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tive mating map to the same candidate gene, wingless (Kronforst et al., 2006). Finally, the YUP locus and other QTL affecting flower color and shape differences between the monkeyflowers Mimulus cardinalus and Mimulus lewisii have pleiotropic effects on assortative mating through their attractiveness to alternative pollinators (Schemske and Bradshaw, 1999; Bradshaw and Schemske, 2003).

DIVERGENT SELECTION AND POSTZYGOTIC ISOLATION

The genetics of postzygotic isolation in ecological speciation has received little attention. Ecological speciation uniquely predicts the evolution of ecologically based postzygotic isolation (also called “extrinsic,” or “environment-dependent” reproductive isolation). As populations in different environments evolve toward different adaptive peaks, intermediate forms, including hybrids, increasingly fall between the peaks and suffer reduced fitness in both parental environments (Hatfield and Schluter, 1999; Rundle and Whitlock, 2001; Schluter, 2001). Such ecologically dependent hybrid fitness has been detected in a few study systems, including pea aphids (Via et al., 2000) and threespine stickleback (Rundle, 2002). The fitness of hybrids between divergently adapted populations may also be affected by heterosis (hybrid vigor), especially in the F1 hybrids (Barton, 2001), and this has not been controlled in most field estimates of ecologically based postzygotic isolation. However, if heterosis is strong the impact of ecologically based postzygotic isolation is diminished (Lowry et al., 2008).

Most of the populations and species that have been studied from an ecological perspective are young and appear to manifest little “intrinsic” (environment-independent) postzygotic isolation. Exceptions include mine populations of Mimulus guttatus, which possess an unidentified copper tolerance gene or gene complex that is favored by ecological selection but that interacts with 1 or more other loci to cause the death of F1 hybrids in crosses to nonmine populations (MacNair and Christie, 1983). Another exception involves sympatric dwarf and normal forms of lake whitefish (Coregonus clupeaformis), which show elevated mortality of embryos (Rogers and Bernatchez, 2006). In a laboratory experiment, Dettman et al. (2007) recorded intrinsic postzygotic isolation affecting growth rate and frequency of meiosis in hybrids between yeast populations that had evolved for 500 generations in 2 distinct environments. Without more evidence on the mechanism of selection we are unable to determine which, if any, of the genes recently discovered to underlie intrinsic postzygotic isolation in Drosophila, yeast, and mice (Coyne and Orr, 2004; Brideau et al., 2006; Lee et al., 2008; Mihola et al., 2009; Tang and Presgraves, 2009) fixed as a result of ecologically based divergent natural selection.



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