The effects of competition and predation on diversification in a model adaptive radiation

Much of life's diversity is thought to have arisen through successive rounds of adaptive radiation-the rapid diversification of a lineage into a range of ecologically and phenotypically distinct species. Both resource competition and predation have been suggested as mechanisms driving this process, although the former is better studied than the latter. Here we show experimentally how predation by a protist, Tetrahymena thermophila, affects diversification in a model adaptive radiation of the bacterial prey, Pseudomonas fluorescens. We estimate the frequency-dependent fitness functions of competing niche-specialist prey in the presence and absence of predation, and use these to test hypotheses about the extent (measured as the number of new genotypes) and rate of diversification. Competition and predation independently generated diversifying selection that we show is capable of driving prey diversification to similar extents but at different rates, diversification being markedly delayed in the presence of predators. The cause of this delay stems from weaker diversifying selection due to the reduction in prey density caused by predation. Our results suggest that predation may play an under-appreciated role in driving adaptive radiations.