Sexual selection on phenotypic traits in a hybrid zone of Littorina saxatilis (Olivi)

Abstract

Step clinal transitions in inherited character(s) between genetically distinct populations are usually referred to as hybrid zones. An example is found in the population of the intertidal snail Littorina saxatilis in Galicia (NW Spain). We studied the shape of the overall fitness surface for sexual selection in this hybrid zone, and the position of hybrids and pure morphs on this surface. We found that sexual divergent selection acted on a combination of phenotypic traits separating the pure morphs, and therefore that sexual selection contributed to morph differentiation. The average fitness of hybrids as a group was not significantly different from that of the pure morphs, but they did show divergent sexual selection in some traits. These results are in agreement with a model of divergent selection favouring both the pure morph as well as those hybrids most resembling each morph. The finding of divergent selection is remarkable because quadratic selection gradients are usually weak in nature.

Genetic variation for shell traits in a direct-developing marine snail involved in a putative sympatric ecological speciation process

“…The existence of a significant genetic variation for the main traits involved in the morphological differentiation between the ecotypes is important, as the contribution of some of these traits to the variation for fitness has been well established (Rolá nAlvarez et al 1997;Cruz et al 2001Cruz et al , 2004b. The finding of a substantial amount of genetic variation for shell size is particularly relevant, as this is the trait assumed to be responsible for the pre-mating reproductive isolation between the two ecotypes (Rolá n- Alvarez et al 2004;Cruz et al 2004a).…”

Natural selection and divergence in mate preference during speciation

“…It is not yet possible to determine conclusively whether divergence in pheromonal communication is linked to adaptation to different host plants, because only two population pairs have been examined. However this level of replication is highly typical of studies of the traits underlying sexual isolation (Ryan et al 1990;Nagel and Schluter 1998;Cruz et al 2001;Jiggins et al 2001;Fordyce et al 2002;Vines and Schluter 2006). Two lines of evidence are highly suggestive.…”

A simple procedure for the comparison of covariance matrices

“…The two morphs occupy different shore levels but form a hybrid zone in the midshore (see [36] for a detailed description of the hybrid zone). Seven shell measures were available (Figure 8, see [37] for more information), all of them taken on pure morph snails from the midshore (only a proportion of the individuals in this area are hybrids) of three locations (Centinela 42º04’N, 8º53’W; Corrubedo, 42º32’N, 9º20’W and Senin, .42º02’N, 8º53’W) and log transformed. The numbers of Rb and Su individuals from each location were 160 and 135, 150 and 112, and 123 and 111.…”