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the process of evolutionary change, then evolution is indeed a change in genotype frequencies.

In summary, population genetics provides an essential framework for understanding how evolution occurs, grounding us in reality by clarifying the pathways that are open to evolutionary exploitation. To quote Carroll (2005a) again, “Simplification may indeed be necessary for news articles, but it can distort the more complex and subtle realities of evolutionary patterns and mechanisms.”

INTERNAL VERSUS EXTERNAL EVOLUTIONARY FORCES

The literature is permeated with dogmatic statements that natural selection is the only guiding force of evolution, with mutation creating variation but never controlling the ultimate direction of evolutionary change (for a review, see Stoltzfus, 2006a). This view derives from two types of arguments. First, hundreds of artificial selection experiments have generated changes in mean phenotypes well beyond the observed range in the base population in just a few dozen generations (Falconer and Mackay, 1996), inspiring the view that quantitative variation is distributed over an effectively infinite number of loci with minuscule effects (Kimura, 1965; Lande, 1975; Bulmer, 1980). Second, much of the earliest work in theoretical population genetics downplayed the ability of mutation to overcome the force of selection (Fisher, 1930; Haldane, 1932). Both arguments ignore significant complications that arise in finite populations, and it is now known that genome composition is governed by biases in mutation and gene conversion, some of which (e.g., mobile-element proliferation) operate via internal drive-like mechanisms (Lynch, 2007).

The notion that mutation pressure can be a driving force in evolution is not new (Darwin, 1859b, 1866; Morgan, 1925; Dover, 1982; Nei, 1987, 2005; Cavalier-Smith, 1997; Yampolsky and Stoltzfus, 2001; Stoltzfus, 2006b), and the conditions that must be fulfilled if mutation is to alter the direction of evolution relative to adaptive expectations are readily derived. Consider two alternative states at a locus, A and a, with the mutation rate of aA being m times that of Aa, but with type A having a fractional selective advantage s over type a. Further progress requires that we specify the effective number of gene copies per locus at the population level. This quantity, Ng, which is equivalent to the effective size of a haploid population and approximately twice that for an outcrossing diploid species, is influenced by many factors, including the breeding system, temporal fluctuations in population size, and the level of recombination (which influences the sensitivity of a locus to spurious hitch-hiking effects), and is generally orders of magnitude smaller than the absolute number of reproductive adults in a population (Lynch, 2007). With these definitions in



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