Small genetic steps lead to mechanical isolation in hummingbird-pollinated gingers

Interactions with pollinators are a potent source of natural selection driving the spectacular array of flowering plant diversity on Earth (Kay & Sargent, 2009; Van der Niet et al., 2014). Floral traits play a central role in this process: reliable and effective pollination by animal pollinators depends on complex floral features, including traits that determine pollinator attraction and reward, as well as the mechanics of pollen transfer. Pollinators specify mating events between individuals, and thus differences in flowers have the potential to generate reproductive isolating barriers (floral isolation). A compelling case of floral isolation comes from spiral gingers (Costus), where hummingbird-adapted species have evolved distinct pollen placement strategies (on the bills vs. foreheads of pollinators) due to differences in flower shape and the arrangements of flower parts. This difference in pollen placement causes a mechanical barrier to cross-pollination. In this issue of Molecular Ecology, Kay and Surget-Groba (2022) dissect the genetic basis of these floral differences using a quantitative trait locus (QTL) mapping approach. They find small-effect QTLs that influence multiple correlated traits and allelic effects that suggest a history of directional selection. Their results indicate mechanical isolation reflects adaptive divergence that has built up piecemeal over time.