A Primer on the Tree of Life (Part 4): Homology in Crisis
Homology in Crisis
As Mayr suggests, there are other examples where genetic similarity appears in unexpected places. Biologically functional similarity that is thought to be the result of inheritance from a common ancestor is called "homology."
The concept of "homology" has been thrown into a crisis via observations, like those of Mayr, that the same genes control the growth of non-homologous body parts. Pax-6 is just one example. Another is the fact that the same gene controls the development of limbs in widely diverse types organisms that have wholly different types of limbs, where their common ancestor is not thought to have a common type of limb.14 The methodology used to infer homology was also challenged when it was discovered that different developmental pathways control the growth of body parts otherwise thought to be homologous. As the textbook Explore Evolution observes:
In sharks, for example, the gut develops from cells in the roof of the embryonic cavity. In lampreys, the gut develops from cells on the floor of the cavity. And in frogs, the gut develops from cells from both the roof and the floor of the embryonic cavity. This discovery--that homologous structures can be produced by different developmental pathways--contradicts what we would expect to find if all vertebrates share a common ancestor. ... To summarize, biologists have made two discoveries that challenge the argument from anatomical homology. The first is that the development of homologous structures can be governed by different genes and can follow different developmental pathways. The second discovery, conversely, is that sometimes the same gene plays a role in producing different adult structures. Both of these discoveries seem to contradict neo-Darwinian expectations.15Perhaps this evidence is just the result of what Mayr called "hidden potentials of the genotype," or perhaps it contradicts neo-Darwinian expectations because neo-Darwinism is wrong.
[14.] Paul Nelson and Jonathan Wells, "Homology in Biology," in Darwinism, Design, and Public Education, John Angus Campbell and Stephen C. Meyer eds. (East Lansing: Michigan State University Press, 2003).
[15.] Stephen C. Meyer, Scott Minnich, Jonathan Moneymaker, Paul A. Nelson, and Ralph Seelke, Explore Evolution: The Arguments For and Against Neo-Darwinism, pgs. 44-45 (Hill House, 2007)."