The term "homology" was defined in 1843 by Richard Owen, a noted British paleontologist, as the "same organ under every variety of form and function." Thus homologous structures can be defined in an evolutionary context as elements whose similarity in various taxa derives from their common origin in a shared ancestor. Homology may be based on:

1. similarities in structure, or how an organ is shaped;
2. topography, or the location of an organ;
3. associations with other structures, an example of which would be bone-muscle relationships;
4. development, including shared expression patterns of homologous genes.

The concept of homology is fundamental to comparative biology and phylogenetics systematics. Homology has historically been defined in terms of inheritance of a structure, with more or less modification, from a common ancestor. In this sense, attributes of two organisms are homologous when they are derived from an equivalent characteristic of the common ancestor. For example, whale...