Macroalgae show diverse patterns of alternation of generations. Especially notable is the diversity of relative size during haploid and diploid phases. In diploid-dominant heteromorphic life cycle species, the diploid phase is large-bodied while the haploid phase is small, whereas in others the opposite is observed. In contrast to heteromorphic life cycle species, both haploid and diploid phases in species with isomorphic life cycles are similar in size and morphology.

To clearify the conditions under which each life cycle is advantageous, we analyze genetic models for life cycle evolution in macroalgae. We derive the fixation probabilities of mutant alleles introduced into a resident population. We next generalized the model to allow dispersal by an individual-based model on a one-dimensional lattice.

We find that species with isomorphic life cycles tend to evolve when the advantages of large size from greater fecundity exceed the advantage of small size. However if fertilization success is low in the water column but increased near the boundary layers, the diploid-dominant life cycles are favored. Under the ecological conditions of our model, haploid-dominant life cycles are not favored.