Figure 1.
A schematic representation of the epistasis model. There are two patches, 1 and 2, that experience uniform selection for the same optimum. Symmetrical migration between patches occurs at rate m. Each patch is initially fixed for the ancestral alleles a and b. During the course of the simulations, two new mutations arise, denoted as A and B. Both new mutations confer a selective advantage over the ancestral genotype. A is the most favoured allele, and B is more favoured than b, but less favoured than A (table 1). ‘Hybrid’ individuals carrying both A and B exhibit reduced fitness because these two alleles are incompatible. Mutations may arise simultaneously or in staggered fashion such that mutation B arises first in patch 1, reaches a frequency qthresh in at least one of the patches, following which mutation A arises in patch 2. Scenarios of initial allopatric divergence were also examined by setting m initially to zero for tcontact generations.