THE EVOLUTION OF ALTERNATIVE MATING STRATEGIES IN VARIABLE ENVIRONMENTS

We assessed the influence of phenotypic plasticity in age at maturity on the maintenance of alternative mating strategies in male Atlantic s almon, Salmo salar. We calculated the fitness, r, associated with the parr and the anadromous strategies, using age-specific survival data f rom the field and strategy-specific fertilization data from the labora tory. The fitness of each strategy depended largely on mate competitio n (numbers of parr per female, i.e. parr frequency) and on age at matu rity. Fitness declined with increasing numbers of parr per female with equilibrium frequencies (at which the fitnesses of each strategy are equal) being within the range observed in the wild. Equilibrium parr f requencies declined with decreasing growth rate and increasing age at maturity. Within populations, the existence of multiple age-specific s ets of fitness functions suggests that the fitnesses of alternative st rategies are best represented as multidimensional surfaces. The points of intersection of these surfaces, whose boundaries encompass natural variation in age at maturity and mate competition, define an evolutio narily stable continuum (ESC) of strategy frequencies along which the fitnesses associated with each strategy are equal. We propose a simple model that incorporates polygenic thresholds of a largely environment ally-controlled trait (age at maturity) to provide a mechanism by whic h an ESC can be maintained within a population. An indirect test provi des support for the prediction that growth-rate thresholds for parr ma turation exist and are maintained by stabilizing selection. Evolutiona rily stable continua, maintained by negative frequency-dependent selec tion on threshold traits, provide a theoretical basis for understandin g how alternative life histories can evolve in variable environments.