Demographic heterogeneity -- unmodeled variation in the propensities of individuals in a population to survive, reproduce, and grow -- has attracted much attention in recent years. Empirical studies have documented substantial heterogeneity in a number of populations. Theoretical studies have shown that it can have substantial impact on demographic variance (and by inference, on extinction risk); some kinds of demographic heterogeneity can also have large effects on the mean population growth rate. Nevertheless, it is difficult to understand all the results because it is not obvious how results from different models are related to one another. This is compounded by the fact that the terminology authors (including ourselves) have used is subtle and inconsistent.
Our goal is to begin the process of uniting these studies; to that end we present a conceptual framework to organize research -- theoretical and empirical -- on demographic heterogeneity. The framework is centered on characterizing the phenotypic correlation structure of a population: correlations can occur within individuals over time, between individuals at a given time, and/or between individuals at different times (including different generations). The special case where all of these are zero is what we called "unstructured heterogeneity" in our earlier work.
Results/Conclusions
This framework leads to some important insights; in particular, natural selection is a particular case of demographic correlations between individuals at a given time, and the heritability requirement for an evolutionary response means that additionally there are correlations between individuals in different generations. Beyond these insights, this framework allows us to make sense of theoretical results that may otherwise seem inconsistent with one other. Finally, our framework helps point to important aspects of demographic heterogeneity that are as yet poorly studied.