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to discriminate ecological from mutation-order speciation solely from examination of features of the underlying genes.

Evidence from laboratory experiments and field studies suggests that reproductive isolation accumulates more rapidly between populations adapting to different environments (ecological speciation) than between populations adapting to similar environments (potentially, mutation-order speciation). This difference might have a genetic explanation. There is a higher probability that the same or equivalent mutations will fix under parallel selection than under divergent selection, which will slow the rate of divergence between populations experiencing similar selection pressures. Alternatively, divergent selection may simply act on more genes.

Other genetic processes during ecological speciation become apparent when there is gene flow between populations, either continuously during their divergence or after secondary contact. With significant gene flow, premating isolation is unlikely to arise or persist unless the genes underlying it somehow overcome the antagonism between selection and recombination, such as by pleiotropy or by reduced recombination with genes under divergent natural selection. No such genetic features are predicted for premating isolation under either ecological or mutation-order speciation in the absence of gene flow.

Finally, for similar reasons the role of standing genetic variation in speciation is likely to be greater under ecological speciation than under mutation-order speciation. Selection from standing variation will increase the chances that separate populations experiencing similar selection pressures will fix the same rather than different alleles, inhibiting divergence. The repetitive origin of species under the transporter process proposed herein seems possible only under ecological speciation. The reason is that while a derived population with a distinct set of strongly interacting alleles built under a mutation-order process and conferring significant reproductive isolation can persist after secondary contact in the face of some gene flow with an ancestral population (Barton, 2001), the export of individual alleles will be resisted by these same interactions. Individual alleles that succeed in spreading to the ancestral population may be favored there, in which case differentiation at the locus will be eliminated.

ACKNOWLEDGMENTS

We thank J. Avise and F. Ayala for their kind invitation; J. Avise, R. Barrett, P. Nosil, S. Via, and another reviewer for helpful comments on the manuscript; and P. Colosimo (Stanford University) and D. Kingsley (Stanford University) for the Eda sequences. Research in D.S.’s laboratory is supported by the Natural Sciences and Engineering Research Council (Canada) and the Canada Foundation for Innovation.



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