The recent recognition that evolution occurs on ecological time-scales is transforming biology. Yet recent advances in our understanding of the mechanisms that maintain species diversity assume that species are fixed, and so rarely consider the influence of rapid, dynamic changes in species-level traits on the outcome of species interactions. Therefore, to understand how rapid evolution influences species coexistence we conducted field experiments using a powerful new empirical system based on the world’s smallest flowering plants – duckweeds. Using this system, we manipulated the ability of two competing species (Lemna minor and Spirodela polyrhiza) to coevolve across approximately 15 generations in the field. We did this by regularly replacing competing populations in one treatment using individuals from single-species stock cultures. At multiple time points during the experiment we quantified the extent of evolutionary change in our treatments using molecular genetics. We then quantified the influence of rapid evolution on the outcome of the competitive interaction by comparing the trajectories of the competing populations in the presence vs. absence of coevolution. Finally, to identify the mechanistic basis of the effects on dynamics, we parameterized models of competitive population dynamics to quantify changes to niche and fitness differences caused by coevolution.
Results/Conclusions
Our results demonstrate that coevolution alters the strength of coexistence on ecological timescales. There were significant differences in the trajectories of the competing species – and a reversal in relative abundance – when the species could coevolve. Most of these effects occurred because of evolutionary change in one species, L. minor. Without coevolution, this species came to be dominated by two genotypes, whereas in the presence of coevolution this species had high genotypic evenness. This result alone suggests a role for coevolution in the maintenance of genetic diversity. However, the effects of coevolution on L. minor caused its population size to decline in relative abundance. Indeed, while coevolution caused L. minor to double its low-density growth rate, concomitant increases in L. minor’s sensitivity to intra- and interspecific competition fully negated any benefits of evolution for its interspecific competitive ability. Ultimately, coevolution created a net advantage for the previously inferior competitor because of changes in average fitness differences. Our results provide rare experimental support for the role of rapid evolution in driving the outcome of competition in the field and suggest that understanding the maintenance of species diversity will require an explicit accounting for the effects of rapid evolution.