The previous chapters testify to rapid population differentiation as a result of selection. Certain populations have also been geographically isolated for extremely long periods of time. Guppy populations are however only weakly reproductively isolated. This chapter asks why guppies have not speciated. In doing so, it reconsiders the ideas raised by Haskins and Liley (after Mayr) in the light of new data. It begins by estimating evolutionary rates using changes in male morphology, behaviour, and life histories. Next, it reviews the evolution of reproductive isolation and assesses the potential for this to arise at each point in the reproductive sequence: pre-mating isolation; post-mating, pre-zygotic isolation; and post-zygotic isolation. The emergence of isolating mechanisms is discussed against the backdrop of population differences in mating tactics, mating opportunities, and predation risk. New data reveal high levels of promiscuity in both males and females. The implications of this for reproductive isolation, and the significance of gene flow in impeding reproductive isolation are also assessed.
Keywords: speciation, pre-mating isolation, sexual coercion, gametic isolation, post-zygotic isolation, learned mate recognition, sympatric speciation