What is the type of natural selection, stabilizing?
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Using a web search tool would have taken even less effort than writing this no-question.\nOh, well...\nGeneral info\nhttp://www.globalchange.umich.edu/globalchange1/current/lectures/selection/selection.html\nMore specific\nhttp://www.bookrags.com/sciences/genetics/stabilizing-selection-wog.html
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Evolutionary biologists have written extensively about how natural selection could produce new species. For instance, in the model called allopatry, developed by Ernst Mayr of Harvard University, if a population of organisms were isolated from the rest of its species by geographical boundaries, it might be subjected to different selective pressures. Changes would accumulate in the isolated population. If those changes became so significant that the splinter group could not or routinely would not breed with the original stock, then the splinter group would be reproductively isolated and on its way toward becoming a new species. Natural selection is the best studied of the evolutionary mechanisms, but biologists are open to other possibilities as well. Biologists are constantly assessing the potential of unusual genetic mechanisms for causing speciation or for producing complex features in organisms. Lynn Margulis of the University of Massachusetts at Amherst and others have persuasively argued that some cellular organelles, such as the energy-generating mitochondria, evolved through the symbiotic merger of ancient organisms. Thus, science welcomes the possibility of evolution resulting from forces beyond natural selection. Yet those forces must be natural; they cannot be attributed to the actions of mysterious creative intelligences whose existence, in scientific terms, is unproved.\n\nSpeciation is probably fairly rare and in many cases might take centuries. Furthermore, recognizing a new species during a formative stage can be difficult, because biologists sometimes disagree about how best to define a species. The most widely used definition, Mayr’s Biological Species Concept, recognizes a species as a distinct community of reproductively isolated populations—sets of organisms that normally do not or cannot breed outside their community. In practice, this standard can be difficult to apply to organisms isolated by distance or terrain or to plants (and, of course, fossils do not breed). Biologists therefore usually use organisms’ physical and behavioral traits as clues to their species membership. Nevertheless, the scientific literature does contain reports of apparent speciation events in plants, insects and worms. In most of these experiments, researchers subjected organisms to various types of selection—for anatomical differences, mating behaviors, habitat preferences and other traits—and found that they had created populations of organisms that did not breed with outsiders. For example, William R. Rice of the University of New Mexico and George W. Salt of the University of California at Davis demonstrated that if they sorted a group of fruit flies by their preference for certain environments and bred those flies separately over 35 generations, the resulting flies would refuse to breed with those from a very different environment. \n\nSome Creationists say that evolutionists cannot point to any transitional fossils—creatures that are half reptile and half bird, for instance. Actually, paleontologists know of many detailed examples of fossils intermediate in form between various taxonomic groups. One of the most famous fossils of all time is Archaeopteryx, which combines feathers and skeletal structures peculiar to birds with features of dinosaurs. A flock’s worth of other feathered fossil species, some more avian and some less, has also been found. A sequence of fossils spans the evolution of modern horses from the tiny Eohippus. Whales had four-legged ancestors that walked on land, and creatures known as Ambulocetus and Rodhocetus helped to make that transition [see “The Mammals That Conquered the Seas,” by Kate Wong; Scientific American, May]. Fossil seashells trace the evolution of various mollusks through millions of years. Perhaps 20 or more hominids (not all of them our ancestors) fill the gap between Lucy the australopithecine and modern humans. Creationists, though, dismiss these fossil studies. Th
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survival of the fittest... darwins's  origin of species ... anyways here\nhttp://www.bartleby.com/11/4001.html\nhttp://en.wikipedia.org/wiki/Survival_of_the_fittest
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it's called population control...fits right in with natural selection!
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Genetic variability allows a group to adapt to changes in habitat, climate, etc- so, for example, if it became suddenly dry for many years, frogs that carried genes that helped them adapt to that would survive, and thus save that population of frogs.  If you look, for example, at humans- some humans are extremely efficient at storing fat- in days gone by, there were probably periods when there was no food for months.  Most people would have died, but these people (who nowadays are obese and have a hard time) would have survived and saved that population of humans.  Diversity is the single best strategy for dealing with environmental stresses that would otherwise wipe out a population.\n*edit* in response to your email- There are a lot of genes that are there for sepcific stress conditions- times when things are not normal, like a period when there isn't any food, or water, or a long period of cold, or apecific diseases.  The more of these kinds of genes a population has in it, the better the chance that at least  few of the population will survive.  For example, say there are 100 diseases that can kill frogs.  There are a thousand frogs in the population- say each frog only has a few of the 100 genes that protect from the diseases.  So, if you have a major disease come through, maybe half the population will die (the ones that didnt have the gene to protect against that disease).  Now, right away, there is a period of drought, and another half die (cause they didnt have the gene to help them survive drought)...and on and on- you can see how, having a lot of different genes (high diversity) would help the whole population survive getting hit with different environmental stresses like disease, parasites, hot and cold weather, etc.  If a population of frogs had only the genes they needed to live in their little pond, in exactly the right weather, with exactly the right amount of food (in other words, they had low genetic diversity) then if any disease, or drought or cold snap came through, it would wipe them out.
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To become "selected against" the individual organism must die before it is able to reproduce. If it manages to reproduce, it will escape natural selection and perpetuate its DNA. Hence, not only the strongest survive. Only the very weakest and unfeasible of organisms die out.
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Everything in nature has a cause, but I would contest it having a purpose. Natural selection could be described as "purposeful" in a sense because it benefits those species that can adapt to changing conditions and compete for limited resources. But the mechanism is pretty well explained and doesn't require conscious, supervisory intent. \nHurricanes happen for pretty well known causes too, and to impute a purpose in this case is a much longer stretch. But if it is absolutely required to define a purpose, you could say that hurricanes aid evolution by placing unusually high stresses on most life forms that experience them. If overpopulation has reached resource-threatening levels, this may be ultimately beneficial to a species, aiding the selection process if it doesn't wipe them out.
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DNA is a way of storing information.  But that is far from intelligence.  Nothing senses changes in the environment and *decides* that it needs to change the organism to survive.\n\nYou are on the right track that there is an *element* of "luck" involved.    Randomness creates *variation*.  But there are two corrections you should know:  (1) Mutation is not the only source of randomness; and (2) Randomness is not the only thing driving evolution ... i.e. EVOLUTION IS NOT BLIND LUCK.\n\n1. Random mutations are improbable, but given huge numbers of replications, and huge amounts of time, there are *many* chances for mutations to occur, and those that produce some slight advantage tend to get propagated slightly better into future generations.  \n\nBut the invention of sexual reproduction took that randomness to a whole new level.   DNA from two different individuals could produce *huge* numbers of different combinations.  This allows improvements to propagate much faster into future generations, and allowed much faster adaptations to a changing environment.\n\n2. It's really important to know that EVOLUTION IS NOT JUST RANDOMNESS (BLIND LUCK).  Evolution is driven by 3 ingredients: variation + selection + time.  Randomness produces the *variation* ... but the selective pressures are NOT random.  \n\nFor example, a mammal may have several litters of offspring, say 20 in total.  Randomness produces the *variation* that some of these will be more hairy than their siblings.  But there is *nothing* random about the fact that those offspring with more hair may survive better in a colder environments and therefore produce more babies.  (E.g. if the environment is getting colder, or if some members want to, say, push a little further north.)  Add the ingredient of *time* and a much hairier subspecies can arise and get separated from the parent species ... and even more *time* and the subspecies can completely lose the ability to mate with the parent species, and you have two different species.\n\nSummary: randomness supplies the sparks of evolution, but selective pressures in the environment + time, provide the fuel.
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It is very questionable that humans are evolving, at least in terms of largescale selections.  Where we are today with technology, rather than nature selecting the most fit of us, oftentimes we alter nature to allow everyone to live.  This obviously isnt true in all cases, but we are at least tampering and slowing down human evolution
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Hi. First off, its "deScent with modification".\nThat was Darwin's term for what is now known as genetic variability in a population, and a stage of his proposed evolutionary mechanism, natural selection.\nHe did not have the genetic background to explain the variations that he saw generation after generation (Mendel's works had been ignored at the time), but he noticed that in every new generation, the individuals showed varied characteristics: some traits where similar to their parents, but others were novel or maybe similar to those of older ancestors. \nThese variations occur because of 1) spontaneous mutations in the parental gametes; 2) recombination between homologous chromosomes during gamete formation (crossing over); 3) the combination of the parents' genotypes to form the genotype of the offspring (this is not strictly new material, only a new combination of preexisting ones).\n\nOnce there is a new generation of individuals that are growing and living together, some of those characteristics may result in better adjustment to the environment. The individuals with those "adaptively advantageous" traits will have higher probabilities of growing, maturing and reproducing more, with respect to other individuals. \nIf those advantageous characteristics are heritable, that is, if they are determined by the individual's combination of alleles (=the two versions of the same gene in each of us), when the individual is able to "pass them on" to more descendants those alleles will spread in the population, and eventually be present in most individuals. This, in a nutshell, is natural selection. NS leads to gradual adjustment of the population with respect to its environment.\nFrom a population genetics perspective, evolution is defined as _the change in allele frequencies_; thus, "descent with modification", coupled with evolutionary factors, leads to evolution of the population. \nIn addition to mutation and natural selection, this change in allele frequencies may be occasioned by migration (genetic flux among different populations), genetic drift (a random process) and selective mating (which strictly speaking only alters the proportion of combinations, but does not produce new material).\n\nSorry for the lengthy answer, but evolutionary processes need a little context to be understood. Good luck with your explanation!
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Actually- just because we are at the top of the food chain doesn't mean that we aren't still subject to selection pressures.  It is true that predation is one selection pressure that we no longer have to worry about, but disease is a selection pressure that we cant fully avoid.  \n\nWhile science has made tremendous progress in terms of preventing/curing "old-world" epidemics like polio and smallpox, new viruses and pathogens are constantly emerging through an evolutionary process of their own.... some diseases are such that they're only fatal to certain groups of people (for example, those with compromised immune systems), as well.  So theoretically, if such a disease were to grow out of control, those groups of people would be selected against...   \n\nAlso, culturally we're seeing that certain groups of people are reproducing more frequently and in greater numbers than others, while some populations have a negative birth rate.  If these sort of culturally-based reproduction trends continue, it's logical to assume that humans will look significantly different in the future. \n\nIt is safe to argue that we as a species have broken free of most of the evolutionary constraints that were once upon us- but there are still some factors that limit our evolutionary fitness.
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Should natural disaster occurs, natural selection occurs, as the species that adaptable enough to survive the natural disaster will continue their existance and those which not adaptable enough to survive these will be wiped out from the area.\nThose which able to continue their existance will inherit the traits that enable them to survive the natural disaster and of course, those who cannot survive will not have a chance to inherit their traits. \nThose will results to the increase of the "resistance" of the surviving species to the same natural disaster.
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Most hypothesis for evolutionary mechanisms are not opposed to basic darwinism, rather they're complementary, e.g. punctuated equilibrium.\nI'd say the neutral theory of evolution (by Kimura) is an alternative because it puts the accent on neutral mutations instead of natural selection.
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i havent heard that example, but its possible. that would possibly fall into the category of microevolution...like when birds beaks get bigger or smaller to crack different nuts. its still a bird, with slightly different attributes. that would be an example of artificial rather than natural selection though, since it would be people choosing what they think is attractive, rather than nature killing off people who were less well equipped to deal with the environment.
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This is Darwin's Theory of Evolution known as natural selection:\n\n1. Species have great fertility. They make more offspring than can grow to adulthood. \n\n2. Populations remain roughly the same size, with modest fluctuations. \n\n3. Food resources are limited, but are relatively constant most of the time. From these three observations it may be inferred that in such an environment there will be a struggle for survival among individuals. \n\n4. In sexually reproducing species, generally no two individuals are identical. Variation is rampant. \n\n5. Much of this variation is heritable.
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Two potentially unresolved questions in evolutionary biology are how and why did sex evolve (the big unresolved issue), and why only two as opposed to more sexes is stable. theory allegedly predicts three is more stable and better than two for example.\n\nYour text book anwsers, like recombination to break the linkage between "good" and "bad" genes and to avoid parasites (red queen hypothesis) explain why sex, ONCE it has evolved, is stable and selected for, BUT not how or why it evolved.\n\nPerhaps you should consider work by Lynn Marguilas et al. on the symbiont theory. Although now largely accepted that organelles like the mitochondria and chloroplast are degenearte symbionts, and less accepted that the nucleus is too, extrapolation of this theory would allow (account for) for the evolution of diploidy and reductive meisois (and thus sexual reproduction) by having two organisms (one aerobic the other anerobic) merging and seperating according to the oxygen levels (oxygen was at first a highly toxic compound to life untill aerobic metabolism evolved). \n\nBy the way hemaphrodites are still sexual, even selfing ones.\n\nAnd additionaly please note and remember for future that sexual discrimination, like homosexual discrimination, and so on, is neither a prerequisite nor a product of sexual reproduction.
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Genetic information is increased by mutation of the genetic material inside an organism. Natural selection simply removes the less advantageous mutations. An increase is not necessarily an improvement but is simply an increase.\n\nDo not think of one current species becoming another, instead think about one common ancestor to both and then how the mutations occurred over million of years and generations to present very different animals.\n\nCertainly, there is a consciousness that can be applied to the selection. Genocide is a good example of somebody thinking they can direct the development of a whole species. Surely the Final Solution is evidence that humans can think in this way. It doesn't mean we should but it proves that we can.