Testing macroevolutionary predictions of the Grant-Stebbins model in the origin of Aeschynanthus acuminatus.

The Grant-Stebbins model predicts that a plant species encountering different pollinators across its range may undergo local adaptation and, subsequently, ecological speciation. We tested whether this could explain the origin of Aeschynanthus acuminatus (Gesneriaceae), a species phylogenetically derived from sunbird specialist ancestors. A. acuminatus is widespread throughout mainland E Asia but also occurs in Taiwan, beyond the range of sunbirds, where it is pollinated by generalist passerines. We hypothesized that A. acuminatus originated from an ancestral lineage that colonized Taiwan, rapidly adapted to its novel pollinator fauna, and secondarily spread to the mainland. We tested among evolutionary scenarios by integrating studies of phylogeography, pollination, and floral morphology. Phylogeographic analysis of genome-wide SNPs revealed a mainland origin. Pollinator observations showed varied visitation by both sunbirds and generalist passerines across mainland Asia. The origin of A. acuminatus likely involved a pollinator niche expansion to include generalist passerines, an ecological shift that enabled its subsequent range expansion. Hypothetical pollinator-mediated fitness models suggest that the derived floral morphology of A. acuminatus represents an adaptive optimum for generalist passerine pollination rather than an intermediate phenotype. Our research illustrates how the evolution of pollinator niches can influence the origin and range dynamics of plant species.