Homology (biology)

Homology (biology)

Homology forms the basis of organization for comparative biology. In 1843, Richard Owen defined homology as "the same organ in different animals under every variety of form and function". Organs as different as a bat's wing, a seal's flipper, a cat's paw and a human hand have a common underlying structure of bones and muscles. Owen reasoned that there must be a common structural plan for all vertebrates, as well as for each class of vertebrates.

Homologous traits of organisms are due to sharing a common ancestor, and such traits often have similar embryological origins and development. This is contrasted with analogous traits: similarities between organisms that were not present in the last common ancestor of the taxa being considered but rather evolved separately. An example of analogous traits would be the wings of bats and birds, which evolved separately but both of which evolved from the vertebrate forelimb and therefore have similar early embryology.

Whether or not a trait is homologous depends on both the taxonomic and anatomical levels at which the trait is examined. For example, the bird and bat wings are homologous as forearms in tetrapods. However, they are not homologous as wings, because the organ served as a forearm (not a wing) in the last common ancestor of tetrapods. By definition, any homologous trait defines a clade—a monophyletic taxon in which all the members have the trait (or have lost it secondarily); and all non-members lack it.

A homologous trait may be homoplasious (i.e., it has evolved independently, but from the same ancestral structure), plesiomorphic (present in a common ancestor but secondarily lost in some of its descendants) or (syn)apomorphic (present in an ancestor and all of its descendants).

A homologous trait is often called a homolog (also spelled homologue). In genetics, the term "homolog" is used both to refer to a homologous protein, and to the gene (DNA sequence) encoding it.