SELECTION IN EXPERIMENTAL POPULATIONS. I. LETHAL GENES

Abstract

THE main concerns of population genetics are the frequencies of genes in populations and the forces that alter them. Central in importance among these forces is natural selection. The analysis of selection in experimental populations began with the work of L’H~RITIER and TEISSIER in 1934 and is today a standard laboratory procedure. Despite the attention of geneticists for thirty years, however, certain important aspects of selection remain undemonstrated. The purpose of this article is to investigate in detail two particular aspects of selection: how it varies, and how it is partitioned among its more important components. Lethal genes have been chosen for this inquiry into the mechanisms of selection, since for them the algebra of selection is greatly simplified. We shall consider selection which acts on two alleles, one of them lethal, at a single locus on one of the autosomes. A lethal allele is one which, when homozygous, causes death before reproduction. The selection against the homozygotes for the lethal allele is thus complete; our task is to estimate the selection on heterozygotes for the lethal allele (LETH) and a non-lethal allele (NL) . COMPONENTS OF SELECTION We shall first develop an algebraic model for the study of lethal genes. The model will involve discrete generations; that is, it will be constructed with specific, non-overlapping periods for reproduction. Let us call the total selective value of organisms carrying two non-lethal alleles, W. “W” is assigned to the heterozygotes in accordance with the usual convention in discussions o€ lethal genes. It measures the average number of off spring produced by heterozygotes relative to the number produced by organisms carrying two non-lethal alleles. The homozygotes for a lethal produce no offspring, so their selective value is 0. The number of offspring, of course, determines the number of alleles present in the next generation. Thus the selective values determine the frequencies of the alleles. In order to reproduce, a fly must live to reproductive age; thus, the total selective value W is composed of all the various types of selection which occur from the formation of zygotes in one generation to the formation of zygotes in the next. The terms “fitness” and “adaptive value” are frequently used as synonyms for selective value. Consider a population in which we determine the frequencies of the genotypes among the adults of each generation before they are transferred to fresh food and