NOTE 18N: Although this question was set in terms of a haploid organism with two alleles at a single locus, it could equally well apply to two alleles in a diploid, with complete dominance (giving two phenotypes), or two asexual genotypes, or two species. The existence of different ecological niches (here, patches A and B), with competition within each, can maintain polymorphism as well as allowing different species to coexist. See p. 624.