So far in this course we have dealt almost entirely with the evolution of characters that are controlled by simple Mendelian inheritance at a single locus. We talked some about gametic disequilibrium and how allele frequencies change at two loci simultaneously, but in every other example we've considered we've imagined that we could understand something about evolution by examining the evolution of a single gene. That's the domain of classical population genetics.
For the next few weeks we're going to be exploring a field that's actually older than classical population genetics, although the approach we'll be taking to it involves the use of population genetic machinery. If you know a little about the history of evolutionary biology, you may know that after the rediscovery of Mendel's work in 1900 there was a heated debate between the ``biometricians'' (e.g., Galton and Pearson) and the ``Mendelians'' (e.g., de Vries, Correns, Bateson, and Morgan).
Biometricians asserted that the really important variation in evolution didn't follow Mendelian rules. Height, weight, skin color, and similar traits seemed to
Since variation in such quantitative traits seemed to be more obviously related to organismal adaptation than the ``trivial'' traits that Mendelians studied, it seemed obvious to the biometricians that Mendelian geneticists were studying a phenomenon that wasn't particularly interesting.
Mendelians dismissed the biometricians, at least in part, because they seemed not to recognize the distinction between genotype and phenotype. It seemed to at least some of them that traits whose expression was influenced by the environment were, by definition, not inherited. Moreover, the evidence that Mendelian principles accounted for the inheritance of many discrete traits was incontrovertible.
Woltereck's experiments on Daphnia helped to show that traits whose expression is environmentally influenced may also be inherited. He introduced the idea of a norm of reaction to describe the observation that the same genotype may produce different phenotypes in different environments. When you fertilize a plant, for example, it will grow larger and more robust than when you don't. The phenotype an organism expresses is, therefore, a product of both its genotype and its environment.
Nilsson-Ehle's experiments on inheritance of kernel color in wheat showed how continuous variation and Mendelian inheritance could be reconciled. He demonstrated that what appeared to be continuous variation in color from red to white with blending inheritance could be understood as the result of three separate genes influencing kernel color that were inherited separately from one another. Fisher, in a paper that grew out of his undergraduate Honors thesis at Cambridge University, set forth the mathematical theory that describes how it all works. That's the theory of quantitative genetics, and it's what we're going to spend the next three weeks discussing.