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Abstract

Speciation generally involves a three-step process—range expansion, range fragmentation and the development of reproductive isolation between spatially separated populations1,2. Speciation relies on cycling through these three steps and eachmay limit the rate atwhich new species form1,3. Weestimate phylogenetic relationships among all Himalayan songbirds to ask whether the development of reproductive isolation and ecological competition, both factors that limit range expansions4, set an ultimate limit on speciation. Based on a phylogeny for all 358 species distributed along the eastern elevational gradient, here we show that body size and shape differences evolved early in the radiation, with the elevational band occupied by a species evolving later.These results are consistentwith competition for niche space limiting species accumulation5. Even the elevation dimension seems to be approaching ecological saturation, because the closest relatives both inside the assemblage and elsewhere in the Himalayas areon average separated bymore than fivemillion years,which is longer than it generally takes for reproductive isolation tobe completed2,3,6; also, elevational distributions are well explained by resource availability, notably the abundance of arthropods, and not by differences in diversification rates in different elevational zones.Ourresults imply that speciation rate is ultimately set by niche filling(that is, ecological competition for resources), rather than by the rate of acquisition of reproductive isolation.