Parallel speciation with allopatry

Response from JohannessonSchluter et al.1xParallel speciation with allopatry. Schluter, D. et al. Trends Ecol. Evol. 2001; 16: 283–284Abstract | Full Text | Full Text PDF | Scopus (17)See all References1 refer to new and interesting information 2xHistorical contingency and ecological determinism interact to prime speciation in sticklebacks, Gasterosteus. Taylor, E.B. and McPhail, J.D. Proc. R. Soc. London B. Biol. Sci. 2000; 267: 2375–2384Crossref | Scopus (213)See all References2 in one of the cases that I referred to in my review of parallel speciation 3xParallel speciation: a key to sympatric divergence. Johannesson, K. Trends Ecol. Evol. 2001; 16: 148–153Abstract | Full Text | Full Text PDF | PubMed | Scopus (96)See all References3, namely the threespine sticklebacks Gasterosteus spp. They argue that the traits responsible for reproductive isolation probably evolved in allopatry rather than in sympatry. However, as pointed out by both of us, the molecular data show conflicting results of the population history. Furthermore, it is not easy to infer under what circumstances the ecological differentiation leading to reproductive isolation occurred.Even under the scenario supported by Schluter et al.1xParallel speciation with allopatry. Schluter, D. et al. Trends Ecol. Evol. 2001; 16: 283–284Abstract | Full Text | Full Text PDF | Scopus (17)See all References1, where benthic forms were the first invaders followed by limnetic forms at later periods, the evolution of ecological differentiation that resulted in reproductive isolation might not have been completed during the allopatric period. It seems probable that late invaders would also try to use the benthic niche as it is presumably more favorable. However, the second group of invaders might have been out-competed by populations already established in the benthic niche and therefore they began to evolve ecological traits more suitable for the limnetic habitat. If this scenario is correct, the reproductive isolation evolved at least partly during the sympatric stage. Schluter et al. have previously supported this view, stating: ‘premating isolation between ecomorphs arose initially as a simple by-product of divergent natural selection on key traits and was later reinforced in sympatry’ 4xNatural selection and parallel speciation in sympatric sticklebacks. Rundle, H.D. et al. Science. 2000; 287: 306–308Crossref | PubMed | Scopus (393)See all References4.However, deterministic rather than stochastic processes are the main reasons behind the evolution of reproductive barriers among these species of sticklebacks, and the work by Schluter et al. presents nice evidence for this.The point I raise in my review 3xParallel speciation: a key to sympatric divergence. Johannesson, K. Trends Ecol. Evol. 2001; 16: 148–153Abstract | Full Text | Full Text PDF | PubMed | Scopus (96)See all References3 is that parallel speciation could be used to support sympatric speciation in cases where there is unambiguous molecular data of monophyly of contrasting ecomorphs living in sympatry, but where an earlier stage of microallopatry could not be rejected. Here, parallel evolution of the same reproductive barriers in independent systems supports ecological rather than stochastic forces as being motors of speciation, and we do not need to invoke physical barriers to explain the reproductive isolation. In light of the new data on microsatellites 2xHistorical contingency and ecological determinism interact to prime speciation in sticklebacks, Gasterosteus. Taylor, E.B. and McPhail, J.D. Proc. R. Soc. London B. Biol. Sci. 2000; 267: 2375–2384Crossref | Scopus (213)See all References2, I agree that sticklebacks might not be the most clear-cut example of parallel speciation in sympatry. The support for reproductive isolation as a consequence of ecological differentiation is still very strong (by again referring to parallel evolution of similar barriers) and this shows indirectly that speciation can take place without a physical barrier, that is, in a non-allopatric situation.