nation assorts variation within and among chromosomes, and genetic drift ensures that gene frequencies will deviate a bit from generation to generation independent of other forces. Given the century of work devoted to the study of evolution, it is reasonable to conclude that these four broad classes encompass all of the fundamental forces of evolution.
Second, all four major forces play a substantial role in genomic evolution. It is impossible to understand evolution purely in terms of natural selection, and many aspects of genomic, cellular, and developmental evolution can only be understood by invoking a negligible level of adaptive involvement (Kimura, 1983; Lynch, 2007). Because all three nonadaptive forces of evolution are stochastic in nature, this conclusion raises some significant technical challenges. It is tempting to think that stochastic processes have no implications for the direction of evolution. However, the effects of mutation and recombination are nonrandom, and by magnifying the role of chance, genetic drift indirectly imposes directionality on evolution by encouraging the fixation of mildly deleterious mutations and discouraging the promotion of beneficial mutations.
Third, the field of population genetics is now so well supported at the empirical level that the litmus test for any evolutionary hypothesis must be its consistency with fundamental population-genetic principles. Grounded in basic Mendelian processes and sampling theory, many of these principles were laid down before the elucidation of the structure of DNA. Shortly after the genetic code was cracked, a series of technological breakthroughs advanced our ability to reveal molecular variation: protein sequencing in the 1950s, surveys of protein variants in the 1960s, ribosomal RNA sequencing in the 1970s, gene sequencing in the 1980s, and whole-genome sequencing in the 1990s. Each of these episodes brought the need for new methods for analysis and interpretation, and in each case the framework was drawn largely from preexisting population-genetic theory. Thus, although we do not yet fully understand the connections between evolution at the molecular and phenotypic levels, we can be confident that the machinery to do so is in place.
Fourth, some attempts to marginalize the contributions of population genetics to our understanding of evolution have pointed to the “bean bag” genetics debate that occurred in the middle of the last century (see Felsenstein, 1975). However, this is a misunderstanding, as the tensions during this period were not about the population-genetic basis of evolutionary change, but about the need to incorporate epistasis into the existing framework, something that population geneticists have now invested heavily in (Wolf et al., 2000; Carter et al., 2005). From the standpoint of its phenotypic products, evolution is more than a change in gene frequencies. Organisms are more than the sum of their parts, just as genes are more than the sum of their functional components. But if we are concerned with