Evolutionary demography of structured two-sex populations and sex ratios

Abstract

Males and females may differ in stage-specific survival, maturation, fertility, or mating availability. These demographic differences, in turn, affect population growth rates, equilibrium structure, and evolutionary trajectories. Models considering only a single sex cannot capture these effects, motivating the use of demographic two-sex models for sexually reproducing populations. I developed a new two-sex modeling framework that incorporates population structure and multiple life cycle processes through transition rate matrices. These models can be applied to a variety of life histories to address both ecological and evolutionary questions. Here, I apply the model to the effects of sex-biased harvest on populations with various mating systems. Demographic considerations also affect evolutionary projections. I derived matrix calculus expressions for key evolutionary quantities in my two-sex models, including the invasion fitness, selection gradient, and second derivatives of growth rates (which have many applications, including the classification of evolutionary singular strategies). I used these quantities to analyze the evolution of the primary sex ratio, under various sex- and stage-specific offspring costs and maternal conditions. Demographic two-sex models lend insight into complex, and sometimes counterintuitive, results that are not captured by models lacking population structure. These findings highlight the importance of demographic structure in ecology and evolution.