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Evolution threatens Darwin

Dengue ("breakbone") fever was much in the news last month in Australia last month, when I was there for the Applied Evolution Summit. The mosquitoes that spread this deadly disease are currently found in Queensland, where we were, but not further west, in the city of Darwin. They were found in Darwin in the early 1900s, however. Could some combination of climate change, human activity, and evolution put Darwin at risk once again? That was one of the questions discussed by Ary Hoffmann at the summit, largely based on a paper titled "Integrating biophysical models and evolutionary theory to predict climatic impacts on species' ranges: the dengue mosquito Aedes aegypti in Australia", published in Functional Ecology last year.

The mosquitoes typically breed in buckets and rainwater-storage tanks -- reduced use of these tanks may explain the retreat of dengue mosquitoes from Darwin -- so they modeled water-temperature cycles in those containers as a function of climate. They also modeled possible evolutionary changes in two key traits: cold tolerance and drying tolerance of eggs.

Climate change alone was not predicted to allow dengue mosquitoes to reach Darwin anytime soon. Evolution alone could threaten Darwin with dengue within 24 years. A combination of climate change and evolution was predicted to shorten the dengue-free period to 19 years.

Indirect effects of climate change, via changed human behavior, could also be important. In particular, decreases in rainfall, whose direct effects might limit mosquito survival, could trigger increased use of water storage tanks. This would provide renewed opportunities for dengue-transmitting mosquitoes to spread to Darwin and elsewhere.

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