The geography of evolutionary opportunity: hypothesis and two cases in gastropods

Evolutionary innovations that require or provide increased per-capita energy budgets and competitive performance should appear at times and in places where resources are abundant and accessible and where predators and competitors impose intense selection. By contrast, innovations without functional benefits should become established in productive environments under conditions of weak selection from enemies. I confirmed these predictions in comparative studies of two types of innovation in gastropod shells. The labral tooth, a structure facilitating predation, appeared in 45 Cenozoic clades of marine gastropods, with the highest concentrations originating during the early Miocene, late Miocene, and Pliocene, all times and sites of high planktonic productivity, large expanses of warm shallow water, and diverse predator-rich biotas. Left-handed coiling, a condition with no obvious survival benefits, arose 19 times independently in right-handed clades. Most left-handed clades, including eight arising in regions outside the tropics, evolved in productive regions where the larva and/or the adult is shielded from intense predation. The times and places of origin of new traits in clades thus offer insights into the geography of evolutionary opportunity.