The genetics of speciation: are complex incompatibilities easier to evolve?

Reproductive isolation can evolve readily when genotypes containing incompatible alleles are connected by chains of fit intermediates. Experimental crosses show that such Dobzhansky-Muller incompatibilities (DMIs) are often complex (involving alleles at three or more loci) and asymmetrical (such that reciprocal introgressions have very different effects on fitness). One possible explanation is that asymmetrical and complex DMIs are 'easier to evolve', because they block fewer of the possible evolutionary paths between the parental genotypes. To assess this argument, we model evolutionary divergence in allopatry and calculate the delays to divergence caused by DMIs of different kinds. We find that the number of paths is sometimes, though not always, a reliable predictor of the time to divergence. In particular, we find limited support for the idea that symmetrical DMIs take longer to evolve, but this applies largely to two-locus symmetrical DMIs (which leave no path of fit intermediates). Symmetrical complex DMIs can also delay divergence, but only in a limited region of parameter space. In most other cases, the presence and form of DMIs have little influence on times to divergence, and so we argue that ease of evolution is unlikely to be important in explaining the experimental data.