roversial idea is that some adaptations might increase the ability of organisms to generate genetic diversity and adapt by natural selection (increasing organisms' evolvability).[170][171][172][173][174]



A baleen whale skeleton, a and b label flipper bones, which were adapted from front leg bones: while c indicates vestigial leg bones, suggesting an adaptation from land to sea.[175]
Adaptation occurs through the gradual modification of existing structures. Consequently, structures with similar internal organisation may have different functions in related organisms. This is the result of a single ancestral structure being adapted to function in different ways. The bones within bat wings, for example, are very similar to those in mice feet and primate hands, due to the descent of all these structures from a common mammalian ancestor.[176] However, since all living organisms are related to some extent,[177] even organs that appear to have little or no structural similarity, such as arthropod, squid and vertebrate eyes, or the limbs and wings of arthropods and vertebrates, can depend on a common set of homologous genes that control their assembly and function; this is called deep homology.[178][179]

During evolution, some structures may lose their original function and become vestigial structures.[180] Such structures may have little or no function in a current species, yet have a clear function in ancestral species, or other closely related species. Examples include pseudogenes,[181] the non-functional remains of eyes in blind cave-dwelling fish,[182] wings in flightless birds,[183] and the presence of hip bones in whales and snakes.[175] Examples of vestigial structures in humans include wisdom teeth,[184] the coccyx,[180] the vermiform appendix,[180] and other behavioural vestiges such as goose bumps[185][186] and primitive reflexes.[187][188][189]

However, many traits that appear to be simple adaptations are in fact exaptations: structures originally adapted for one function, but which coincidentally became somewhat useful for some other function in the process.[190] One example is the African lizard Holaspis guentheri, which developed an extremely flat head for hiding in crevices, as can be seen by looking at its near relatives. However, in this species, the head has become so flattened that it assists in gliding from tree to tree—an exaptation.[190] Within cells, molecular machines such as the bacterial flagella[191] and protein sorting machinery[192] evolved by the recruitment of several pre-existing proteins that previously had different functions.[144] Another example is the recruitment of enzymes from glycolysis and xenobiotic metabolism to serve as structur