Ease of fixation of a change in coiling: computer experiments on chirality in snails

Abstract

This paper deals with chirality of snails. It explores the ease of fixation of a change in coiling direction, caused by an invasion of snails carrying a mutant chirality allele into a normal, homogeneous population, by using Monte-Carlo simulation. Additionally, single-gene speciation on the basis of chirality is discussed. Six factors are studied in particular, namely: (i) the absolute and relative population size; (ii) the mating success, which is known to be related to shell shape, especially the height/width ratio; (iii) the maternal effect, underlying chirality; (iv) the (low) mobility of the snails; (v) fitness differences (heterosis); and (vi) the invading mutant allele being either dominant or recessive. The impact of these factors is quantified. Small populations with not too few invaders and dominance of the mutant chirality allele are of paramount relevance for its occasional fixation. In comparison to this, the maternal effect turns out to be considerably less significant, whereas the mobility of the snails plays only a minor role. The simulations confirm the expectation that heterosis can be very effective. Logically, the mating success is also a very important factor. This reflects the observation in nature that among snails with slender shells, when mating between mirror-image individuals is still possible to a certain extent, reverse populations have originated far more frequently than among snails with globular shells, where such matings are impossible. Only the latter cases, which are very rare in nature, may concern single-gene speciation.