thought to have been involved in speciation. They reveal extensive prezygotic ecologically based isolation, with little or no isolation attributable to postzygotic genetic incompatibilities (Schluter, 2001; Via, 2001).

In the 2-stage model of speciation described here, allopatric speciation can occur without stage 1, but sympatric speciation cannot. There is essentially only 1 path for purely sympatric speciation: rapid divergence at genomic regions harboring QTL for traits under divergent selection, leading to significant ecologically based reduction of successful interbreeding between incipient species and ecological allopatry by the end of stage 1. Then, during stage 2, genetic incompatibilities can accumulate to reinforce the ecologically based isolation and make it permanent.

In contrast, allopatric speciation cannot be divided into distinct stages, because the accumulation of DMIs by independent responses to uniform or balancing selection can occur at the same time as the evolution of ecologically based isolation driven by divergent selection. In allopatry, any combination of divergent selection, uniform selection, and genetic drift could produce speciation (Fig. 1.6C). Because the rapid divergence under selection that characterizes ecological speciation with gene flow is not required when populations are geographically isolated (although it can happen), allopatric speciation will often take much longer than speciation with gene flow.

Divergence hitchhiking neutralizes the most long-standing criticism of sympatric speciation, the difficulty of maintaining linkage disequilibrium between genes involved in resource use and those that produce assortative mating [e.g., Coyne and Orr (2004, pp. 127–137); Felsenstein (1981); Hawthorne and Via (2001); Via (2001)]. Although it has been clear for some time that this problem is mitigated if the traits under divergent selection for resource use also affect mate choice (Schluter, 2001), or if there is pleiotropy or physical linkage between the 2 classes of genes (Felsenstein, 1981; Hawthorne and Via, 2001; Via, 2001), these observations have done little to quell the controversy.

By providing a simple mechanism by which combinations of genes that produce assortative mating can accumulate and be protected from recombination, divergence hitchhiking removes the major constraint on sympatric speciation that prevented its acceptance for so long. The controversy over sympatric speciation has occupied a tremendous number of researchers over the past 50 years. If additional studies in other taxa show that divergence hitchhiking is a general phenomenon, we may finally be able to put this issue behind us.