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.