nervous system, fear-processing circuits in the  amygdala , the reward pathway from the  ventral tegmental area  (VTA) to the  nucleus accumbens  and prefrontal cortex. All of these circuits heavily rely on neurotransmitters and their precursors, but there has been the most research support for dopamine and serotonin pathways:


Previous studies show that genes account for at most 50 percent of a given trait. [ 1 ]  However, it is widely accepted that variance in gene sequence affect behavior, and genes are a significant risk factor for  personality disorders . [ 20 ]  With the growing interest in using molecular genetics in tracing the biological basis of personality, [ 8 ]  there may be more gene-trait links found in the future.


Varying  polymorphisms  and sequence repeats in the gene for  dopamine receptor D4  and serotonin transporter gene  5-HTTLPR , have both been found to influence the extraversion trait in adults. Specifically, study participants with at least one copy of the 7-repeat variant of the dopamine receptor D4 gene had higher scores of self-reported extraversion. [ 8 ]  This suggests that dopamine and serotonin interact to regulate the conflicting behavioral traits of careless exploration vs. cautious inhibition. [ 19 ]


Synaptic plasticity  refers to the ability of neurons to strengthen or weaken the connections between them. According to  Hebbian theory , these connections are strengthened and maintained through repeated stimulation between neurons. Specifically, there is an emphasis on  long-term potentiation  (LTP), which is the prolonged strengthening of synaptic connections that facilitate learning from experience.


On a larger scale, there are many pathways and brain regions that are interdependent and contribute to a cohesive, stable personality. For example, the amygdala and  hippocampus  of the  limbic system  mediate emotional intensity and consolidate memory of these experiences. But the basic mechanism by which these pathways and brain regions perform these functions, is synaptic plasticity. Ultimately, it boils down to this feature of neurons that allows the brain to learn from repeated experiences, retain memories, and ultimately maintain personality. [ 21 ]   Joseph LeDoux , an award-winning neuroscientist, asserts that although humans share the same brain systems, it is the unique wiring of neurons that is different in each person and makes their personality. [ 21 ]


Over the past two decades,  structural magnetic resonance imaging  (sMRI) and  functional magnetic resonance imaging  (fMRI) techniques have been used to study associations between neural activations in the brain and personality traits and other cognitive, social, and emotional processes that characterize personality. Using MRI-based methods for such studies has become increasingly popular due to the non-invasive nature of MRI and the high resolution of MRI.


The use of structural magnetic resonance imaging (sMRI) to understand the neurobiological basis of personality and sociocognitive functioning involves assessing the relationship between individual differences in these factors and individual differences in measures of brain structure, such as  gray matter  volume,  cortical  thickness, or structural integrity of  white matter tracts .


Studies have shown that brain volume is meaningfully correlated with four of the  Big Five  personality measures. Extraversion was associated with increased volume of medial  orbitofrontal cortex , a region associated with processing reward-related stimuli. Conscientiousness was associated with increased volume in the lateral  prefrontal cortex , a region involved in planning and the voluntary control of behavior. Agreeableness was associated with increased volume in regions involved in  mentalizing , which is the ability to infer the intentions and mental states of other individuals. Neuroticism was associated with increased volume of brain regions associated with threat, punishment, and  negative emotions . Openness/Intellect was not significantly correlated with the volume of any brain structures. [ 22 ]  In another study, neuroticism was negatively correlated with the gray matter volume of the right amygdala, whereas extraversion was positively correlated with gray matter volume of the left amygdala. [ 23 ]  A separate study also reported a significant association between neuroticism scores and gray matter volume of the left amygdala. [ 24 ]  In one MRI study, [ 25 ]  Novelty Seeking correlated with increased  grey matter  volume in regions of the  cingulate cortex , Harm Avoidance correlated with decreased grey matter volume in the orbitofrontal, occipital, and parietal cortex. Reward Dependence correlated with decreased grey matter volume in the  caudate nucleus .


A separate but similar line of research has used  diffusion tensor imaging  to measure the structural integrity of white matter in the brain. One study has shown that neuroticism is negatively correlated with the structural integrity of white matter tracts that connect various brain regions, such as the  prefrontal cortex ,  parietal cortex ,  amygdala , and other regions in the subcortex. On the other hand, Openness and Agreeableness are positively associated with the structural integrity of these white matter tracts. Openness was also positively associated with the structural integrity of white matter interconnecting dorsolateral prefrontal cortex in both hemispheres. [ 26 ]


Functional magnetic resonance imaging (fMRI) involves the indirect measurement of neural activity by measuring disturbances in local magnetic fields in the brain. These local disturbances are linked to differential amounts of blood flow to the brain, which is linked to neural activity. Early work using fMRI has studied whether individual differences in personality traits and sociocognitive functioning are associated with individual differences in neural activations in certain brain regions during certain tasks. Such studies have demonstrated associations between single brain regions' neural responses to certain tasks and individual differences in a wide range of sociocognitive functioning, such as approach/avoidance behavior, [ 27 ]  sensitivity to rejection, [ 28 ]  conceptions of the self, [ 29 ] [ 30 ]  and susceptibility to persuasive messages. [ 31 ]  A small collection of fMRI studies have also demonstrated a significant relationship between brain responses to certain tasks and personality survey measures, such as extraversion and neuroticism. [ 32 ] [ 33 ]


Over time, neuroscience researchers have recognized that brain regions do not operate in isolation. In fact, the synchronization of firing rates of neurons across different brain regions helps mediate the integration and processing of information across the brain. [ 34 ] [ 35 ]  Thus, studies relating neural activation in single regions to personality measures and associated sociocognitive functioning ignore information about how personality and sociocognitive functioning relate to neural activations across multiple regions in the brain. For example, it is unlikely that neural activation in a single brain region is unilaterally associated with individual differences in personality measures, such as the tendency to down-regulate  negative emotions . However, the  functional connectivity , or the synchronization of neural activity, between two brain regions can be related to individual differences in personality and sociocognitive functioning. For example, one study found that in an emotion regulation task, coupling of neural responses in the amygdala and the prefrontal cortex was significantly associated with more successful regulation of negative emotions. [ 36 ]  Other studies shown that neuroticism is associated with relatively low functional connectivity between amygdala and anterior cingulate cortex during a variety of tasks, such as viewing negative emotional stimuli [ 37 ] [ 38 ]  and during a classical conditioning reward task. [ 39 ]


Functional connectivity can also be measured at rest, during which individuals are not explicitly engaged in any task. [ 40 ]  These  resting-state functional connectivities  can also be related to personality measures and other sociocognitive functioning. For instance, one study found that functional connectivity patterns originating from the amygdala are predictive of neuroticism and extraversion scores. [ 41 ]  However, personality measures and sociocognitive functioning are not subserved solely by the functional connectivity between two given brain regions. Indeed, examining functional connectivity across the brain may shed more light on the neurobiological basis of personality and sociocognitive functioning. [ 42 ]  For example, a recent line of research has demonstrated that individual differences in functional  connectomes , which are characterized by patterns of spontaneous synchronization of neural activations across the entire brain, are predictive of individual differences in personality and sociocognitive functioning, such as openness to experience, [ 43 ]  fluid intelligence, [ 44 ]  and trait levels of paranoia. [ 45 ]  The use of functional connectomes to predict individual differences is known as "functional connectome fingerprinting" and allows researcher to construct models of personality and sociocognitive functioning based on neural activity across the whole brain rather than within single regions (if using neural activations) or single pairs of regions (if using functional connectivity). [ 46 ]


Functional connectomes can be distilled into constituent intrinsic brain networks that are present during sleep, at rest, and during tasks. [ 47 ]  These brain networks can also reliably be mapped onto cognitive systems. [ 48 ] [ 49 ]  The  default mode network , for example, is composed of regions such as the  medial prefrontal cortex ,  angular gyrus ,  temporoparietal junction , and the  hippocampus , to name a few. One study has shown that Extraversion and Agreeableness are positively correlated with overall neural activity in the default mode network. [ 50 ]  Assessing the relationship between neural activity in brain networks and personality traits is an important first step for identifying  where  the neurobiological basis of personality traits may be localized. However, this approach does not offer a complete mechanistic explanation of  how  and  why  individual differences in these brain networks are related to individual differences in personality. [ 42 ]  To address this gap, neuroscience researchers have begun to leverage  graph theoretical  approaches to better understand characteristics of these brain networks, such as their  assortativity ,  efficiency , and  modularity . For example, one study has demonstrated that individual differences in anxiety-related harm avoidance behavior was associated with relatively low efficiency (i.e., high  path length ) in the  insular - opercular  brain network at rest. This finding suggests that trait anxiety may be associated with relatively slow and inefficient transfer of information within the insular-opercular brain network. [ 51 ]  Another study used a graph theoretical approach to demonstrate that high trait impulsivity was associated with relatively high modularity of resting-state brain networks, such that brain networks exhibited relatively high within-system density of functional connectivity but relatively low between-system density of functional connectivity. [ 52 ]  A separate study has also demonstrated that high Conscientiousness is associated with high local clustering and high betweenness centrality within the default mode network and the  fronto-parietal network  (FPN). [ 53 ]  Given the role of the FPN in cognitive control, these findings suggest that people high on Conscientiousness may exhibit higher cognitive control. Furthermore, heightened interconnectivity within the DMN also provides convergent evidence that highly conscientious individuals may be adept at high-level cognitive tasks, such as complex planning, given that the DMN is strongly associated with high-level executive function and working memory. [ 53 ]


The  bloodline theory  ( simplified Chinese :  血统论 ;  traditional Chinese :  血統論 ) or  blood lineage theory  was a political theory associated with the " Loyalist Faction " ( Baohuang Pai ) of the  Red Guards  during the early phase of the  Cultural Revolution  in the  People's Republic of China . Opponents included the  "Rebel Faction"  ( Zaofan Pai ) of the Red Guards.


According to the bloodline theory, the defining factor in a person's  class standing  was their family's class position. It was expressed by the  bloodline couplet , "from a revolutionary father a hero, from a reactionary father a bastard." Although this position was politically-discredited, it continued to have a political impact during the Cultural Revolution.


According to the bloodline theory, the defining factor in a person's class standing was their family's class position. [ 1 ]  Regardless of a person's current position, they could not be considered as among the revolutionary people unless their family background was that of the poor or middle peasants, proletariat, or soldiers and would instead be considered among the so-called  Five Black Categories . [ 1 ]  In some instances, the criteria for Red Guard membership was so exacting that for purposes of the bloodline theory, family background would be traced back to grandparents or distant relatives. [ 2 ]


The essence of the bloodline theory was summarized by the slogan, "from a revolutionary father a hero, from a reactionary father a bastard." [ 3 ]  Other permutations of this bloodline couplet included the phrase, "it is basically like this" or the phrase "it is exactly like this," suggesting that advocates of the bloodline theory may have disagreed about the weight that it should have. [ 2 ]


In the period after the success of the  Chinese Communist Revolution , the newly-founded People's Republic of China had to address the building of socialist governance, norms, and order. [ 4 ] : 415   These state building tasks involved questions of who the next leaders would be and where they would be found (i.e., among families that had already included revolutionary leaders, or among the youth). [ 4 ] : 415–416


Under normal circumstances, the  Chinese Communist Party  (CCP)'s policy was that individuals should not be judged on family class background alone, but rather by their political performance. [ 5 ] : 114   At the Cultural Revolution's outset, questions of legacy or succession reached their peak, [ 4 ] : 416   with some children of senior  cadre  criticized the emphasis on political performance as part of  Peng Zhen's  "revisionist class line." [ 5 ] : 114


The bloodline theory began in secondary schools and spread from there to universities. [ 5 ] : 114–115   In late July 1966, variations of the bloodline couplet began appearing among Red Guard groups, with several different student groups claiming to have originated it. [ 2 ]  In the view of these early Red Guards (also known as "Old Red Guards"), class origin was the most important criteria for group membership. In descending order of status, these were: (1) children of army officials, (2) children of civilian state cadres, (3) children with working class family backgrounds, and (4) children from peasant backgrounds. [ 2 ]  Anyone not from "red origins" would be excluded, and those with the "purest bloodline" were still viewed in a hierarchy. [ 2 ]


The bloodline couplet caused major controversy. [ 5 ] : 114–115   Shortly after its appearance, arguments broke out primarily within the ranks of Red Guard students over how to interpret the principle. [ 2 ]  The bloodline theory was initially widespread among student activists during the Cultural Revolution, but was then strongly criticized by the  Maoists . [ 3 ]  Significant public opposition to the bloodline theory began in late 1966. [ 2 ]


Chen Boda  was the first Maoist leader to criticize the bloodline couplet, stating, "A theory of 'born-redness' has become popular lately. Those advancing this fallacy actually have attacked and marginalized the children of workers and peasants . . . . They confuse some students and encourage them to present the couplet, 'If the father is a hero, the son is also a hero.'" [ 2 ]   Jiang Qing  famously inverted the bloodline theory's slogan, and argued that if parents were revolutionaries then their children should follow their example, but if parents were reactionaries, then their children should rebel. [ 3 ]  In 1966, middle school student  Yu Luoke  wrote a popular pamphlet,  On Class Origins , that played a significant role in discrediting the bloodline theory. [ 3 ]


In the view of academic Alessandro Russo, the bloodline theory was a form of "biological classism" and "ideological trick" which ultimately failed because of how widespread political participation was during the early phase of the Cultural Revolution. [ 3 ]  Russo writes that even after the theory was politically discredited, it continued to have an impact during the Cultural Revolution. [ 3 ]


Historian Rebecca Karl observes that the bloodline theory had the "curious effect of casting suspicion on the vast majority of the old revolutionaries. After all, the nucleus of the CCP back in the 1920s and 1930s had been urban, educated youths along with some offspring of landlord or rich peasant families (for example, Mao himself)." [ 1 ]


Familial sleep traits  are  heritable  variations in sleep patterns, resulting in abnormal sleep-wake times and/or abnormal sleep length.