Experimental evolution of protozoan traits in response to interspecific competition.

Decades of experiments have demonstrated the ecological effect of competition, but experimental evidence for competitive effects on trait evolution is rare. I measured the evolution of six protozoan traits in response to competitors from the inquiline community of pitcher plants. Replicate populations of Colpoda, a ciliated protozoan, were allowed to evolve in response to intra- and interspecific competition for 20 days (approximately 100 generations), before traits were measured in two common garden environments. Populations that evolved with interspecific competition had smaller cell sizes, produced fewer cysts and had higher population growth rates relative to populations grown in monoculture. The presence of interspecific competitors led to differential lineage sorting, most likely by increasing the strength of selection. These results are the first to demonstrate protozoan evolution in response to competition and may have implications for species coexistence in this system.