Similarities among different species can show two different kinds of relationships, both of which support evolution and natural selection. These similar structures are known as homologous structures and analogous structures.
Similarities shared by closely related species (species who share many characteristics) are homologous, because the species have descended from a common ancestor which had that trait. Homologous structures may or may not serve the same function. Figure below shows the forelimbs of mammals, considered homologous because all mammals show the same basic pattern: a single proximal bone joins a pair of more distal bones, which connect to bones of the wrist, “hand,” and digits. With this basic pattern, bats build wings for their lives in the air, whales form fins for their lives in the sea, and horses construct long, hoofed legs for speed on land. Therefore, homologous structures support common ancestry.
