Genet Research 45(3):299-314 (June 1985)
Polymorphism from environmental heterogeneity: models are only robust if the heterozygote is close in fitness to the favoured homozygote in each environment
Rolf F. Hoekstra, R. Bijlsma, A. J. Dolman

SUMMARY

The lack of robustness of models of the maintenance of polymorphism in a heterogeneous environment which has been pointed out by Maynard Smith & Hoekstra (1980), applies also to models based on habitat selection, on temporal variation and on density-regulated selection. Only if (partial) dominance 'switches' between environments such that the fitness of the heterozygote is always close to the favoured homozygote, is there reasonable robustness. This is true for all models considered. It is argued that there are good reasons for supposing that the favourable allele at a locus may show dominance, although the experimental evidence is still scanty.

1. INTRODUCTION

There is a large body of theoretical work on the problem of how genetic variation can be maintained in a varied environment, which has been reviewed by Felsenstein (1976) and Hedrick, Ginevan & Ewing (1976). Broadly speaking, temporal variation in selection coefficients at a single two-allelic locus in a diploid population brings about protected polymorphism (i.e. both alleles increase in frequency when rare) if there is geometric mean overdominance (averaged over time), while spatial fitness variation leads to protected polymorphism if there is harmonic mean overdominance (averaged over the subpopulations or niches). In the latter case, conditions for protected polymorphism become broader with decreasing amounts of migration between the subpopulations. The theory thus shows that environmental variation in fitness may maintain genetic polymorphism in the absence of (arithmetic mean) overdominance, which has been known for a long time as a potential variation preserving mechanism (Fisher, 1922), but which is also well known for a remarkable lack of empirical evidence concerning its operation in natural populations (Lewontin, 1974).

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