The combined effects of anthropogenic and natural processes are rapidly modifying patterns of environmental variation in ways that alter the traits of individuals, populations, and species. Recent interest has turned the effect that this induced variation might have on mating systems, which can govern many ecological and evolutionary outcomes through their pervasive effects on the magnitude and intensity of conflicts among family members, including mates, parents and offspring, and siblings. These conflicts arise from the relatedness asymmetries that are a fundamental consequence of sexual reproduction, and their magnitude critically depends on the structure of the mating system. All else being equal, multiple mating by females increases the likelihood that brood mates will be half- rather than full-sibs, so that any benefit at the expense of a sibling entails diminishing inclusive fitness costs as the degree of polyandry rises. In this way, polyandry enhances the scope for both parent-offspring and sibling conflict. For many marine and terrestrial organisms, there are ample opportunities for mating systems to vary extensively and for offspring to influence parental investment after fertilization. Thus, the conditions required for relatedness asymmetries to influence life-history evolution are likely to be widely met across many species and in many environments.
Results/Conclusions
Marine invertebrates exhibit an unrivalled range of variation life history traits, including egg and offspring size, with major fitness consequences for both parents and offspring. By combining field observations, laboratory manipulations and genetic analysis, we established that there is extensive variation in both the degree of multiple mating and the degree of maternal resource allocation within and between capsules, clutches, individuals and populations of the marine whelk, Nucella ostrina. The relatedness of siblings within natural egg capsules significantly predicted the mean of offspring size. Experimental manipulations of mating system revealed a positive correlation between levels of polyandry and offspring size variation within a capsule, a pattern not explained by the genotype of the father. In addition, crosses between populations with divergent mating systems exhibited asymmetric reproductive incompatibilities consistent with models of conflict-driven speciation. These results demonstrate how mating system variation alone can induce significant variation in life history traits, and underscore the need for a more complete understanding of mating system evolution in nature.