Speciation and Gene Flow between Snails of Opposite Chirality
Figure 7
Proportion of Sinistral Snails Amongst Offspring of Sinistrals Compared with Frequency of Sinistrals in the Population, with Reduced Mating Frequency or Fertility for the Rare Chiral Morph
We assumed a strong frequency-dependent mating disadvantage against the rare morph, no interchiral mating (α = 1) and equal sexual selection on the two sexes. The graph shows eight runs that start close to the border separating two alternative outcomes, which runs approximately vertically down the middle of x-axis of the graph (when the frequency of sinistrals is about 0.5). As there was no interchiral mating, then if the starting condition is two distinct populations (sinistral snails have only sinistral alleles and vice versa), then the two chiral morphs stay separate: the points along the top horizontal line represent this, indicating competition between two reproductively isolated types, or single-gene speciation (y = 1; as in Figure 6, this is also the second, unstable equilibrium that is referred to in the main text). In all other circumstances, the model shows that the population moves to equilibrium, so that there is extensive gene flow between different chiral morphs. For further information, see Results or section 3.i in Protocol S1.