Résumé
Sexual reproduction leads to an alternation between haploid and diploid
phases, whose relative length varies widely across taxa. Previous
genetical models showed that diploid or haploid life cycles may be
favored, depending on dominance interactions and on effective
recombination rates. By contrast, niche differentiation between haploids
and diploids may favor biphasic life cycles, in which development occurs
in both phases. In this paper, we explore the interplay between genetical
and ecological factors, assuming that deleterious mutations affect the
competitivity of individuals within their ecological niche, and allowing
different effects of mutations in haploids and diploids (including
antagonistic selection). We show that selection on a modifier gene
affecting the relative length of both phases can be decomposed into a
direct selection term favoring the phase with the highest mean fitness
(either due to ecological differences or to differential effects of
mutations), and an indirect selection term favoring the phase in which
selection is more efficient. When deleterious alleles occur at many loci
and in the presence of ecological differentiation between haploids and
diploids, evolutionary branching often occurs and leads to the stable
coexistence of alleles coding for haploid and diploid cycles, while
temporal variations in niche sizes may stabilize biphasic cycles.