Inter-Genotypic Competition in Plant Populations II. Maintenance of Allelic Polymorphisms with Frequency-Dependent Selection and Mixed Selfing and Random Mating.

EPORTS by investigators in recent years indicate that substantial amounts of allelic polymorphism are often maintained in plant populations that exhibit various levels of outcrossing. The reason for this phenomenon is an important consideration in evolutionary theory, particularly for those organisms that exhibit a high degree of self-pollination (where the mating system would seem to impose a severe restsiction on the amount of genetic recombination that can occur). Evidence is accumulating, however, that self-pollinators often possess more genetic flexibility than would be expected on the basis of observed levels of outcrossing, and that the distinction between outcrossers and selfers is, in this respect, not so great as previously supposed. JAIN and ALLARD (1960) and ALLARD and JAIN (1962) analyzed an experimental population of barley and found that substantial heterozygosity remained after 18 generations of natural selection under a mating system with approximately 99% self-pollination. ALLARD and WORKMAN (1963) reported similar results for three lima bean populations with about 95,% self-pollination. These authors speculated that the primary agency acting to maintain heterozygosity in the population was selection favoring heterozygotes. HARDING, ALLARD and SMELTZER (1966) subjected one of the lima bean populations studied by ALLARD and WORKMAN to detailed analysis and concluded that selective values of the genotypes in the population were frequency-dependent. They further postulated that the pattern of frequency dependency suggested a “neighborhood effect” in which the relative fitness of heterozygotes is affected by competition from other genotypes growing in proximity. Excess heterozygosity and frequency-dependent selection does not appear to be limited to self-pollinated plants. LEWONTIN and HUBBY (1966) found a high proportion of polymorphic loci in several populations of Drosophila pseudoobscura