Analogous structures

Analogous structures are features that resemble each other in function, and often in shape, in organisms that did not inherit them from a common ancestor that had that feature. The wings of insects, birds and bats all serve for flight, yet an insect wing is a thin sheet of cuticle with no bones, while bird and bat wings are built on a bony forelimb. Other examples are the streamlined bodies and fins of sharks and dolphins, and the camera-like eyes of octopuses and vertebrates.

Analogies arise through convergent evolution: unrelated lineages facing similar demands, such as moving through air or water, are shaped by natural selection toward similar solutions. They contrast with homologous structures, which share an origin and underlying plan even when their uses differ, like the human arm, the whale flipper and the bat wing. One feature can be both. As wings, the flight surfaces of birds and bats are analogous, because flight arose separately in each line, but as forelimbs they are homologous, because both inherited the same set of arm bones.

Telling analogy from homology matters when building phylogenetic trees. Mistaking a convergent trait for an inherited one can make distant species look like close relatives, so biologists compare fine structure, embryonic development and DNA.