Evolution under competition increases phytoplankton production by reducing the density-dependence of net energy fluxes and growth

Competition can drive rapid evolution but forecasting how species evolve in communities remains difficult. Life history theory predicts that evolution in crowded environments should maximise population production, with intra- and inter-specific competition producing similar outcomes if species compete for similar resources. Despite its appeal, this prediction has rarely been tested in communities. To test its generality and identify its physiological basis, we experimentally evolved four species of marine phytoplankton (spanning three orders of magnitude in cell size) alone or together in a community for 4.5 months. We then quantified changes in their metabolism, demography, and competitive ability at two timepoints ([~]60 and 120 generations) in common garden experiments. One species was outcompeted during the evolution experiment. For the other three, we found the same evolutionary outcome: species evolved greater biovolume production regardless of competition treatment but did so either by increasing max. population size or individual cell size. Biovolume production increased because of the differential evolution of photosynthesis and respiration under intense competition. These metabolic changes meant that intraspecific competition decreased and cells maintained higher rates of net energy production and growth as populations neared the stationary phase. Overall, these results show that intra- and inter-specific competition influence physiological and population parameters similarly in species that compete for essential resources. Life history theory thus provides a valuable base for predicting how species evolve in communities, and our results show how these predictions connect with the evolution of metabolism and competitive ability.