As a lineage splits and diversifies, species’ traits diverge in different ways. For example, as anoles diversified in the Caribbean, trunk-ground anoles’ bodies become muscular and stocky, trunk-crown anoles’ heads become long and thin, and grass anoles’ tails become long and slender. This process of adaptation to different environments seems simple and intuitive, but the evolution of traits is not so simple.
Most traits don’t evolve independently – changes in one trait are often correlated with changes in another trait, which can constrain a species’ response to selection. This correlation between traits is represented by the genetic variance-covariance matrix (G matrix). The size, shape, and orientation of the G matrix determine the speed and direction of morphological change, and defines the “line of least genetic resistance” along which a species can evolve. But of course, as species diverge and their traits shift, the correlations between these traits themselves may not stay constant – that is to say, the G-matrix itself can evolve. Which means that G represents both a constraint on evolutionary change, as well as a product of evolution itself. So does the G matrix evolve along with species divergence, or does it limit morphological evolution?
In his talk at Evolution 2017, Joel McGlothlin (Virginia Tech) described his efforts to address these question in anoles. As a poster child of adaptive radiation, Anolis provides an excellent opportunity to explore the dynamics of G matrix evolution and evolutionary constraint. To that end, McGlothlin and colleagues estimated G matrices for seven anole species (no easy task), including representatives from three ecomorph categories. He laid out the following question: has the G matrix evolved as Anolis diversified? Or do we see a signature of constraint conserved across anoles?
First, McGlothlin and colleagues found that the G matrix has indeed evolved in the course of Anolis diversification: the shape, orientation, and size of the G matrix was different for each species studied. More closely related species had more similar G matrices, and there was a weak link between ecomorph and G matrix structure, but overall, G was clearly different across the seven anole species. This suggests that trait correlations (and therefore species’ potential responses to selection) are not necessarily constant across the anole radation.
However, despite this overall divergence, one important aspect of the G matrix – its orientation – was similar across all anole species sampled. This suggests that the line of least genetic resistance has remained constant throughout the diversification of anole ecomorphs, and is deeply conserved. So even though individual species’ trait correlations have changed as anoles have diverged, the signature of morphological constraint has persisted. The study provides a fascinating illustration of the complexity of morphological evolution, and provides a fresh new link between micro- and macro- evolutionary processes in Anolis lizards.