Theoretically exploring the likelihood for sympatric speciation: Interactions between snail shell-coil direction and anti-predation morphology

Background: A single locus in snail genomes encodes alternative shell-coil-direction phenotypes (left or right), termed 'chirality'. Opposite-coil morphs that attempt mating encounter difficulties aligning their genitals and thus experience partial to complete reproductive failure. Individuals with the rarer coil morph in a population thereby incur fitness disadvantages. Gittenberger (1988) suggests, and Ueshima and Asami (2003) document, that snail shell chirality produces sympatric speciation based on a single locus. Predation may also play a role: snakes possess asymmetric jaws, which allow them to prey more effectively on snails with right-coil shells (Hoso et al., 2007). Hypothesis: In a population containing mostly individuals with right-coil shells, the rarer left-coil morph gains a fitness advantage because it more effectively resists predation from snakes that prey preferentially on right-coil morphs. Question: Can anti-predation advantages in snail shell morphology compensate for - or overwhelm - any reproductive disadvantages of a particular shell morphology so as to prevent or promote the fixation of shell-coil-direction morphs? Methods: Computer simulations of a computational model. Results: Low predation rates in small demes can fix the left-coil morph, which in turn increases the proportion of left-coil morphs in the general population. Intermediate predation rates in larger demes can maximize the chance of fixation of the left-coil morph. But high predation rates reduce the chance of their fixation. Conclusions: Differences in resistance to predation might explain the shell-coil dimorphisms observed naturally.