The divergence between ecotypes in a Littorina saxatilis hybrid zone is aligned with natural selection, not with intra-ecotype variation

Abstract

Multivariate covariation matrices occupy a central place in the analysis of evolutionary divergence among populations. They may constrain the multivariate space directions available to the divergence, and at the same time, their structure may be affected by the forces driving the divergence, and may thus provide information about the action of these forces in the past. Unfortunately, the relationship between covariation and divergence is complex and its theoretical analysis has not been completely resolved. To advance in the understanding of this relationship it is therefore necessary to gather information from case studies in which the evolutionary dynamics is as well known as possible. Here I analyse the covariation and divergence in a well-studied hybrid zone of the marine snail Littorina saxatilis. I compare the covariation matrix of seven shell measures with their multivariate divergence both between the two ecotypes in the hybrid zone and among different localities. I also propose an eigenvector-based procedure to predict the multi-character divergence expected under a null model of divergence proportional to within population covariation. The divergence between ecotypes was clearly not proportional to the covariation. Instead, it was in the same direction as the divergent selection pressures estimated in previous studies of this hybrid zone. In addition, the phenotypic divergence between ecotypes, but not between localities, was much larger than expected in terms of Fst estimates already published for this hybrid zone. Finally, I found no evidence of depletion in variance for the traits contributing the most to the divergence. I conclude that the covariation matrix has no direct relationship with the divergence between ecotypes, as it did not seriously constrain the direction of this divergence, nor was influenced in any obvious way by its development in the past.