Ecological diversification preceded geographical expansion during the evolutionary radiation of Cataglyphis desert ants

Biological diversity often arises as organisms adapt to new ecological conditions (i.e. ecological opportunities) or colonise suitable areas (i.e. spatial opportunities). Cases of geographical expansion followed by local ecological divergence are well described; they result in clades comprising ecologically heterogeneous subclades. In contrast, nothing is known about evolutionary radiation events in which ecological opportunities preceded spatial spread. Here, we show that the desert ant genus Cataglyphis likely originated in open grassland habitats in the Middle East ~18 million years ago and became a taxon of diverse species specialising in prey of different masses. Around 9 million years ago, southern Europe and northern Africa experienced aridification and were colonised by Cataglyphis, which was preadapted to the harsh environmental conditions. The result was the rapid accumulation of species, and the appearance of local assemblages containing species from different lineages that still displayed ancestral foraging specialties. These findings highlight that, in Cataglyphis, ecological diversification happened before the genus geographically spread into newly arisen suitable habitats, resulting in a clade composed of ecologically homogeneous subclades.