Figure 4.

Two scenarios for establishing the conditions that favor the spread of an underdominant inversion in parapatry. The population occupies two habitats, indicated by the stippled and open regions. Alleles A and B are deleterious in combination in both habitats. Allele A is favored in the habitat to the left and allele B in the habitat to the right. The most common genotype is shown in the top corners. Dashed curves show the frequency of Ab and solid curves the frequency of aB. Asterisks show the appearance of a new mutation that spreads by selection in one of the habitats. (Left side) One advantageous mutation appears in each of the two populations. (Right side) Both advantageous mutations appear in the right-hand population. (Both sides, bottom) An inversion appearing in either habitat that captures the most common haplotype will spread by the local adaptation mechanism.