Repeated evolution of terrestrial lineages in a continental lizard radiation

The ‘early-burst’ model of adaptive radiation predicts an early increase
in phenotypic disparity concurrent with lineage diversification. Although
most studies report a lack of this coupled pattern, the underlying
processes are not identified. The continental radiation of
<i>Hemidactylus</i> geckos from Peninsular
India includes morphologically diverse species that occupy various
microhabitats. This radiation began diversifying ~36 Mya with an early
increase in lineage diversification. Here, we test the ‘early-burst’
hypothesis by investigating the presence of ecomorphs and examining the
pattern of morphological diversification in a phylogenetic framework. Two
ecomorphs— terrestrial and scansorial species— that vary significantly in
body size and toepad size were identified. Unlike the prediction of the
‘early-burst’ model, we find that disparity in toepad morphology
accumulated more recently ~14 Mya and fit the OU model. Ancestral state
reconstruction of the two ecomorphs demonstrates that terrestrial lineages
evolved independently at least five times from scansorial ancestors, with
the earliest diversification in terrestrial lineages 19–12 Mya. Our study
demonstrates a delayed increase in morphological disparity as a result of
the evolution of terrestrial ecomorphs. The diversification of terrestrial
lineages is concurrent with the establishment of open habitat and
grasslands in Peninsular India, suggesting that the appearance of this
novel resource led to the adaptive diversification.