EASE OF FIXATION OF A CHANGE IN COILING - COMPUTER EXPERIMENTS ON CHIRALITY IN SNAILS

This paper deals with chirality of snails. It explores the ease of fix ation of a change in coiling direction, caused by an invasion of snail s carrying a mutant chirality allele into a normal, homogeneous popula tion, by using Monte-Carlo simulation. Additionally, single-gene speci ation 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, underlyi ng chirality; (iv) the (low) mobility of the snails; (v) fitness diffe rences (heterosis); and (vi) the invading mutant allele being either d ominant or recessive. The impact of these factors is quantified. Small populations with not too few invaders and dominance of the mutant chi rality allele are of paramount relevance for its occasional fixation. In comparison to this, the maternal effect turns out to be considerabl y less significant, whereas the mobility of the snails plays only a mi nor role. The simulations confirm the expectation that heterosis can b e very effective. Logically, the mating success is also a very importa nt factor. This reflects the observation in nature that among snails w ith slender shells, when mating between mirror-image individuals is st ill 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.