Diversity and the coevolution of competitors, or the ghost of competition past

That niches of competitors in ecological communities are shaped by mutual coevolution, which thus allows many species to coexist, is a commonly-held view. Two species must live together consistently to coevolve, so since predators (or parasites) are dependent upon their prey, they will necessarily co-occur with them and so should coevolve. In contrast, competing species, which are not dependent on each other, need not consistently co-occur or coevolve. Increased diversity, by reducing the consistency of co-occurrence, also reduces the chance of coevolution. To demonstrate coevolutionary divergence of competitors one must show: 1) that divergence has actually occurred: this has been done for some fossil sequences but not for any extant competitors; 2) that competition, rather than some other mechanism, is responsible; and 3) that it has a genetic basis. To demonstrate 2) and 3) for natural populations requires appropriate field experiments, which are suggested in the paper. This has been done, in part, in only one case. Thus the notion of coevolutionary shaping of competitors' niches has little support at present. Theory and evidence suggest that it is probable only in low diversity communities.