LIFE LINES 31

HOW DOES EVOLUTION WORK AT THE LEVEL OF THE GENE?


When Darwin worked out the evidence for a past evolution of life and presented a theory of natural selection to account for that evolution, he did not have the advantage of a theory of heredity. That would not enter evolutionary studies for another 35 to 50 years. What Darwin described were things he saw which we call phenotypes. My hazel eyes and full head of hair at age 77 are phenotypes. It would take a genetic analysis to reveal the genes involved in those two phenotypes and today that is largely done at a molecular level, the actual genes being isolated, sequenced, and their functions worked out in the cells where they are expressed. For most of the traits Darwin studied, he assumed that change was very gradual to bring about increases in size, change in intensities of color, developing visual acuity, or other biological functions necessary for survival. In the late 1890s most biologists thought such factors involved in character formation were numerous each with a slight effect and things like height would be a consequence of these numerous factors sorting themselves out into bell-shaped curves. It was the prevailing idea from the 1920s to the 1970s for measuring human intelligence with IQ tests.

What was surprising to me at the 74th Cold Spring Harbor laboratory Symposium on evolution that ended in early June 2009 was how different we interpret such changes in appearance today. Instead of thousands of barely observable changes that distinguish a dachshund from a wolf only about a dozen major genes are involved. Similarly the difference between an eyeless albino cave fish and one that has full sight and pigmentation is also due to a dozen or so genes. In both cases one can deconstruct the recent and bring it back to the past by genetic breeding in only a fraction of a human life time. I was discussing this with my former student, Ron Sederoff at the meeting when Jim Watson joined us and said, “What this conference has shown is that gradualism is dead”. Sederoff and I continued our discussion and we agreed that small numerous changes leading to domestic breeds of animals and plants were not true. But there are traits like coat color in cereal grains and human skin color where about five pairs of genes can distribute color in bell-shaped curve. Such quantitative traits, like the beak size and shape of finches in the Galapagos may depend on a similar small number of factors that form such bell-shaped curves. What is exciting about the analysis of domesticated forms or the evolutionary changes in cave fish where no human intervention was involved, is the small number of genetic changes that led to the “degeneration” of the eyes and coat color of the fish and the small number of changes that led to the development of more acute sensing of pressure in the dark cavern waters they live in.

What this illustrated to me is the fallacy we often fall into of assuming that what we see (the phenotype) corresponds to our imagined mechanism of how it came to be (the genotype). Genotypes are resolved by breeding analysis or by molecular analysis of the genes involved. The wonderful thing about science is that analysis and experimentation are far superior to doctrine and logic as guides to the past, whether that doctrine is “creationism” in any of its forms or “Darwinian gradualism” in any of its premolecular forms.