Protected polymorphism in the two-locus haploid model with unpredictable fitnesses

In an unpredictable environment, the distributions of alleles from which po lymorphism can be maintained forever belong to a certain set, the C-viabili ty kernel. Such a set is calculated in the two-locus haploid model, as well as the corresponding fitnesses at any time which make this maintenance pos sible. The dependence of the C-viability kernel on the set U of admissible fitnesses and on the recombination rate r is studied. Notably, the C-viabil ity kernel varies rapidly in the neighborhood of equal fitness of AB and ab ; it becomes empty when ab has a fitness below a certain function, which is delineated, of the recombination rate. The properties of the two-locus mod el under constraints, out of equilibrium and with unpredictable selection a re thus presented.