Evolutionary implications of Hubbell's neutral theory of community assembly

Evolution occurs in an ecological context. Repeated and rapid radiations of organisms, such as for some groups of scleractinian corals onto reefs, are often accompanied by an extensive and parallel diversification of aut- and community ecologies. This pattern is a major feature of evolution. Yet, there exist few ties between phylogenetics and ecology. For example, the most widely-used null models of diversification (e.g., S Nee et al 1994 Phil Trans Roy Soc B 344: 77) do not incorporate fundamental parameters of population-level analyses. However, if evolutionary diversification is time-integrated ecology, then interactions in ecological communities are expected to affect phylogenetic tree topology and branch-length distributions. Similarly, given sufficient evolutionary conservation of ecologically important traits, there should be phylogenetic structure to community membership (CO Webb 2000 Am Nat 156: 145). The few studies to date on this latter issue have generally found that community members are indeed nonrandomly dispersed across a phylogeny, indicating an as yet little explored phylogenetic component to community ecology (key review in Webb et al 2002 Ann Rev Ecol Syst 33: 475). Our simulations of random diversification incorporating per capita speciation and extinction rates (sensu SP Hubbell 1997 Coral Reefs 16: S9) indicate that phylogenetic distance between common species is a function of population size. In the simulations, common species appear phylogenetically overdispersed because they more often give rise to new taxa than do rare species. Population size, an important ecological parameter, directly affects evolutionary distance between community members. This is interesting because it offers an alternative to the traditional explanation of phylogenetic and taxonomic overdispersion of communities as resulting from competitive exclusion by evolutionarily closely allied species (e.g., Graves and Gotelli 1993 PNAS 90: 1388). We are currently testing this result using phylogenies of marine invertebrates and flowering plants. Below shows a sample output of a small, simulated community of evolving species with equivalent ecological abilities. The simulation is seeded with a single species that fills all community space (25 individuals). Over the 500 generations (iterations) of the simulation shown, each individual has a small probability P of speciating or dying. A) Species abundance Ni. Per capita mortality m and speciation v occurs with P(m) = P(v) = 1/Sum(Ni). B) Phylogenetic tree of the community. The tips of the tree give species abundances at the 500th generation.