 Although natural selection is the only evolutionary force that can account for adaptation, several evolutionary forces, acting alone or in combination, can sustain a genetic polymorphism, at least transiently. Thus, assigning causal agency to a single evolutionary force is often a more difficult problem when explaining polymorphism within species than it is for adaptations. In these studies of conspicuous polymorphisms, natural selection was “privileged” among the four evolutionary forces as an explanation for their maintenance, at least in part, because one of the founders of ecological genetics, E. B. Ford, collaborated extensively with R. A. Fisher (see Ford 1975). In the early period (1928–1950), much of the problem of assigning causal agency to the maintenance of genetic polymorphism was resolved by definition rather than by empirical observation (see section 2 below:  Classical Ecological Genetics and Polymorphism).  That is, only those polymorphisms where the rarest form was deemed “too common” to be accounted for by recurrent mutation were considered suitable for study. Moreover, if the species was abundant, then random genetic drift could similarly be eliminated as a relevant process. Indeed, the longest standing controversies in ecological genetics centered on whether natural selection could account for the observed frequencies of polymorphism or whether the variation in those frequencies indicated a significant role of random genetic drift (see, for example, Schemske and Bierzychudek [2001] regarding the interpretation of flower color polymorphism in the desert annual, Linanthus parryae).