Neutral theory in ecology, with its symmetry assumption that species have, to a first approximation, identical per capita vital rates, allows us to ask a fundamental question: ''How many of the patterns we observe in ecological communities result from species similarities, rather than from species differences?" The traditional approach in ecology is to focus on species differences, and ask how these differences contribute to the assembly of ecological communities and to the coexistence of their member species. Neutral theory takes the opposite tack, and asks: ''What would communities be like if all species followed the mean?'' Modern neutral theory in ecology, often called the unified neutral theory (UNT), is a stochastic theory and its parameters are the familiar ones in population biology: probabilities of birth and death, dispersal, emigration and immigration, and speciation. Neutral theory makes predictions about patterns of biodiversity on local to global spatial and temporal scales, including relative species abundance, species-area relationships, phylogeny, and phylogeography. Symmetric neutral theory is an idealized theory about what to expect in the absence of demographic differences among species. It plays a theoretical role in ecology analogous to Boyle's ideal gas law in physics - although there are no perfect gases, Boyle's law works well as an approximation. Having a quantitative neutral theory is valuable because it provides a formal sampling theory and a set of testable null expectations against which to compare the properties of actual communities.
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