Abstract:
It is often hypothesised that ecological opportunity or release, for example after colonisation of new biomes lacking competitors, promotes rapid evolution. We test for this in the Gehyra variegata group, a diverse but taxonomically intractable gecko radiation in Australia. We compare diversity, diversification rates and morphological evolution across two biomes: the Australian monsoonal tropics (AMT) and arid zone (AAZ). We predict ecological release and elevated rates of lineage, ecological and phenotypic evolution in the AAZ as opposed to the AMT for two reasons. First, species in the Gehyra variegata group are largely rock restricted and overlap a larger bodied clade of Gehyra in the AMT, but in the AAZ have no congeners and occur across trees and rocks. Second, paleoclimatic data suggests that the latter biome is relatively young, having expanded from the late Miocene. We use intense geographic sampling, high-throughput, phylogenomic data (547 loci across 42 candidate lineages), and multispecies coalescent methods to determine the number of species and sub-specific lineages, and to estimate a dated, highly-resolved phylogeny. We confirm higher diversity of independently evolving lineages in the AAZ (28 lineages across 11 recognised species) than the AMT (12 lineages across five species). Multiple shifts between rock and tree dwelling have also occurred in the arid zone, but not the AMT. However, we do not find a change in rates of morphological evolution (body size and shape) between the AAZ and AMT; although we do find consistent (but moderate) shifts in head depth between rock and tree taxa. Across both biomes, most diversity was concentrated in two similar aged late-Miocene radiations, and in rocky habitats. Higher lineage diversity in similar aged radiations without shifts in rates of morphological change suggests that, rather than ecological release, physical geography may have played the dominant role in higher rates of AAZ diversification, with complex rocky habitats such as the Pilbara of Western Australia promoting isolation and localised persistence of lineages through climatic oscillations.