Evolution has been long-studied to identify the causes of trait variation within and among species, but its role in explaining community-level patterns of species diversity and coexistence has only recently been explored. Because more closely related species often share ecologically-important traits, ecologists initially predicted that competitive exclusion would occur more frequently between recently diverged species. However, ecologists now recognize that the phylogeny-coexistence relationship is not as simple as once thought, in part because the confounding effects of multiple processes cannot be teased apart using field-based observational data. In this poster, we present the theoretical background of ongoing empirical work, which tailors existing expectations of phylogeny-coexistence relationships to include more complex processes that likely contribute to phylogenetic patterns in nature. We combine mathematical theory with examples from our work in annual plant communities to propose new hypotheses for testing the role of evolution on ecological interactions, and how these interactions are modified by species’ historical interactions [evolution in sympatry vs. allopatry]. Our goal is to provide a theoretical framework for future tests of the relationship between evolutionary history and coexistence by evaluating how species interactions contribute to the phylogenetic patterns observed in natural communities.
The ability of phylogenetic relationships to dictate competitive outcomes ultimately depends on the level of conservatism in the ecologically-important traits. Within this constraint, our work shows that trait divergence can in fact lead to both species coexistence and exclusion. Previous conceptual models have stated that niche differences, or the degree to which species limit their own population growth rates relative to that of other species, promote coexistence by reducing the amount of resource-use overlap between competitors. By contrast, differences in competitive ability lead to the exclusion of inferior competitors by dominant species unless they are countered by sufficiently large niche differences. Using similar arguments, our work demonstrates that extant competitors originating from the same biogeographic region (i.e., evolve in sympatry) always have a faster rate of niche divergence compared to the divergence of competitive ability. This niche divergence may manifest by species co-occurring or not, depending on whether specialization occurs for limiting factors within or between habitat-types. When competitors originate from different biogeographic regions, rates of divergence are unpredictable based solely on phylogenetic relationships. This unpredictability is consistent with observed patterns of species invasions, where some species fail to invade altogether while others have spectacularly negative impacts on native diversity.