Limited by the roof of the world: mountain radiations of Apollo swallowtails controlled by diversity-dependence processes

Abstract

Mountainous areas contain a substantial part of the world species richness, but the evolutionary origins and diversification of this biodiversity remain elusive. Diversification may result from differences in clade age (longer time to diversify), net diversification rates (faster speciation rate) or carrying capacities (number of niches). The likelihood of these macroevolutionary scenarios was assessed for six clades of Apollo swallowtails ( Parnassius ) that diversified mainly in the Himalayan–Tibetan region. The analyses suggest that neither the clade age nor the speciation rate could explain the mountain butterfly diversification. Instead diversity-dependence models were strongly supported for each group. Models further estimated clades' carrying capacities, which approximate to the current number of species, indicating that diversity equilibrium has been reached (or close to being reached). The results suggest that diversification of mountain butterflies was controlled by ecological limits, which governed the number of niches, and provide macroevolutionary justification for regarding mountains as islands.