Behavioral Ecology Vol. 14 No. 2: 201-211
© 2003 International Society for Behavioral Ecology
On the evolution of polygyny: a theoretical examination of the polygyny threshold model
a Department of Biological Sciences, 371 Serra Mall, Stanford University, Stanford, CA 94305, USA b Santa Fe Institute, 1399 Hyde Park Road, Santa Fe, NM 87501, USA
Address correspondence to S.E. Ptak. E-mail: susan{at}charles.stanford.edu.
The polygyny threshold model states that if costs incurred are less than the benefits gained from mating polygynously in terms of breeding-situation quality, then polygyny is favored and could evolve. We constructed mathematical models and computer simulations to evaluate this hypothesis. In the basic model, there is a single locus with two alleles, which regulates whether the female is receptive to polygyny. There are two breeding situations of differing quality on which males randomly assort. Females then select a mate based on the associated breeding situation and whether the male already has mates. This basic model is extended mathematically to include a cost for the initial female of a male with multiple mates and again to include gene expression in males. The computer simulations extend the basic model to multiple loci and alleles and to multiple breeding situations. The results presented here suggest that the polygyny threshold model is valid in a population genetic context: if the fitness of females that actually mate polygynously is greater than the fitness of monogamous females on poorer breeding situations, polygyny evolves. However, this approach reveals interesting dynamics not apparent from the verbal model. If the trait is expressed in males and females, then polygyny can evolve even if females mating polygynously have a lower fitness than females mating monogamously. In the multiple breeding-situations model, the polygyny allele increases to some equilibrium value above which it experiences no selection. Surprisingly, as the cost of polygyny increases, the equilibrium frequency of the polygyny allele also increases. The difference between this evolutionary model and the ideal free distribution is discussed.
Key words: evolution, polyandry, polygamy, polgyny, polgyny threshold model.