Ecological and Evolutionary Stochasticity Shape Natural Selection

The distribution of biodiversity depends on the combined and interactive effects of ecological and evolutionary processes. The joint contribution of these processes has focused almost exclusively on deterministic effects, even though mechanisms that increase the importance of random ecological processes are expected to also increase the importance of random evolutionary processes. Here we manipulate the sizes of old field fragments to generate correlated sampling effects for a focal population (a gall maker) and its enemy community. Traits and communities were more variable in smaller patches. However, because of the preference of some enemies for some trait values (gall sizes), random variation in population mean trait values exacerbated differences in community composition. The random distribution of traits and interactions created predictable but highly variable patterns of natural selection. Our study highlights how stochastic processes can affect ecological and evolutionary processes structuring the strength and direction of selection locally and at larger scales.