Ecological and evolutionary consequences of individual heterogeneity

Amy B. Forsythe, Troy Day and William A. Nelson, Biology, Queen's University, Kingston, ON, Canada, Troy Day, Mathematics & Statistics, Queen's University, Kingston, ON, Canada

Background/Question/Methods
Individual vital rates, such as mortality and birth rates, determine how individuals survive, develop, and reproduce throughout their life. Accumulating empirical evidence reveals that among-individual variation in these vital rates exists in nearly all populations, even when the individual members are genetically identical. However, little is known about the ecological and evolutionary consequences of this nongenetic individual heterogeneity. Even less recognized is the idea that different types of nongenetic individual heterogeneity can modify population dynamics in distinct ways.
Here, we simulate random genotypes and ask how different forms of nongenetic individual heterogeneity influence mean fitness and rates of evolution by natural selection in age- and stage-structured populations. To this end, we distinguish nongenetic individual heterogeneity in which intrinsic vital rates are permanent throughout an individual’s life (fixed condition) from those that can change at any time (dynamic condition).
Results/Conclusions
Across a wide range of assumptions, we find that mean fitness is greater when nongenetic individual heterogeneity is structured as fixed condition compared to dynamic condition. Conversely, rates of evolution by natural selection are slower with fixed condition compared to dynamic condition. Increasing the ratio of nongenetic relative to genetic variation strengthens the differences between populations with nongenetically determined fixed versus dynamic condition, both in mean fitness and in rates of evolution by natural selection.
This work contributes to resolving the paradox of stasis, which occurs when there is heritable genetic variation in a trait and apparent phenotypic selection on that trait, yet the trait does not change over time. In particular, our results suggest that populations with fixed condition are more likely to experience evolutionary stasis than populations with dynamic condition. More generally, our findings emphasize the importance of accounting for different structures of individual heterogeneity when investigating the ecological and evolutionary dynamics of populations.