Population Size, Natural Selection and the Genetic Load

Abstract

How great a genetic load can a population tolerate ? This subject has recently been discussed by several authors^1–5. We believe that a most important point can be stated briefly: because most organisms produce far more offspring than are necessary to maintain a constant population density, and because population densities remain, very roughly, constant, many individuals die before they are mature; it does not matter whether they die of starvation, accidents or from genetic ailments; the population can still maintain itself. It is not so much that there is a genetic load which might threaten the species, but that there is an ecological load, resulting from density regulation, which because it must, as Darwin noted, produce natural selection, gives rise to the apparent genetic load. A population will be able to tolerate what seems to us a considerable genetic load, without being, on that account, in any danger of extinction. The genetic load is, for the most part, merely an expression of the fact that not all genotypes are equally viable when the population becomes crowded. Many individuals have to die in the process of density regulation, and if those which die differ genetically from those which survive, we will observe a “genetic load”. This is the crux of the Malthus—Darwin concept of selection. This does not of course apply to genetic conditions which are markedly disabling at all population densities; there must be a decided limit (although a fairly high one, for the dead individuals simply leave more food or space for others, which otherwise would die) to the number of these which a population can contain. We suggest the term “loaded” for this last kind of selection.