Evol Ecol Res 20: 317-329 (2019)     Full PDF if your library subscribes.


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

Christopher Degagne and Jonathon Stone

Department of Biology, McMaster University, Hamilton, Ontario, Canada

Correspondence:J. Stone, Department of Biology, McMaster University, Life Sciences Building, Room 215, 1280 Main Street West, Hamilton, Ontario L8S 4K1, Canada.
email: jstoner@mcmas ter.ca


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.

Keywords: chirality, computer simulation, evolution, speciation.

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