Pollinator and habitat-mediated selection as potential contributors to ecological speciation in two closely related species

In ecological speciation, incipient species diverge due to natural selection that is ecologically based. In flowering plants, different pollinators could mediate that selection (pollinator-mediated divergent selection) or other features of the environment that differ between habitats of 2 species could do so (environment-mediated divergent selection). Although these mechanisms are well understood, they have received little rigorous testing, as few studies of divergent selection across sites of closely related species include both floral traits that influence pollination and vegetative traits that influence survival. This study employed common gardens in sites of the 2 parental species and a hybrid site, each containing advanced generation hybrids along with the parental species, to test these forms of ecological speciation in plants of the genus Ipomopsis. A total of 3 vegetative traits (specific leaf area, leaf trichomes, and photosynthetic water-use efficiency) and 5 floral traits (corolla length and width, anther insertion, petal color, and nectar production) were analyzed for impacts on fitness components (survival to flowering and seeds per flower, respectively). These traits exhibited strong clines across the elevational gradient in the hybrid zone, with narrower clines in theory reflecting stronger selection or higher genetic variance. Plants with long corollas and inserted anthers had higher seeds per flower at the Ipomopsis tenuituba site, whereas selection favored the reverse condition at the Ipomopsis aggregata site, a signature of divergent selection. In contrast, no divergent selection due to variation in survival was detected on any vegetative trait. Selection within the hybrid zone most closely resembled selection within the I. aggregata site. Across traits, the strength of divergent selection was not significantly correlated with width of the cline, which was better predicted by evolvability (standardized genetic variance). These results support the role of pollinator--mediated divergent selection in ecological speciation and illustrate the importance of genetic variance in determining divergence across hybrid zones. The remarkable diversity of the flowering plants has been attributed to evolutionary effects of pollinators since the time of Darwin. Despite that long history, there are few tests of whether differences in appearance between closely related plant species reflect adaptation to different animal pollinators or adaptation to other differences between habitats such as soil types or climatic conditions. This study examines the relationships of plant traits to fitness in field gardens planted with two closely related species of subalpine plants and their hybrids. It presents strong evidence that divergence between the two species is driven by pollinator-mediated natural selection rather than habitat-mediated selection. It focuses on both floral and vegetative traits that show clines, i.e., gradual changes in trait value, across an elevational gradient. Natural selection due to differences in seed production favored long flowers in the site occupied by slender skyrocket (Ipomopsis tenuituba) but short flowers in the site occupied by scarlet gilia (Ipomopsis aggregata), a signature of divergent selection mediated by pollinators. Similar results were obtained for another reproductive trait, position of the anthers. In contrast, vegetative traits (leaf thickness, leaf hairs, and water-use efficiency during photosynthesis) did not experience divergent selection across the parental sites, thus providing no support for habitat-mediated selection. Across all measured traits, the strength of divergent selection did not correlate with the steepness of the cline, as frequently assumed, which was better predicted by the relative amount of genetic variability in the trait and thus its potential to evolve. Results elucidate the mechanism of plant speciation between these two species. They also illustrate the importance of combining data on spatial patterns in traits and natural selection measurements to understand how species diverge in nature.