The scale-of-choice effect and how estimates of assortative mating in the wild can be biased due to heterogeneous samples

Sexual size dimorphism is widespread among dioecious species but its underlying driving forces are often complex. A review of sexual size dimorphism in marine gastropods revealed two common patterns: firstly, sexual size dimorphism, with females being larger than males, and secondly females being larger than males in mating pairs; both of which suggest sexual selection as being causally related with sexual size dimorphism. To test this hypothesis, we initially investigated mechanisms driving sexual selection on size in three congeneric marine gastropods with different degrees of sexual size dimorphism, and, secondly, the correlation between male/female sexual selection and sexual size dimorphism across several marine gastropod species. Male mate choice via mucus trail following (as evidence of sexual selection) was found during the mating process in all three congeneric species, despite the fact that not all species showed sexual size dimorphism. There was also a significant and strong negative correlation between female sexual selection and sexual size dimorphism across 16 cases from seven marine gastropod species. These results suggest that sexual selection does not drive sexual size dimorphism. There 2 was, however, evidence of males utilizing a similar mechanism to choose mates (i.e. selecting a female slightly larger than own size) which may be widespread among gastropods, and in tandem with present variability in sexual size dimorphism among species, provide a plausible explanation of the observed mating patterns in marine gastropods.

Natural color polymorphisms are widespread across animal species and usually have a simple genetic basis. This makes them an ideal system to study the evolutionary mechanisms responsible for maintaining biodiversity. In some populations of the intertidal snail Littorina fabalis, variation in shell color has remained stable for years, but the mechanisms responsible are unknown. Previous studies suggest that this stability could be caused by frequency-dependent sexual selection, but this hypothesis has not been tested. We analyzed shell color polymorphism in mating pairs and surrounding unmated individuals in two different populations of L. fabalis to estimate sexual fitness for color, as well as assortative mating. The estimated effective population size from neutral markers allowed us to disregard genetic drift as the main source of color frequency changes across generations. Shell color frequency was significantly correlated with sexual fitness showing a pattern of negative frequency dependent selection with high disassortative mating for color. The results suggested a contribution of male mate choice to maintain the polymorphism. Finally, the implementation of a multi-model inference approach based on information theory allowed us to test for the relative contribution of mate choice and mate competition to explain the maintenance of color polymorphism in this snail species.

Littorina fabalis is an intertidal snail commonly living on the brown algae Fucus vesiculosus and showing frequent shell-color polymorphisms in the wild. The evolutionary mechanism underlying this polymorphism is currently unknown. Shell color variation was studied in mated and non-mated specimens of this species from different microareas in one locality from NW Spain, in order to estimate sexual selection and assortative mating that may (still) be operating in this population. The analyses across microareas allowed us to investigate frequency-dependent selection and assortative mating components, mechanisms that could maintain the polymorphism. The presence of shell scars caused by crab attacks, an environmental variable not related with sexual selection or assortative mating, was used as experimental control. This study provides new evidence of significant disas-sortative mating and some degree of sexual selection against some shell colors, supporting the results found 21 years ago in a similar study, i.e. in the same species and locality. The similarity of these estimates during the studied period suggests that this experimental approach is consistent and valid to be extended to other populations and organisms. In addition, sexual selection and assortative mating estimates did not change across microareas differing in shell color frequencies, suggesting than the polymorphism can not be maintained by a frequency-dependent (sexual selection-based) mechanism. Our main hypothesis is that negative assortative mating could contribute to the maintenance of the polymorphism, perhaps by males showing distinct female color preferences when searching for mates.