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Mal-(adaptation) to Urban Heat Islands

Speaker: Dr. Sarah E. Diamond, Case Western Reserve University

This seminar will examine the evolution of thermal tolerance and performance in response to the urban heat island effect using an acorn-dwelling ant species. Cities have expanded globally over the last century and these replicated urban heat islands provide an unprecedented opportunity to explore how populations cope with rapid increases in temperature. Within-generation phenotypic plasticity is one possible coping strategy, but there is growing evidence that contemporary evolution of temperature-sensitive traits is more common than previously appreciated.

In a majority of cities, but not all, acorn ants have evolved higher heat tolerance, but lost their tolerance of cold temperatures. These changes were not due to indirect effects of body size, which showed no evolutionary divergence between urban and rural populations, nor maternal effects, as the phenotypic patterns of population divergence persisted from F1 to F2 generations. The evolution of higher heat tolerance appears to be adaptive owing to evidence of positive directional selection on heat tolerance coupled with evidence of local adaptation. In particular, urban populations produced more sexual reproductives under warmer laboratory rearing temperatures, whereas rural populations produced fewer under these conditions.

Despite these adaptive shifts, other traits including whole-colony metabolic rate exhibit potentially maladaptive evolutionary divergence, with urban populations exhibiting a higher metabolic rate compared with rural populations. These findings are then put in a broader meta-analytical framework examining thermal tolerance responses to cities across a number of different species. This work highlights how cities can be used to explore contemporary convergent evolution of thermal traits in response to a common agent of selection, the urban heat island effect, but also potential constraints on thermal adaptation.