Homology
Homology is the study of structurally similar biological systems which share a common origin but may or may not share a similar function. Although homology was originally conceived as mere structural similarity, it is now a tightly combined notion that refers to structural similarity due to common ancestory. Two or more structures that are similar because of shared ancestry are called homologous structures.
Similarities can be due to evolutionary ancestry, where the structures were found in a common ancestor and through developmental ancestry (where structural differences between different sexes of the same species might be called homologous). An example of shared evolutionary ancestry would be the flippers of a penguin and the wings of other birds, while an example of common developmental ancestry would be the ovaries of a human female and the testes of a human man.
Analogy, the opposite of homology, refers to the similarity of structures in which the cause is not shared ancestry; this is also called homoplasy. An example of analogy would be the wings of bats, birds, and insects, which are not due to shared ancestry, but rather through convergent evolution through different developmental pathways. With analogous systems, emphasis is ususally on a common function, while in homologous systems the emphasis is on structure and evolutionary history.
In genetics, homology is also used to refer to similarity in protein or DNA sequences that come from a common ancestor. This is why we can say that certain primate DNA is nearly 99% homologous with human DNA. Homologous chromosome pairs refer to the matched pairs of chromosomes in diploid cells, each of which is inherited from a different parent.
Web Resources On Homology
Homology, Homotopy and Applications
Evolution Makes Sense of Homologies
Book Resources On HomologyHomology: The Hierarchical Basis of Comparative Biology by Brian K. Hall
Speciation by Jerry A. Coyne
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