As regular readers of this site will know, anoles are remarkable for the repeated, independent evolution of ecomorphs on the four islands of the Greater Antilles. Each ecomorph is defined by a suite of ecological and morphological traits that appear to be shaped by natural selection.
In a recent paper, Kolbe et al. ask whether those suites of morphological traits are actually suites. In other words: is convergence in form across islands reached by evolving the same sets of characters in a similar manner? Do all trunk-ground ecomorphs, for example, achieve relatively long limbs by growing both the femur and the humerus (i.e. those traits covary together)? Or do some trunk-ground anoles achieve long limbs by only growing the tibia and the radius while others grow the femur and radius etc.?
Understanding whether and how different sets of traits vary together can give a good understanding of how natural selection and evolutionary history combine to explain the convergent evolution of Anolis ecomorphs.
The authors ask several questions in this paper.
Does the pattern of trait variation and covariation differ among 8 species of anoles? The authors found that there are differences among species in the way that traits vary together but that the variation was not correlated with the evolutionary history. In other words, species that are closely related don’t necessarily show the same covariance in their traits. However, there was some similarity in the trait variances and covariances for the three anoles that belong to the trunk-ground ecomorph class (A. sagrei, A. gundlachi, A. sagrei; see the figure above). That these distantly related species have the same sets of traits varying together provides more evidence that natural selection has shaped ecomorph evolution in anoles.
Do some functionally based character sets show stronger integration than others? Does integration vary among species? One might expect that suites of traits important for certain activities, say running, or eating, might evolve together. Depending on the life history and ecology of a species, some functions may be more important than others and suites of traits related to those functions may covary more tightly together (i.e show stronger integration). For example, trunk-ground anoles are active and run frequently, so limb traits may be strongly integrated. Crown-giant anoles, on the other hand, eat large prey, so head and jaw traits may be strongly integrated.
Kolbe et al. found that some functionally based trait sets are more integrated than others (see the figure above). Limb elements, for example, are strongly integrated. This makes sense, given their shared function during locomotion. However, there is also variation in the strength of integration among ecomorphs suggesting that ecology and natural selection influence the degree of integration. Trunk-ground ecomorphs have the strongest limb integration (perhaps important for all the running and jumping they do) while twig and grass-bush anoles have strong integration for head shape (with long, narrow, streamlined heads, perhaps this is important for moving through cluttered environments).
In conclusion, this study suggests that natural selection has been important in shaping convergent patterns of covariance among traits in Anolis ecomorphs. Such trait integration is yet another way that anoles have evolved convergently in the Greater Antilles. But you don’t have to take my word for it. This is a rich study that could only be summarized here; I invite you to read it for yourself!
JASON J. KOLBE, LIAM J. REVELL, BRIAN SZEKELY, EDMUND D. BRODIE III, AND JONATHAN B. LOSOS (2011). CONVERGENT EVOLUTION OF PHENOTYPIC INTEGRATION AND ITS ALIGNMENT WITH MORPHOLOGICAL DIVERSIFICATION IN CARIBBEAN ANOLIS ECOMORPHS. Evolution, 65, 3608-3624 DOI: 10.1111/j.1558-5646.2011.01416.x