Alpine floras (plants in the vegetation belts above the climatic treelines) experience cold climates, particularly in temperate mountains during winter, but they are generally species-rich. Yet, biogeographers have not determined whether these floras represent evolutionarily independent but convergent assemblages drawn from their regional floras, or whether they originated from particular clades pre-adapted to harsh conditions. Here, we analyse the evolutionary relationships of angiosperm (flowering plant) species in 63 alpine floras worldwide (~7,000 species) in comparison with their regional floras (~94,000 species) and with the entire global flora. We find that each of the alpine floras represents an assemblage of more closely related species than their respective regional floras. The degree of phylogenetic clustering of species in alpine floras in tropical mountains exceeds that in temperate mountains. However, in relation to the global flora, temperate alpine floras are phylogenetically closely related subsets of floras that colonized cold temperate areas during interglacial periods. We conclude that alpine floras include a few dominant families that have evolved tolerance to low temperature, and that evolutionary niche conservatism explains their phylogenetic clustering, compared with species in their regional species pools.