COMPARATIVE ANATOMY
The study of comparative anatomy entails examining the morphology of various anatomical structures among species and noting similarities and differences, in an effort to establish a relationship between species. Characteristics that are shared among a large number of similar species generally are viewed as ancestral, while those limited to one or few species are viewed as derived (or more advanced or specialized). The comparison of a variety of characteristics possessed by similar species allows us to differentiate between species that are truly closely related and those that have converged in appearance owing to similar habitats. Characteristics that show evidence of common ancestry are called homologous features. Those that have similar function but appear to be derived from differing ancestral characters are called homoplastic features. Sometimes structures that were used in an ancestor lose their use in more modern forms. We refer to these reduced structures as vestigial.