Comparative anatomy is the study of physical structures within and across species, genera, and higher level classifications of life. The concepts of homologous, analogous, and vestigial structures are fundamental to and underpin the field. In addition to zoology, it has close ties to phylogeny, which deals with the evolution of species, as well as evolutionary biology and paleontology. It also informs cladistics, the predominant methodology used to identify and classify ancestor and descendant species of organisms into evolutionary groups.
The 16th century Flemish scientist Andreas Vesalius laid the foundation for the development of comparative anatomy with the 1534 publication of De Humani Corporis Fabrica Libri Septem — i.e., The Seven Books on the Structure of the Human Body. The publication of Fabrica, as it’s known for short, launched a new tradition of empirical study of anatomy and the emergence of modern comparative anatomy, the founding of which is attributed to the 17th century English scientist and physician Edward Tyson.
Differences and similarities in comparative anatomy, now augmented with studies of molecular biology, are the basis for establishing evolutionary relationships among and between species. The study of comparative anatomy has produced strong evidence supporting the theory of evolution. Organisms that have similar anatomical structures from embryo to adult forms are believed to be closely related in evolutionary terms. It is also hypothesized that they share a common ancestor.
One of comparative anatomy’s fundamental concepts is that of homologous structures. Homologous anatomical structures are those found in organisms that are very similar in embryological development and form but different in function. The existence of homologous structures is believed to imply that the organisms are evolutionarily closely related and share a common ancestor. The forelimbs of mammals — such as the dorsal fins of whales and dolphins, the forelegs of cats and dogs, and the arms and legs of humans and other primates — are examples of homologous structures.
Anatomical structures found in organisms that are different in terms of development and morphology yet similar in function are known as analogous structures. The differences in embryonic development and ultimate form imply that no close evolutionary relationship exists between the two organisms and they do not share a common ancestor. An example of analogous structures is the wings of insects and birds.
Another important concept is that of vestigial structures. Anatomical features found in an organism that no longer serve any function are termed vestigial structures. They are often reduced in size, such as the human appendix. The presence of the feature in one organism and a vestigial one in another implies that the two share a common evolutionary ancestor and are closely related in an evolutionary sense. Whales, for example, have vestigial hind leg bones similar to those of land mammals.