Taxonomy

Most modern taxonomists agree that classification systems should reflect evolutionary relationships. Thus, they agree that one should distinguish between homologous features and analogous features in constructing a classification system.

Homologous features have a common evolutionary origin, although they may differ in superficial morphology. For example, the arms of a human, fins of a whale, and front legs of a dog are all homologous. The underlying skeletal system of these appendages is similar and these three species had a common mammalian ancestor having appendages with similar skeletal structure.

Analogous features are superficially similar, but the similarity is not a result of common evolutionary origin. For example, the wings of a bird, bat, and butterfly are all analogous. Although they all have a similar superficial morphology and function, their underlying structure is very different. In other words, birds, bats, and butterflies did not share a common ancestor that had wings with a common design. The wings of birds, bats, and butterflies are similar merely because the laws of physics places certain restrictions on the shape of an appendage that can support flight.

Although taxonomists agree on the distinction between analogous and homologous features, they sometimes disagree about which specific method should be used to classify species. The three most commonly used methods are phenetics, cladistics, and evolutionary taxonomy. Some taxonomists use a combination of several of these different methods.