Lichens are classic models of symbiosis, and one of the most frequent nutritional modes among fungi. The ecologically and
geographically widespread lichen-forming algal (LFA) genus Trebouxia is one of the best-studied groups of LFA and
associates with over 7000 fungal species. Despite its importance, little is known about its diversification. We synthesized
twenty years of publicly available data by characterizing the ecological preferences of this group and testing for
time-variant shifts in climatic regimes over a distribution of trees. We found evidence for limited shifts among regimes, but
that disparate lineages convergently evolved similar ecological tolerances. Early Trebouxia lineages were largely forest
specialists or habitat generalists that occupied a regime whose extant members occur in moderate climates. Trebouxia then
convergently diversified in non-forested habitats and expanded into regimes whose modern representatives occupy
wet-warm and cool-dry climates. We rejected models in which climatic diversification slowed through time, suggesting
climatic diversification is inconsistent with that expected under an adaptive radiation. In addition, we found that climatic
and vegetative regime shifts broadly coincided with the evolution of biomes and associated or similar taxa. Together, our
work illustrates how this keystone symbiont from an iconic symbiosis evolved to occupy diverse habitats across the globe.
