Miosis to Variation
Evolution can also occur as a result of genes being transferred from one population to another. This gene flow occurs when there is migration. The loss or addition of people can easily change gene pool frequencies even if there are no other evolutionary mechanisms operating. For instance, if all red haired people were to leave Scotland, the next generation there would likely have very few people with this trait. The Scottish population would have evolved as would the populations into which the red haired people migrated.
Gene flow can also occur without migration. When people travel to another area and successfully mate with people in the population there, a transfer of genes occurs between the populations even though the traveler returns home. For example, when U.S. soldiers had children in Southeast Asia with Vietnamese women during the war there in the 1960's and early 1970's, they altered the gene pool frequencies of the Vietnamese population.
Genes may occasionally also flow between species. For instance, segments of DNA may be transferred from one species to another by viruses as they invade the cells of animals or plants. This apparently rare form of gene flow has been documented for some species of insects, fish, reptiles, mammals, and especially microorganisms, but it has not been conclusively demonstrated for humans. However, it has been suggested that 40-50% of human DNA sequences may have been transported from other species by viruses. This is yet to be verified.
Gene Flow — also called migration — is any movement of individuals, and/or the genetic material they carry, from one population to another. Gene flow includes lots of different kinds of events, such as pollen being blown to a new destination or people moving to new cities or countries. If gene versions are carried to a population where those gene versions previously did not exist, gene flow can be a very important source of genetic variation. In the graphic below, the gene version for brown coloration moves from one population to another.
An adaptation is a feature that is common in a population because it provides some improved function. Adaptations are well fitted to their function and are produced by natural selection.
Adaptations can take many forms: a behavior that allows better evasion of predators, a protein that functions better at body temperature, or an anatomical feature that allows the organism to access a valuable new resource — all of these might be adaptations. Many of the things that impress us most in nature are thought to be adaptations.
Mimicry of leaves by insects is an adaptation for evading predators. This example is a katydid from Costa Rica.
The creosote bush is a desert-dwelling plant that produces toxins that prevent other plants from growing nearby, which reducing nutrients and water.
Echolocation in bats is an adaptation for catching insects.
So what's not an adaptation? The answer: a lot of things. One example is vestigial structures. A vestigial structure is a feature that was an adaptation for the organism's ancestor, but that evolved to be non-functional because the organism's environment changed.
Fish species that live in completely dark caves have vestigial, non-functional eyes. When their sighted ancestors ended up living in caves, there was no longer any natural selection that maintained the function of the fishes' eyes. So, fish with better sight no longer out-competed fish with worse sight. Today, these fish still have eyes — but they are not functional and are not an adaptation; they are just the by-products of the fishes' evolutionary history.