EVOLUTION OF THE ALTERNATION OF HAPLOID AND DIPLOID PHASES IN LIFE-CYCLES .2. MAINTENANCE OF THE HAPLO-DIPLONTIC CYCLE

The relative duration of the haploid and the diploid phases during the reproductive cycle varies greatly between organisms. This paper addre sses the question of the evolution of haploid, diploid, and haplo-dipl ontic life cycles. When the life span of haploid and diploid individua ls is constant whatever their cycle, we show that the haplo-diplontic cycle has an advantage, which depends on the sex-ratio in anisogamous species and on the probability of fertilization in isogamous species. This is because meiosis and fertilization occur half as often in the h aplo-diplontic cycle as in haploid or diploid cycles, for the same num ber of generations of individuals. This argument is demonstrated using a model which considers a genetic determination of the cycle, and fix ed haploid and diploid fitnesses. The relevance of measures of fitness of haploid and diploid individuals in predicting the evolution of lif e cycles is discussed. Measures obtained in algae are compared with th eoretical predictions.