dominance relationships at individual loci. This extra component of postzygotic isolation is intrinsic in that it is manifested in both environments, and it represents a Dobzhansky-Muller incompatibility (Coyne and Orr, 2004) because it results from an interaction within hybrids of genes that are favored in the genetic background of the parent populations. Barton (2001) pointed out that the model might not be able to account for cases of very strong hybrid incompatibility, such as the lethality of gene combinations in hybrids between mine and nonmine populations of M. guttatus.

Surprisingly, Barton (2001) also found that intrinsic postzygotic isolation evolves just as readily between populations experiencing parallel selection as between populations under divergent selection. This finding conflicts with empirical studies that typically find faster evolution of reproductive isolation, including intrinsic postzygotic isolation (Dettman et al., 2007), between populations adapting to different environments than between populations adapting to similar environments (Rice and Hostert, 1993; Schluter, 2000, 2001). Broad comparative studies have found that intrinsic postzygotic isolation is highest between species that are ecologically differentiated (Bolnick et al., 2006; Funk et al., 2006). This discrepancy may be accounted for by features of nature not incorporated in the model. For example, Barton’s model assumes that adaptation occurred entirely from new mutations of which an infinite variety is available. Yet, the number of advantageous mutations for a given trait may be restricted, with the result that the same mutations occur and fix repeatedly under parallel selection (Palmer and Feldman, 2009; Unckless and Orr, 2009), preventing divergence. The same is expected if populations adapt from the same standing genetic variation (Colosimo et al., 2005; Barrett and Schluter, 2008) and if there is gene flow between separate populations evolving under parallel selection (Morjan and Rieseberg, 2004). Finally, divergent selection may be more effective than parallel selection simply because it acts on more traits and on more genes (Nosil et al., 2009). These factors help to explain why ecological speciation may generally be faster than mutation-order speciation even though selection is driving alleles to fixation under both mechanisms.

Speciation occurs from standing genetic variation when reproductive isolation between 2 or more populations evolves from alleles already present within the common ancestral population, rather than from new mutations. Theory to describe the buildup of Dobzhansky-Muller incompatibilities often explicitly assumes that speciation occurs from new mutations [e.g., Barton (2001) and Gavrilets (2003)] rather than from standing