Phenotypic covariance matrix in life-history traits along a latitudinal gradient: a study case in a geographically widespread crab on the coast of Chile

Geographically widely spread species can cope with environmental differences among habitats by genetic differentiation and/or phenotypic flexibility. In marine crustaceans, intraspecific variations in life-history traits are pervasive along latitudinal clines. Replicated latitudinal clines are of evolutionary interest because they provide evidence of the occurrence of natural selection. If the means of traits along the latitudinal gradient are expected to be the result of natural selection, there is no reason why variances and covariances will not also be subject to selection, since selection is essentially a multivariate phenomenon. We studied life-history changes in means, variances, and covariances (i.e. P matrix) in 6 populations of the endemic crab Cyclograpsus cinereus (Decapoda: Grapsidae) along a latitudinal gradient over 19 degrees on the Chilean coast. Trait means differed among localities for all traits analyzed (i.e. female size, number and size of eggs, and reproductive output), and the variation displayed a clinal pattern. In general, the main result that emerged from planned comparisons of P matrices is that, when detected, differences between localities mainly reflect differences in the magnitude of phenotypic variation (i.e. eigenvalues), rather than in the relationships between traits (i.e. eigenvectors). Sea-surface temperature was only correlated with the covariance between egg numbers and reproductive output. Matrices comparisons for Flury and jackknife methods were highly linked, with limits of biogeographic provinces described for the coast of Chile. Our study strongly highlights the importance of estimating the P matrix, not only mean values, in order to understand the evolution of life-history traits along a latitudinal gradient. Furthermore, the study of the variation in the P matrix might provide important insights into those evolutionary forces acting on it.