 Despite the not infrequent occurrence of small population sizes where drift would be expected to be most efficacious, random genetic drift was still considered an irrelevant evolutionary force in ecological genetics because natural selection was viewed as being particularly strong during periods of population decline. That is, the strength of natural selection was negatively correlated with population density. Thus, as population size and \(N_e\) declined owing to environmental stress, natural selection gained in strength so that the boundary between domains  (Fig. 1)  always favored selection. The smallest populations showed little phenotypic variation, which was seen as evidence that they were the most fit or most finely adapted populations. Thus, the lack of phenotypic variation in small populations was owing to it having been eliminated by natural selection during the immediately prior period of decline. Conversely, under periods of population increase, environmental stresses were minimal, so that natural selection was weaker and more permissive of variation. This concept of relaxed selection provided Ford with a cause for the increase in observations of rare phenotypic variants in large and growing natural populations. If selection pressure increases inversely to population size, then the role of random genetic drift in evolution must be greatly restricted.