Generalists link peaks in the shifting adaptive landscape of Australia's dragon lizards

The adaptive landscape is one of biology's most flexible and intuitive general concepts. This multidimensional space enables us to map individuals, populations, and species to an evolutionary topography, teasing apart what distinguishes entities from one another. A common exercise is clustering species into adaptive peaks which highlight regions of explored trait space, often considered trait optima or adaptive zones. Typically this reveals broad regions of unpopulated and suboptimal trait space that must be traversed between peaks, however, these evolutionary pathways remain poorly understood. Here we use new sequence-capture and linear morphological datasets to investigate the evolution of the adaptive landscape in a group of highly visible and charismatic reptiles: Amphibolurinae dragon lizards (Family Agamidae). We generate a well-supported time-calibrated phylogenomic tree from >5,000 nuclear markers for more than 360 specimens across 176 dragon species, including undescribed diversity, and provide new insights into the diversification of a group that includes iconic species like the frilled-neck lizard, thorny devil, and bearded dragon. By investigating the multivariate morphological evolution of dragon lizards we show that the colonization of Australia was followed by rapid morphological expansion. While the ancestral amphibolurine was likely tree-living, the evolution of the morphological landscape was facilitated by a transition to a generalist form and then subsequent transitions to more specialized ecomorphs. These results provide evidence of an evolutionary pathway for adaptive diversification in a continental radiation.