Review
The Unified Neutral Theory of Biodiversity and Biogeography at Age Ten

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A decade has now passed since Hubbell published The Unified Neutral Theory of Biodiversity and Biogeography. Neutral theory highlights the importance of dispersal limitation, speciation and ecological drift in the natural world and provides quantitative null models for assessing the role of adaptation and natural selection. Significant advances have been made in providing methods for understanding neutral predictions and comparing them with empirical data. In this review, we describe the current state-of-the-art techniques and ideas in neutral theory and how these are of relevance to ecology. The future of neutral theory is promising, but its concepts must be applied more broadly beyond the current focus on species–abundance distributions.

Section snippets

The mystery of biodiversity

Imagine yourself deep in a tropical rain forest or floating over a coral reef surrounded by thousands of interacting species. How can so many species coexist? The traditional answer is that all species differ in important ways [1], so that each species is limited by a unique set of factors. Adaptive trade-offs prevent the evolution of super-species that are better at doing everything. Classic ecological niche theory captures the unique roles of species and, consequently, is often complex and

The history of neutral theory

Neutral theory makes a controversial ‘neutrality assumption’: all individuals within a particular trophic level have the same chances of reproduction and death regardless of their species identity [2]. Kimura pioneered the idea in a different context with his neutral theory of molecular evolution [3]. His intent was to model changes in allele frequency at a locus where all the alleles were selectively ‘neutral’, so that substituting one allele for another did not affect the fitness of an

Interpreting fits to empirical data

Most empirical evaluations of neutral models have focused on fitting the classic neutral model to species abundance distributions [2] (Box 1). Substantial improvements in the fitting methods have been developed since the early work (Box 2); in fact, one great benefit to the classic neutral model is that its simplicity allows a likelihood-based framework for fitting data 11, 12, 13, 14. Here, we do not attempt to enumerate the many fits that have been carried out [15], but instead focus on what

The other assumptions of the model

Classic neutral theory includes several ‘auxiliary assumptions’, unrelated to neutrality [30], which can cause the model to fail for reasons other than its neutrality and draw attention away from the key issues. It is therefore desirable to remove these auxiliary assumptions.

The future of neutral theory

The field has progressed considerably during the past 10 years, but what developments do the next 10 years hold in store? In this section, we discuss the possibilities.

Discussion

History has shown, not only in physics, but also in ecology, that one can make considerable progress with simple models. The ideal gas law, for example, is a good approximation, but there are no ‘ideal gases’ in reality. The success of the birth–death model of diversification [76], MacArthur and Wilson's theory of island biogeography [10] and Levins’ metapopulation model [83] are all testimony to the utility of simple models in ecology and evolution. Neutral theory follows this tradition.

Acknowledgements

We thank the editor Paul Craze and two anonymous reviewers for their helpful comments and suggestions. We also thank Bart Haegeman, David Alonso, Egbert Leigh, Jr., Evan Economo, Fangliang He, Jonathan Eastman, Luke Harmon, Matt Pennell, Stephen Cornell and members of the Harmon and Rosenblum lab groups for their useful comments and discussions on the paper. JR was funded by the EPSRC (grant EP/F043112/1); RSE acknowledges financial support from NWO.

Glossary

Beta-diversity
also known as distance-decay; the probability as a function of distance between two individuals that they will be of the same species.
Coalescence
a technique developed in population genetics for simulating or analytically solving properties of a sample of individuals by tracing their ancestry back to their common ancestors.
Dispersal kernel
a statistical distribution describing stochastic dispersal events by giving the probability of dispersal as a function of distance.
Dispersal

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