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Genotypes of 68,230 individuals taken from 10 generations (F(4)-F(6), F(14)-F(17), F(24)-F(26)) of an experimental population of barley were determined for four esterase loci. The results show that frequencies of gametic ditypes changed significantly over generations and that striking gametic phase disequilibrium developed within a few generations for each of the six pairwise combinations of loci which were monitored. The complex behavior of these four enzyme loci in the population is attributed to interactions between selection and restriction of recombination resulting from the effects of linkage and/or inbreeding.

Changes in gene and genotypic frequencies at four esterase loci were monitored over 25 generations in Composite Cross V, an experimental population of barley, to obtain experimental evidence concerning the balance of forces responsible for: (1) the marked differences in allelic frequencies among barleys from different ecogeographical regions of the world; and (2) the extensive allelic variation found within local populations of barley. Analyses of the highly significant changes in allelic frequencies which occurred in CCV showed they were due to directional selection favoring particular alleles and not to mutation, migration or genetic drift. The results show that intense balancing selection, featuring consistent excesses of heterozygotes, also occurred in CCV. It is concluded that among the factors of neo-Darwinian evolution, natural selection plays the predominant role in determining the observed patterns of allelic variation in the barley species as a whole.