Evolution of Threespine Stickleback ( <i>Gasterosteus aculeatus</i> ) Defensive Traits following the Stocking of Brook Trout ( <i>Salvelinus fontinalis</i> )

AbstractIn recent decades, numerous observations have been made of evolution induced by anthropogenic change in natural populations. Evolution in response to harvest, climate change, pollution, landscape change, and introduced invasive species are common. Here, we provide evidence for evolution in a new context by documenting a large increase in the frequency of defensive pelvic spines in two unusual threespine stickleback (<i>Gasterosteus aculeatus</i>) populations previously shown to mostly lack such structures. These populations, in Parc national du Lac-Témiscouata, Québec, Canada, were historically free of predatory fish and consisted nearly entirely of pelvic-spineless stickleback. This phenotypic change coincided with the stocking of the lakes with brook trout (<i>Salvelinus fontinalis</i>), a stickleback predator, and the introductions of other species used by anglers as live bait. The rapid evolutionary change toward a more defensive morph in the populations should prompt increased caution regarding the effects of management practices on native species.