THE POPULATION-GENETICS OF THE SELF-INCOMPATIBILITY POLYMORPHISM IN PAPAVER RHOEAS .8. SAMPLING EFFECTS AS A POSSIBLE CAUSE OF UNEQUAL ALLELE FREQUENCIES

This paper considers the hypothesis that the unequal S-allele frequenc ies in the British populations of Papaver rhoeas we have examined are caused by a sampling effect over and above that due to drift. The resu lts obtained by simulating the polymorphism on the computer show that it takes between 70 and 80 non-overlapping generations before stochast ic equilibrium is achieved with respect to allele frequency in a popul ation of size 3720 containing 31 alleles, both when the frequencies of the alleles are initially as unequal as they were in the R106 sample (Campbell and Lawrence, 1981b) and when these frequencies are exactly equal. These results show that the strength of the frequency-dependent selection that maintains the polymorphism becomes very considerably a ttenuated as the number of alleles in the population increases. A revi ew of the relevant genetical;and ecological evidence suggests that the size of the British populations is considerably larger than the minim um required to maintain the number of alleles they are estimated to co ntain, so that opportunities for sampling effects to perturb the frequ encies of these alleles appear to be limited. The results obtained fro m an investigation of the reproductive biology of the species, however , show that the distribution of progeny size is markedly non-random. I t is possible that this may be the cause of the unequal allele frequen cies observed in the the British populations.