Abstract
Patterns of trait integration reflect the underlying genetic and developmental architecture of morphology and significantly influence the direction of evolution. Nevertheless, the relationship between integration and disparity is complex and unlikely to be uniform across large phylogenetic and ecological scales. To date, there are little data comparing patterns of integration across major ecological transitions, limiting understanding of the processes driving changes in trait integration and their consequences. Here, we investigated patterns of cranial integration and disparity across pinnipeds, three closely related carnivoran families that have undergone a secondary adaptation to the aquatic niche with varying levels of ecological differentiation. With a three-dimensional geometric morphometric dataset of 677 specimens spanning 15 species, we compared five models of trait integration, and examined the effects of sexual dimorphism and allometry on model support. Pinnipeds varied greatly in patterns of cranial integration compared to terrestrial carnivorans. Interestingly, this variation is concentrated in phocids, which may reflect the broader range of ecological and life-history specializations across phocid species, and greater independence from the terrestrial habitat observed in that group, relative to otariids. Overall, these results indicate that major ecological transitions, and presumably large changes in selection pressures, may drive changes in phenotypic trait integration.
Highlights
Analyses of phenotypic integration measure the magnitude to which traits are correlated and dependent, whether& 2019 The Authors
Otariid species cluster on the positive extreme of PC1, but phocids are more dispersed across the two major axes, with three main phocid groupings: a Hydrurga leptonyx cluster with the most positive PC2 scores, an intermediate L. weddellii þ Pagophilus groenlandicus þ Halichoerus grypus þ Phoca vitulina þ Lobodon carcinophaga cluster, and a Cystophora cristata þ Mirounga leonina cluster on the negative end of the phocid PC2 distribution, towards the walrus morphospace
Our results indicate that the secondary adaptation to the aquatic environment in pinnipeds may have driven reorganization of cranial modularity relative to that observed in terrestrial carnivorans
Summary
Analyses of phenotypic integration measure the magnitude to which traits are correlated and dependent, whether& 2019 The Authors. The effect of integration among traits may be dependent on whether the major axis of shape covariation aligns with the direction of selection on those traits [15,16]. Whereas high integration among traits forces most variation to happen along few dimensions (and it constrains variation on other directions), if the direction of the selection vector coincides with the main axis of shape variation high integration may promote high variance along that dimension [15,16]. The opposite is expected to happen when there is a significant difference in direction between the selection and integration vectors, and in this case, high trait correlation may constrain increases in shape variance by preventing the exploration of certain trajectories and morphospace regions [9,17 –19]. Identifying which, if either, of these effects has dominated through organismal evolution is a central question in biology
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