THREE genetically discrete male morphs coexist in Paracerceis sculpta, a Gulf of California marine isopod1-5. The large α males defend harems within intertidal sponges, the smaller β males mimic female behaviour and morphology, and the tiny γ males invade and sequester themselves within large harems. If selection is responsible for maintaining this polymorphism, then the mean fitness of each male morph must be equal over time6-9. Here we report that average reproductive success is equivalent among the three male morphs in monthly population samples collected over two years. We have investigated the total opportunity for sexual selection within and among morphs, and find that < 0.10% of the total opportunity for sexual selection occurs among morphs. Furthermore, alleles responsible for the expression of this polymorphism conform to the Hardy-Weinberg equilibrium, indicating the absence of differential natural selection among morphs. Conditions necessary for stable coexistence of three alternative male reproductive strategies seem to exist in nature.
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