20. Mechanisms of Evolutionary Change

This lecture picks up the discussion of how assumptions of the Hardy- Weinberg equilibrium are violated in natural populations and how these violations are responsible for the occurrence of evolutionary change. An important point of this discussion is that natural selection, as described by Darwin, is not the only cause of evolution. Instead, a number of other factors can cause changes in the gene pool of a population, the most notable of these being genetic drift, or random fluctuations in the gene pool due to small population size. The lecture concludes by pointing out that natural selection can be a stabilizing force, maintaining the current phenotypic profile of a population, as well as a directional force or a diversifying force, changing the average phenotype.