Darwin’s theory of evolution by natural selection requires heritable variation for selection to work on. In Darwin’s time, thinking about heritability centered on the idea of “blending inheritance”—the hypothesis that the offspring receive some average mix of the parental characters. If this were true, each generation would be more average than the last and variation would steadily decline. To make natural selection work, Darwin had to suppose that the frequency of mutations—”sports” as they were often called—was high.
In 1900, the pioneering work of Gregor Mendel was independently rediscovered by Hugo De Vries, Carl Correns, and Erich von Tschermak-Seysenegg. Mendel had proposed that each character is controlled by two genetic factors (genes), one from each parent. Most characters have a dominant form and a recessive form (phenotype). The different phenotypes result from different versions of the gene, called alleles, whether they are the same or different in the two chromosomes. The dominant phenotype is expressed when one or both alleles are dominant; when both alleles are recessive, the recessive phenotype is seen.
It was not immediately obvious that Mendel’s model had anything to say about evolution; biologists assumed that dominant alleles would slowly displace their recessive counterparts, thus inexorably reducing variation. In 1908, however, G. H. Hardy and W. Weinberg independently realized that Mendelian inheritance would, in fact, maintain variation at current levels under most circumstances.
The conditions necessary for this Hardy-Weinberg equilibrium in a sexually reproducing population are:
- The population needs to be large; otherwise, chance effects could lead to genetic drift—a random change in allele frequencies that usually leads to the local extinction of one of the two alleles.
- Mutations must not be common, since mutations produce new alleles.
- There must be no immigration or emigration that alters allele frequency.
- Mating must be random with respect to alleles.
- Reproductive success must be random with respect to alleles.
If any of these conditions is violated, evolution (a change in allele frequency) will occur. Violation of condition 5, of course, is the essence of natural selection. (Condition 4, which Darwin called “sexual selection,” is only a special case of condition 5.)
The Hardy-Weinberg formulation not only accounts for how variation is maintained, it identifies alternative mechanisms for evolution—genetic drift, for instance. What Hardy and Weinberg could not know at the time is that most genes are pleotropic; that is, each has multiple effects of the phenotype. Moreover, most phenotypic characters are controlled by a combination of many genes. Each of these genotype/phenotype interactions helps preserve variation. In addition, sexual recombination operates to produce a nearly infinite number of novel combinations of alleles in each generation.
- Gould, J. L., & Keeton, W. T. (1996). Biological science (6th ed.). New York: Norton.