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Abstract
BackgroundMany groups of land snails show great interspecific diversity in shell ornamentation, which may include spines on the shell and flanges on the aperture. Such structures have been explained as camouflage or defence, but the possibility that they might be under sexual selection has not previously been explored.Presentation of the hypothesisThe hypothesis that is presented consists of two parts. First, that shell ornamentation is the result of sexual selection. Second, that such sexual selection has caused the divergence in shell shape in different species.Testing the hypothesisThe first part of the hypothesis may be tested by searching for sexual dimorphism in shell ornamentation in gonochoristic snails, by searching for increased variance in shell ornamentation relative to other shell traits, and by mate choice experiments using individuals with experimentally enhanced ornamentation. The second part of the hypothesis may be tested by comparing sister groups and correlating shell diversity with degree of polygamy.Implications of the hypothesisIf the hypothesis were true, it would provide an explanation for the many cases of allopatric evolutionary radiation in snails, where shell diversity cannot be related to any niche differentiation or environmental differences.
Highlights
Many groups of land snails show great interspecific diversity in shell ornamentation, which may include spines on the shell and flanges on the aperture
That shell ornamentation is the result of sexual selection
That such sexual selection has caused the divergence in shell shape in different species
Summary
The process of sexual selection is caused by the fact that, in sexually reproducing organisms, not all individuals will be successful in securing mates. Shell diversification in Plectostoma is extreme, with varying degrees and kinds of ornamentation, involving ribs (that often have developed into hollow spines) and apertural flanges (see figure 2 for examples) These two examples show a situation that is often found in land snails: a largely allo- or parapatrically distributed group of species, which show a great diversification in shell ornamentation that is not obviously paralleled by niche-differentation. One may expect that both effect act in opposite directions: high predator abundance (selecting for defensive structures) will result in low prey population density, and this in turn will reduce the number of sexual encounters, relaxing the selection pressure on sexual signalling of shell ornamentation In any case, this complication might be controlled for by selecting sister groups with different degrees of polygamy that share the same habitat, and are experiencing similar predation pressures. Unconnected, allopatric populations experiencing the same environmental pressures are still expected to diverge rapidly in their ornamentations [39]
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