A New Way to Integrate Selection When Both Demography and Selection Gradients Vary over Time

Abstract

When both selection and demography vary over time, how can the long-run expected strength of selection on quantitative traits be measured? There are two basic steps in the proposed new analysis: one relates trait values to fitness components and the other relates fitness components to total fitness. We used one population projection matrix for each state of the environment together with a model of environmental dynamics, defining total fitness as the stochastic growth rate. We multiplied environment-specific, stage-specific mean-standardized selection gradients by environment-specific, stage-specific elasticities of the stochastic growth rate, summing over all relevant life history and environmental paths. Our two example traits were floral tube length in a rainforest herb and the timing of birth in Red Deer. For each species, we constructed two models of environmental dynamics, including one based on historical climate records. We found that total integrated selection, as well as the relative contributions of life-history pathways and environments, varied with environmental dynamics. Temporal patterning in the environment has selective consequences. Linking models of environmental change to relevant short term data on demography and selection may permit estimation of the force of selection over the long-term in variable environments.

The importance of the timescale of the fitness metric for estimates of selection on phenotypic traits during a period of demographic change

“…Especially in temporally variable environments, the ability to partition fitness into demographic components can be essential (Horvitz et al . ).…”

Experimental migration upward in elevation is associated with strong selection on life history traits

“…For example, selection on the timing of birth in red deer was found to act through both juvenile survival and adult reproduction, with both the direction of selection gradients and the relative importance of these vital rates fluctuating among years depending on winter climate conditions (Coulson et al, 2003;Horvitz et al, 2010). Changing environmental conditions can influence not only the relationships between traits and a particular vital rate, but also the relative contributions of vital rates to population growth.…”

“…This integrative approach often alters qualitative conclusions relative to individual selection gradients (Coulson et al, 2003;Ehrlén & Münzbergová, 2009;Gamelon et al, 2011;Horvitz et al, 2010). Trait elasticities integrate the effects of a trait on all vital rates, given the effects of each vital rate on λ, to estimate the net proportional effect of a trait on λ.…”

Extreme climate events and individual heterogeneity shape life‐history traits and population dynamics

“…Therefore, recruitment is a highly plastic trait that if heritable, could potentially have a critical role in the evolution of life histories. Further investigation of how environmental variability affects such patterns is required, as the effect on selection on a life-history trait may depends upon the current environmental state and the stochastic sequence of states in which it is embedded (Horvitz et al 2010).…”