Published online by Cambridge University Press: 05 June 2012
Introduction
Nearly all speciation events in animals seem to involve spatial separation of populations, at least in the initial stages of divergence, and we have few compelling examples of sympatric speciation (Mayr 1947; Coyne & Orr 2004). Given the requirement of geographical isolation, following Mayr (1947), we consider three steps that limit the rate at which new species form. First, gene flow between populations must be restricted. Second, populations diverge in various traits that generate reproductive isolation. Third, populations must expand ranges. This third stage is essential, for without range expansions, newly produced species would remain geographical replacements of one another, severely limiting the total number of species that can be produced from a common ancestor.
Stage 2, that is the generation of reproductive isolation, is usually taken to be synonymous with speciation (Coyne & Orr 2004). Ecological conditions experienced by different taxa can drive and accelerate the rate of attainment of reproductive isolation (‘ecological speciation’, Schluter 2001; Rundle & Nosil 2005), thereby accelerating speciation. This has been the focus of much recent study and is one of the main topics considered in this volume. However, ecological influences on any of the three stages – not just the acquisition of reproductive isolation – can limit the rate at which new species form (Mayr 1947).
In this chapter, we first summarize various ecological and nonecological factors that may limit each of the three stages of speciation.
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