Environmental drivers of phenotypic selection and intraspecific trait variability in damselflies
Variation in morphological and life history traits among populations is widely documented. Although natural selection is often evoked to explain these differences, the environmental drivers of selection are less well understood. Yet, understanding how and why the environment shapes selection among populations is essential to predict how species will respond to future environmental changes. Damselflies in the genus Enallagma (Odonata: Coenagrionidae) exhibit a wide variety of morphological and life history traits throughout their range, making them good organisms to study the environmental drivers of selection on phenotypic traits. We sampled Enallagma from 24 lakes throughout California, ranging from desert to alpine ecosystems. We quantified a number of environmental variables for each lake and estimated growth and mortality rates and conducted morphometric analyses for each damselfly population. We used linear models to determine the relationship between local and regional environmental variables, morphological trait distributions, and estimates of phenotypic selection.
We identified, photographed, and measured morphological traits for over 10,000 damselflies. We found significant intraspecific trait variability among lakes, and evidence for phenotypic selection through both growth and mortality. Interestingly, the strength and direction of selection through growth and mortality was variable among populations, but differences in selection were only weakly correlated with differences in trait distributions among populations. Our analyses also reveal that local biotic conditions such as population density and the presence or absence of fish predators are more important predictors of selection on damselflies than regional environmental factors such as latitude and elevation. Our study suggests that biological interactions may be key drivers of selection in this genus.