Steep difference between ecotypes and intermediate phenotype depression for shell trait integration in a marine snail hybrid zone.

Multivariate analyses of phenotypic integration for a set of characters provide information about biological systems that cannot be obtained in univariate studies of these characters. We studied phenotypic integration for seven shell measures across the phenotypic gradient in a hybrid zone of the marine snail Littorina saxatilis in Galicia, NW Iberia. We found clear differences in the degree of integration between the two ecotypes involved in the hybrid zone, likely related to differences in the strength of natural selection acting on the snails' shells in each ecotype's habitat. We found also evidence of a decrease in integration in the phenotypically intermediate, hybrid snails, consistent with hybridization resulting in a release of multivariate variation and increased evolvability. Across the phenotypic gradient, decreases in overall integration tended to be accompanied by increases in some measures of modularity, but the latter did nor reflect high correlation structure. The increases occurred only in a proportional sense, correlations among modules tending to decrease faster than within modules for low overall integration tiers. Integration analyses based on non partial and partial correlations tended to produce contrasti...Continue Reading