A highly conserved ontogenetic limb allometry and its evolutionary significance in the adaptive radiation of Anolis lizards

A key characteristic of adaptive radiations is the diversification into
ecologically specialized morphologies. However, comparative morphology
indicates that diversifications often proceed along narrow, and
phylogenetically highly conserved, evolutionary trajectories. These
patterns of covariation between characters arise in ontogeny, raising the
possibility that adaptive morphologies might be biased towards trait
covariation that resemble growth trajectories. Anolis lizards are an
excellent system to test this prediction since they have repeatedly
evolved highly specialized locomotor morphologies. Here, we investigate
the covariation of embryonic growth of 13 fore- and hindlimb bones in 15
species, and compare these to the evolutionary covariation of these limb
bones across 267 species of the entire Anolis clade. Our results
demonstrate that species differences in relative limb length are
established already at the time of hatching, and are the result of
differential growth as well as differential sizes of cartilaginous
anlagen. Multivariate analysis of limb bones revealed that Antillean
Anolis share a common allometric pattern that is characterized by positive
allometric growth of the long bones relative to metapodial and phalangeal
bones. This major axis of ontogenetic allometry in limb bones deviated
from the major axis of evolutionary allometry of the Antillean Anolis, and
was equally disparate to the evolutionary allometries of the two clades of
mainland Anolis lizards. These results demonstrate that the remarkable
diversification of locomotor specialists in Anolis lizards are accessible
through changes that are largely independent from ontogenetic growth
trajectories, and therefore likely the result of genetic modifications
that manifest at the earliest stages of limb development.