Seasonal evolution and its effects on competition

Rapid evolution can influence competitive dynamics. While theory and past
work have focused on the competitive consequences of rapid evolution in
response to competitors (e.g., character displacement), abiotic
environmental factors are also responsible for much of the rapid evolution
observed in nature. Yet, we lack knowledge of how evolution in response to
a variable abiotic environment shapes competition between evolving
species. Our knowledge of these effects is limited not only by the absence
of direct quantification of environment-driven evolution’s effects on
competition, but also by the rarity of systems in which environment-driven
evolution itself has been measured across multiple competitors. Here, we
used an outdoor mesocosm experiment with three species of naturally
co-occurring Drosophilid fly competitors to quantify the seasonal
evolution of stress tolerance and reproductive traits and the effects of
this evolution on demographic performance under competition. We found that
rapid seasonal evolution occurred across all three species, across all
measured traits, and across every sampling interval. Throughout the course
of their growing season, species first converged on similar phenotypic
profiles (e.g., high heat tolerance and low fecundity), then evolved in
parallel. This seasonal evolution had clear impacts on pairwise
competition between species. In competition trials in a common garden
greenhouse environment, seasonal evolution had the potential to alter
demographic performance by over 30%. Under seasonally varying field
conditions over the summer, in one species pair, seasonal evolution had 7x
stronger effects on competitive performance than did the changing
environmental conditions themselves. Our work highlights that the rapid,
environment-driven evolution increasingly documented by evolutionary
biologists can have a major, underappreciated influence on ecological
competitive dynamics.