Homology versus Analogy

Do the human’s arm, cat’s limb, bat’s wing, and seal’s flipper have anything in common? There are no similarities with respect to their functions—lifting, walking, flying, and swimming—but careful analysis reveals commonality in their fundamental construction. Each of these mammalian forelimbs (or pentadactyls) consists of a long bone, connected to two smaller bones, linked to a number of even smaller bones, attached to approximately five digits. What about the wings of insects and birds? They all have the same function—flying—but they bear no structural resemblance to one another.

In 1843, the famed but highly controversial English biologist and comparative anatomist Richard Owen sought to explain why similar structures could have dissimilar functions and why similar functions could have dissimilar structures. He referred to the mammalian forelimbs as homologies— that is, the same organ has the same basic structures but different functions in various species. By contrast, the wings of insects, bats, and birds have a similar function but evolved separately by different pathways. Owen’s definitions were modified by Charles Darwin to incorporate an evolutionary explanation. In homologies, the basic structure has evolved from a common ancestor to serve dissimilar functions in an adaptation to different environmental situations. In contrast to homologies, analogous structures have similar characteristics, but have evolved independently from dissimilar ancestors, to meet environmental challenges, a process referred to as convergent evolution.

The wings of birds, bats, and insects are analogous structures, which have similar functions but dissimilar frameworks that evolved independently from very different ancestors to meet a common environmental need.

Vestigial structures are difficult to explain in the absence of a common ancestor. Eye bulbs in blind, cave-dwelling salamanders, the human appendix, and the pelvic girdle of whales have no function in their extant species but are homologous to a functioning structure in an ancestral species.

Homologies exist at both a structural and a molecular level. The genetic code—the sequence of nucleotides in DNA and RNA that determines the order of amino acids in the biosynthesis of proteins —is nearly identical in all organisms from bacteria to humans. Similarly, common genes exist across living organisms. The universal nature of the genetic code and genes provide additional support for evolution from a common ancestor.