Homology is a term used in comparative anatomy and evolutionary biology in reference to traits of organisms that have a common phylogenetic ancestry, but are now dissimilar in their structure, function, or behavior. Homology is based on the observation that there are basic, repeating patterns in the attributes of organisms, and that these are often due to a shared ancestry. However, depending on the environmental circumstances affecting the evolution of particular species within related groups (i.e., differences in natural selection), the attributes may eventually adopt differing forms. Nevertheless, their essential homology is retained, and this can often be discerned during earlier stages of development.

The study of homologies has been an important aspect of biology since the nineteenth century, and numerous examples have been identified in various groups of both closely and distantly related organisms. For example, the three tiny bones of the middle ear of humans...