Fifty years of genetic load

The author's involvement with and his successive reactions to the genetic load concept [whose beginning is identified with Haldane's paper. The effect of variation on fitness] is presented in the form of a personal odyssey. The major change in attitude involved the realization that the density- and frequency-independent selection discussed by most population geneticists has little bearing on events transpiring within natural populations; instead, natural selection should be viewed primarily as a density- and frequency-dependent phenomenon. Under this view, the culling of a large number of young zygotes to the considerably smaller number of adults that can be sustained by the environment is an essential process enabling any population's continued existence; to the extent that genetic variation facilitates culling, a genetic load (in direct opposition to the early view) can enhance a population's persistence through time.