Evidence for Evolution — Comparative Anatomy

Comparative anatomy is the branch of biology that systematically compares the structure of body parts across different species, and it provides a second, largely independent line of evidence for evolution, built around a crucial distinction between two categories of anatomical similarity: homologous organs and analogous organs. Correctly telling these two categories apart is essential, because only one of them — homology — actually indicates a close evolutionary relationship, while the other — analogy — can arise between completely unrelated organisms and reflects nothing more than a shared functional demand imposed by a similar environment or way of life.

Homologous organs are organs that share the same fundamental underlying structure and the same relative position within the body, even though the organs may look quite different externally and may now be adapted to perform quite different functions in different species — this shared underlying structural plan is best explained by the different species having all inherited it, with subsequent modification, from a single common ancestor. The forelimbs of a human (adapted for grasping and manipulating objects), a bat (adapted, with much-elongated finger bones supporting a wing membrane, for powered flight), a whale (adapted, with a shortened, broadened bone structure, for swimming) and a horse (adapted, with reduced and fused bones, for fast running) are the standard textbook example: despite these four limbs looking, and being used, in completely different ways on the outside, an X-ray or skeletal dissection reveals that all four share the identical basic bone arrangement of the pentadactyl (literally 'five-fingered') limb — a single upper bone (the humerus), two forearm bones (the radius and ulna), a cluster of wrist bones (carpals), and five digits — the same basic bony plan simply modified, stretched, shortened or fused differently in each lineage to suit its own particular way of life. Because homology reveals that different species have diverged, over evolutionary time, away from a shared ancestral structure to serve different functions, it is described as evidence of divergent evolution.

Analogous organs, by contrast, are organs that perform the SAME function, and that may even look broadly similar to each other on the outside, but that have a completely different underlying internal structure and evolved completely independently, in lineages that are not closely related at all — such similarity is best explained not by shared ancestry, but by both lineages having independently faced a similar functional challenge and having independently evolved broadly similar solutions to it, a process called convergent evolution. The wings of a bird and the wings of a butterfly are the standard example: both structures serve the identical function of powered flight, and both can look superficially similar as external, elongated, flattened flying surfaces, yet a bird's wing is a modified pentadactyl forelimb, built around the same internal bony skeleton discussed above and covered in feathers, while a butterfly's wing is an entirely different kind of structure altogether — a simple membranous outgrowth of the insect's external exoskeleton, with no internal bones at all and a completely different embryological origin. Because a bird and a butterfly are only very distantly related within the animal kingdom, and because their two kinds of wing are built on such fundamentally different underlying plans despite serving an identical function, the wings of birds and the wings of butterflies are analogous, not homologous, organs. …