Convergent Evolution

Convergent evolution describes the acquisition of the same biological trait in unrelated lineages.

The wing is a classic example of convergent evolution in action. Flying insects, birds, and bats have all evolved the capacity of flight independently. They have "converged" on this useful trait.

The ancestors of both bats and birds were terrestrial quadrupeds, and each has independently evolved powered flight via adaptations of their forelimbs. Although both forelimb adaptations are superficially "wing-shaped," they are substantially dissimilar in construction. The bat wing is a membrane stretched across four extremely elongated fingers, while the airfoil of the bird wing is made of feathers, which are strongly attached to the forearm (the ulna) and the highly fused bones of the wrist and hand (the carpometacarpus), with only tiny remnants of two fingers remaining, each anchoring a single feather. (Both bats and birds have retained the thumb for specialized functions.) So, while the wings of bats and birds are functionally convergent, they are not anatomically convergent.

Traits arising through convergent evolution are termed analogous structures, in contrast to homologous structures, which have a common origin. Bat and pterosaur wings are an example of analogous structures, while the bat wing is homologous to human and other mammal forearms, sharing an ancestral state despite serving different functions. Similarity in species of different ancestry that is the result of convergent evolution is called homoplasy. The opposite of convergent evolution is divergent evolution, whereby related species evolve different traits. On a molecular level, this can happen due to random mutation unrelated to adaptive changes; see long branch attraction. Convergent evolution is similar to, but distinguishable from, the phenomena of parallel evolution. Parallel evolution occurs when two independent but similar species evolve in the same direction and thus independently acquire similar characteristics—for instance gliding frogs have evolved in parallel from multiple types of tree frog.