Data from: High evolutionary constraints limited adaptive responses to past climate changes in toad skulls

Interactions among traits that build a complex structure may be
represented as genetic covariation and correlation. Genetic correlations
may act as constraints, deflecting the evolutionary response from the
direction of natural selection. We investigated the relative importance of
drift, selection, and constraints in driving skull divergence in a group
of related toad species. The distributional range of these species
encompasses very distinct habitats with important climatic differences and
the species are primarily distinguished by differences in their skulls.
Some parts of the toad skull, such as the snout, may have functional
relevance in reproductive ecology, detecting water cues. Thus, we
hypothesized that the species skull divergence was driven by natural
selection associated with climatic variation. However, given that all
species present high correlations among skull traits, our second
prediction was of high constraints deflecting the response to selection.
We first extracted the main morphological direction that is expected to be
subjected to selection by using within- and between-species covariance
matrices. We then used evolutionary regressions to investigate whether
divergence along this direction is explained by climatic variation between
species. We also used quantitative genetics models to test for a role of
random drift versus natural selection in skull divergence and to
reconstruct selection gradients along species phylogeny. Climatic
variables explained high proportions of between-species variation in the
most selected axis. However, most evolutionary responses were not in the
direction of selection, but aligned with the direction of allometric size,
the dimension of highest phenotypic variance in the ancestral population.
We conclude that toad species have responded to selection related to
climate in their skulls, yet high evolutionary constraints dominated
species divergence and may limit species responses to future climate
change.