Ecology and the evolution of biphasic life cycles

Sexual eukaryotes undergo an alternation between haploid and diploid nuclea r phases. In some organisms, both the haploid and diploid phases undergo so matic development and exist as independent entities. Despite recent attenti on, the mechanisms by which such biphasic life cycles evolve and persist re main obscure. One explanation that has received little theoretical attentio n is that haploid-diploid organisms may exploit their environments more eff iciently through niche differentiation of the two ploidy phases. Even in is omorphic species, in which adults are morphologically similar, slight diffe rences in the adult phase or among juveniles may play an important ecologic al role and help maintain haploid-diploidy. We develop a genetic model for the evolution of life cycles that incorporates density-dependent growth. We find that ecological differences between haploid and diploid phases can le ad to the evolution and maintenance of biphasic life cycles under a broad r ange of conditions. Parameter estimates derived from demographic data on a population of Gracilaria gracilis, a haploid-diploid red alga with an isomo rphic alternation of generations, are used to demonstrate that an ecologica l explanation for haploid-diploidy is plausible even when there are only sl ight morphological differences among adults.