The Selection Mosaic and Diversifying Coevolution between Crossbills and Lodgepole Pine

Asymmetrical competition determines which of two seed predators drives the evolution of lodgepole pine (Pinus contorta ssp. latifolia) cones. Red squirrels (Tamiasciurus hudsonicus) are effective preemptive competitors in lodgepole pine forests so that red crossbills (Loxia curvirostra) are uncommon and selection from Tamiasciurus drives cone evolution. When Tamiasciurus are absent, crossbills increase in abundance and coevolve in an evolutionary arms race with pine. Similarly, Tamiasciurus alters the evolutionary trajectories of large-seeded pines, many of which rely on birds (Corvidae) for their seed dispersal. Populations therefore exhibit a selection mosaic with coevolutionary hot spots. In the coevolutionary hot spots, divergent selection on crossbills potentially leads to reproductive isolation and speciation. This results in a subsequent reduction in the geographic mosaic but diversifies the adaptive landscape on which crossbills have radiated. Thus, divergent selection is a double-edged sword. Divergent selection is critical in creating a selection mosaic but erodes the selection mosaic when it promotes reproductive isolation and speciation.