Abstracts (first author)
Demographic heterogeneity, selection, and population response
Demographic heterogeneity (unmodeled variation in traits underlying vital rates) has attracted much attention in recent years. Empirical studies show substantial heterogeneity in many populations. Theory shows that it can have strong impact on demographic variance (and by inference, extinction risk); some kinds of demographic heterogeneity can also have large effects on the mean population growth rate.
Our studies of heterogeneity point to a strong connection with natural selection, and provide new insights on how changes in phenotype distributions can occur in structured populations. The key is an understanding of the within-population phenotypic correlation structure. Correlations can occur within individuals over time, between individuals at a given time, and between individuals at different times.
In matrix models allowing persistent heterogeneity, survival heterogeneity increases the mean population growth rate, and the population becomes dominated by "good survivors" even if the parent-offspring correlation is zero or negative. By contrast, the effect of fertility heterogeneity depends on the sign of the parent-offspring correlation. This is because information transmission across time depends (for reproduction) entirely on the parent-offspring correlation, but for survival information is also transmitted by changes in the population's phenotype structure.
With overlapping generations, then, selection on survival and on reproduction can cause different responses. The demographic response (change in stable phenotype distribution) occurs because of the non-genetic correlation structure of the population. For survival, this response occurs regardless of the parent-offspring correlation, but for reproduction, the response depends strongly on the parent-offspring correlation. This suggests that selection on survival may often lead to larger responses and faster changes than is the case for selection on reproduction.