Local adaptation of tree lizards to canyon dwelling

Abstract

How and under what situations populations adapt to local conditions remains a key question in evolutionary biology. This study tests if the particular morphology of a population of Tree lizards, Urosaurus ornatus, located in a canyon on the margin of the species range represents an adaptation to canyon habitat. Morphology was compared across 40 populations showing that relative hind limb length, tail length, and mass were all outliers for this population. The function of the relatively longer hind limbs, tail, and lower mass was proposed to be for better sprinting ability on the sheer canyon walls that provide the only available habitat structure for this population. Partial least squares regression found significant effects of tail length on top speed on a broad, steep surface. Partial least squares logistic regression identified significant effects of tail length on survival as well in males but not females of this population. Another canyon population of Tree lizards with access to alternative substrates (trees) showed no evidence of selection on the same morphological features. Ancestral state reconstruction using a phylogeny inferred for 21 populations found that the unique morphology of the focal population was evolutionarily derived compared to closely related populations and so likely arose under the present environmental conditions. Population genetic structure also supported the process of adaptive divergence as there was no evidence for migration and/or a recent genetic bottleneck in the focal population. Lizards in this population appear to have responded to selection allowing them to become specialists for running on canyon walls while other canyon populations with access to a greater variety of habitat structure have not.