 Figure 2. The interaction of Random Genetic Drift and Natural Selection, when there is either genotype-by-environment interaction or additive-by-additive epistasis (see text). The selective effect, \(s\), of a single gene changes magnitude as the frequency of the alternative environments, \(f_{E_1}\) and \(f_{E_2}\), connected by gene flow, changes or as the frequency of alternative alleles, \(p_B\) and \(p_b\), change at an interacting locus. Thus, neither the selective effect of a gene nor the effective population size remains constant. As a result, relative to  Figure 1,  the threshold boundary between the domains of Natural Selection and Random Genetic Drift is greatly widened, meaning that both forces play more or less equal evolutionary roles over a broad range of values of \(s\) and \(N_e\). Furthermore, interactions of this sort open the possibility that changes in the relative frequencies of alternative environments or alternative alleles at other loci can move a gene’s selective effect from the domain of selection that of drift or vice versa during the course of its evolution. [An  extended description of figure 2  is in the supplement.] 