Reproductive isolation and the causes of speciation rate variation in nature

Rates of species formation vary widely across the tree of life and contribute to many of the most striking large-scale patterns in biological diversity. For the past few decades, most research on speciation has focused on the evolution of barriers to gene flow between populations. The present review discusses the relationship between these barriers, collectively known as reproductive isolation', and the rate at which speciation occurs. Although reproductive isolation plays a key role in the maintenance of biological diversity, there is little evidence to suggest that any forms of reproductive isolation serve as rate-limiting controls on speciation rates as measured over macroevolutionary timescales. Identifying rate-limiting steps of the speciation process is critical for understanding why we observe the numbers of species that we do and also for explaining why some groups of organisms have more species than others. More generally, if reproductive isolation is not the rate-limiting control on speciation rates, then factors other than reproductive isolation must be involved in speciation and our definition of speciation should be expanded to incorporate these additional processes.