The diversification and evolution of niche breadth across spatial scales in western North American monkeyflowers

Species show remarkable variation in ecological niche breadth, but the
directionality of niche breadth evolution remains a question for niche
axes across spatial scales. Testing the association between niche breadth
evolution and the process of diversification could shed light on the role
of ecology in the formation and maintenance of biodiversity. Here we
applied Cladogenetic State change Speciation and Extinction models in
western North American monkeyflowers (Mimulus sensu lato), with
two states of niche breadth (generalist and specialist). We aimed to
estimate the effects of niche breadth along broad-scale bioclimatic and
local-scale microhabitat axes on diversification rate, and the
evolutionary trend and mode of niche breadth. We found opposite patterns
across spatial scales, with higher speciation rates detected for
generalists along broad-scale bioclimatic axes and for specialists along
local-scale microhabitat axes. Furthermore, we found weak trends towards
specialization for bioclimatic niche breadth, but generalization for
microhabitat niche breadth. Both cladogenetic and anagenetic changes were
comparable and thus important in the evolution of niche breadth in this
group. Together, these findings suggest that the underlying mechanisms of
niche breadth evolution might differ across spatial scales, and that
change in niche breadth could be tightly associated with the
diversification process.