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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.
