Population differences in aggression are shaped by tropical cyclone-induced selection.

Extreme events, such as tropical cyclones, are destructive and influential forces. However, observing and recording the ecological effects of these statistically improbable, yet profound 'black swan' weather events is logistically difficult. By anticipating the trajectory of tropical cyclones, and sampling populations before and after they make landfall, we show that these extreme events select for more aggressive colony phenotypes in the group-living spider Anelosimus studiosus. This selection is great enough to drive regional variation in colony phenotypes, despite the fact that tropical cyclone strikes are irregular, occurring only every few years, even in particularly prone regions. These data provide compelling evidence for tropical cyclone-induced selection driving the evolution of an important functional trait and show that black swan events contribute to within-species diversity and local adaptation.