Comparative anatomy is the study of similarities and differences in the anatomy of organisms. It is closely related to evolutionary biology and phylogeny (the evolution of species).
Two major concepts of comparative anatomy are:
1. Homologous structures - structures (body parts/anatomy) which are similar in different species because the species have common descent. They may or may not perform the same function. An example is the forelimb structure shared by cats and whales.
2. Analogous structures - structures which are similar in different organisms because they evolved in a similar environment, rather than were inherited from a recent common ancestor. They usually serve the same or similar purposes. An example is the torpedo body shape of porpoises and sharks. It evolved in a water environment, but the animals have different ancestors.
The rules for development of special characteristics which differ significantly from general homology were listed by Karl Ernst von Baer (the Baer laws).