Rapid evolution of host repertoire and geographic range in a young and diverse genus of montane butterflies

Evolutionary changes in geographic distribution and larval host plants may
promote the rapid diversification of montane insects, but this scenario
has been rarely investigated. We studied the rapid radiation of the
butterfly genus Colias, which has diversified in mountain
ecosystems in Eurasia, Africa, and the Americas. Based on a dataset of 150
nuclear protein-coding genetic loci and mitochondrial genomes, we
constructed a time-calibrated phylogenetic tree of Colias species with
broad taxon sampling. We then inferred their ancestral geographic ranges,
historical diversification rates, and the evolution of host use. We found
that the most recent common ancestor of Colias was likely
geographically widespread and originated ~3.5 Ma. The group
subsequently diversified in different regions across the world, often in
tandem with geographic expansion events. No aspect of elevation was found
to have a direct effect on diversification. The genus underwent a burst of
diversification soon after the divergence of the Neotropical lineage,
followed by an exponential decline in diversification rate toward
the present. The ancestral host repertoire included the legume genera
Astragalus and Trifolium but later expanded to include a wide
range of Fabaceae genera and plants in more distantly related families,
punctuated with periods of host range expansion and contraction. We
suggest that the widespread distribution of the ancestor of all extant
Colias lineages set the stage for diversification by isolation of
populations that locally adapted to the various different environments
they encountered, including different host plants. In this scenario,
elevation is not the main driver but might have accelerated
diversification by isolating populations.