Gene flow in hybrid zones acts as a barrier to speciation, yet we still see cases in which separate hybridizing species are maintained. Selection for different floral traits at each end of a hybrid zone by pollinators and for different vegetative traits by environmental gradients are two possible sources of selection that could maintain different allele frequencies at different locations within a hybrid zone. In this study we measured five floral traits and three vegetative traits along an elevational gradient in an Ipomopsis hybrid zone and calculated their resultant clines to test predictions about selective forces that drive speciation and maintain species differences between I. aggregata populations at lower elevations and I. tenuituba populations at higher elevations within the hybrid zone.
Results/Conclusions
Corolla length increased with elevation, while corolla width, nectar production, relative redness and specific leaf area (SLA) decreased. The narrowest cline was in corolla length (w=0.12 km), followed by flower color, nectar production and corolla width (w=0.43, 0.97, and 1.4 km, respectively) while the widest cline was in SLA (w=1.7 km). Cline centers were all similarly located along the hybrid zone with the exception of nectar production, which was centered closer to the low elevation I. aggregata sites. The similarity of these clines’ centers for all floral traits besides nectar production is consistent with a shift in the pollination regime approximately midway down the hybrid zone. Combined with past pollination research in this hybrid zone, the narrower clines in floral traits than in vegetative traits suggest that speciation is mainly driven by selection for longer, narrower, paler flowers at higher elevations by hawkmoths and shorter, wider, redder flowers at lower elevations by hummingbirds.