The theory of neutral and weakly selected genes.

The controversy over whether a major fraction of evolutionary allele substitutions and protein polymorphisms in natural populations is due to random drift of neutral mutations remains unresolved; but the discussion has led to extensions of population genetic theory, particularly in stochastic models. The work of Maruyama in seeking quantities that are not strongly dependent on assumptions about population structure is particularly discussed. Under some circumstances, and perhaps quite generally, the probability of eventual fixation of a mutant gene, the total number of heterozygotes in which the gene is involved before its fixation or less, and the total number of heterozygotes during the time the gene has a specified frequency in the entire population have this desirable property. Whether there are many mutants that are so nearly neutral as to be unaffected by selective forces or not, it is clear that there are many loci at which selection is very weak and some of the evolutionary consequences of this are discussed briefly.