 The existence of either genotype-by-environment interaction \((\rG \times \rE)\) or gene-by-gene interaction (epistasis or \(\rG \times \rG\)) greatly complicates the estimation of selection coefficients. Ecological geneticists like Ford postulated interactions of the sort that could change the sign of genic selection coefficients with changes in the environment (including density) or in the genetic background. This kind of reversal of selective effect requires what is known as a “crossing-type” norm of reaction for \(\rG \times \rE\) or additive-by-additive epistasis for \(\rG \times \rG\) (Wade 2002). The simplest model of crossing-type \(\rG \times \rE\), consists of additive selection (i.e., genotypic fitnesses of \(1 + 2s, 1 + s\), and 1 for genotypes AA, Aa, and aa, in one environment and the opposite order in the second environment) in each of two alternative environments, \(E_1\) and \(E_2\), with frequencies, \(f_{E_1}\) and \(f_{E_2}\), respectively. As the two environments fluctuate in frequency, spatially or temporally, the selective effect of an A allele changes in both magnitude and sign (see  Fig. 2).  Depending upon the relative frequencies of the alternative environments and the amount of gene flow or migration between them, the A allele on average can be a “good” gene or a “bad” gene with respect to fitness, a gene of major effect or minor effect on fitness, or even a neutral gene if the two environments are equally abundant. The smaller the amount of migration between the environments, the greater is the degree of local adaptation to each as theory and  Ford suggested (see above).  However, the average selective effect of the gene in the sense of Fisher’s theory must be smaller than the average observation in a particular locality at a particular time because the long-term average contains both positive and negative values of \(s\). Furthermore, to the extent that the local value of \(s\) changes sign owing to continuous fluctuations in local environmental conditions, the average effect on fitness A allele will also move downwards, from the domain of selection to the domain of drift as Wright suggested. Thus, the very kind of population subdivision imagined by Ford, with selection acting in every locality albeit in different directions, creates, rather than eliminates, an increased opportunity for random genetic drift. 