Similar Structures

Homologous structures, or homologies, are structures inherited from a common ancestor or from the same evolutionary origin. Homologous structures do not necessarily look alike—the structures evolve for different functions, so they may look very different from each other and from their common ancestral structure. For example, elephants' tusks and beavers' teeth are homologous structures, though they look quite different.

Conversely, structures that look similar are not necessarily homologous. Similar structures that evolved independently are called analogous structures. They often evolve in response to similar environmental conditions or for similar functions. For example, sharks and dolphins both have stream-lined bodies, flippers and fins that help them swim, but these structures evolved independently of each other.