Arizona to Idaho on the tongue of a moth: Geographic trends of floral form and scent in Oenothera caespitosa ssp. marginata

Plant-pollinator relationships tend to vary geographically, but relatively few studies have quantified how floral phenotypic evolution tracks such variation. The tufted evening primrose ranges across western North America, where it is pollinated by different sized hawkmoths and a variety of bees. I set up a common garden experiment in which populations from across this range were grown together, to separate genetic and environmental sources of variation in floral morphology and odor. I measured aspects of both floral morphology (e.g. corolla diameter, nectar tube length, anther-stigma distance) and floral odor chemistry from all plants and analyzed their variation using principal components analysis. Flower size, particularly tube length, shows a nearly bimodal distribution with increasing latitude, consistent with a shift from long- to short-tongued hawkmoth (or possibly bee) pollination. By removing environmental sources of variation, this study demonstrates that local adaptation to pollinators may leave a genetic footprint on floral reproductive morphology and floral odor.