Identifying climatic drivers of hybridization with a new ancestral niche reconstruction method

Applications of molecular phylogenetic approaches have uncovered evidence
of hybridization across numerous clades of life, yet the environmental
factors responsible for driving opportunities for hybridization remain
obscure. Verbal models implicating geographic range shifts that brought
species together during the Pleistocene have often been invoked, but
quantitative tests using paleoclimatic data are needed to validate these
models. Here, we produce a phylogeny for Heuchereae, a clade of 15 genera
and 83 species in Saxifragaceae, with complete sampling of recognized
species, using 277 nuclear loci and nearly complete chloroplast genomes.
We then employ an improved framework with a coalescent simulation approach
to test and confirm previous hybridization hypotheses and identify one new
intergeneric hybridization event. Focusing on the North American
distribution of Heuchereae, we introduce and implement a newly developed
approach to reconstruct potential past distributions for ancestral
lineages across all species in the clade and across a paleoclimatic record
extending from the late Pliocene. Time calibration based on both nuclear
and chloroplast trees recovers a mid- to late-Pleistocene date for most
inferred hybridization events, a timeframe concomitant with repeated
geographic range restriction into overlapping refugia. Our results
indicate an important role for past episodes of climate change, and the
contrasting responses of species with differing ecological strategies, in
generating novel patterns of range contact among plant communities and
therefore new opportunities for hybridization. The new ancestral niche
method flexibly models the shape of niche while incorporating diverse
sources of uncertainty and will be an important addition to the current
comparative methods toolkit.