Abstract:  In this paper, I examine the dynamics of species richness in a model system in which multiple species compete in a metacommunity (multiple patches linked by dispersal). Patches lie along an environmental gradient, and new species are introduced into the system by speciation of existing species. This model is used to explore how the ecological similarity of species influences the patterns in community structure that result and to determine whether patterns in fossil and systematics data may be signatures for different types of community structure. Making species more similar overall along the entire gradient or making new species that have more similar optimal positions along the gradient to their progenitor both increase the time required to drive species extinction. As a result, making species more similar ecologically to one another increases overall species richness because of an increased frequency of transient species in the system. Having more transient species in systems shifted the longevity distributions of species in the fossil record towards having a greater frequency of shorter duration species, and the age distribution of extant species that would be estimated from molecular phylogenies also had a higher frequency of younger aged species. Comparisons of these results with species longevity distributions extracted from two data sets and with species ages derived from species-level molecular phylogenies strongly suggest that transient species are an important component of real biological communities.