Regional differences in leaf evolution facilitate photosynthesis following severe drought

Characterizing physiological and anatomical changes that underlie rapid
evolution following climatic perturbation can broaden our understanding of
how climate change is affecting biodiversity. It can also provide evidence
of cryptic adaptation despite stasis at higher levels of biological
organization. Here we compared evolutionary changes in populations
of Mimulus cardinalis from historically different climates in the
north and south of the species’ range following an exceptional drought. We
grew seeds produced from pre-drought ancestral plants alongside
peak-drought descendants in a common greenhouse and exposed them to wet
and dry conditions. Prior to the drought, northern ancestral populations
expressed traits contributing to drought escape, while southern ancestral
populations expressed drought avoidance. Following the drought, both
regions evolved to reduce water loss and maintain photosynthesis in dry
treatments (drought avoidance), but via different anatomical alterations
in stomata, trichomes, and palisade mesophyll. Additionally, southern
populations lost the ability to take advantage of wet conditions. These
results reveal rapid evolution towards drought avoidance at an anatomical
level following an exceptional drought, but suggest that differences in
the mechanisms between regions incur different trade-offs. This sheds
light on the importance of characterizing underlying mechanisms for
downstream life-history and macromorphological traits.