Genetic variation is fundamental to Darwin's theory of evolution through natural selection, although when the idea was initially developed the mechanisms of inheritance were not yet known. Selection acts to favor some phenotypes over others, resulting in differences in relative fitness. The extent to which these phenotypes have a genetic basis determines whether those phenotypes that survive will pass on their attributes to their offspring. This process of genetic change through natural selection is termed adaptation. It can most readily be studied in recognizably polymorphic species, i.e., those in which genetic variability can be monitored over space and time. Accordingly, perhaps the best known study of natural selection in the wild is that of industrial melanism in the peppered moth, Biston betularia, in Britain. The typical form (typica) of this moth is light colored with black, pepper-like spots and is well camouflaged on lichen-covered birch trees. Following industrialization of certain areas of England, a darker form (carbonaria) started to appear in great abundance. This prompted the question, were the typical forms more subject to bird predation where the lichen on trees was soot covered and darker, leaving the darker forms in greater numbers? This issue was addressed by Kettlewell, in the 1950s, who manipulated the relative frequency of morphs and monitored their success. His findings were consistent with the hypothesis of predation—typica are eaten from dark backgrounds much more often than from light backgrounds, and vice versa for carbonaria. Further, a more recent reanalysis has demonstrated that, associated with the postindustrial reduction in pollution, there has been a shift to a lower frequency of the darker carbonaria forms. Still, a number of issues remain unresolved, demonstrating the difficulty involved in studying such systems in nature. For example, the carbonaria form was been found to persist even in areas relatively unaffected by pollution and two melanic forms have been shown exist in nature.

Adaptation has been studied using a number of other genetically determined polymorphisms in nature as indicators of variability. These studies include not only the now familiar color polymorphisms, such as are found in butterfly wing patterns, ladybird beetles, walking sticks, and happy face spiders, but also behavioral polymorphisms such as caste structure in social insects. Because of the relative ease by which polymorphisms can be measured phenotypically, and the often relatively simple genetic basis that underlies the variation, studies of polymorphisms have contributed much to the general understanding of natural selection in the field, particularly in demonstrating that selection can be a very powerful force and that balancing selection can maintain genetic variability.

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