Is heterozygous advantage necessary for polymorphism in one-locus two-allele systems?

This paper presents a model of density-dependent selection in a continuously reproducing diploid population with two alleles at one locus. Using differential equations, we assume that the fertility of a mating pair is determined exclusively by the female. Genotypic fitness is defined as the average contribution of an individual of the genotype to the total population growth rate. The conditions for protected polymorphism are derived, and the most interesting new result is that genetic polymorphism does not necessarily require heterozygote superiority in fitness in diallelic one-locus systems if both fertility and viability selection are simultaneously operating in the population.