Longer summers associated with global warming are causing some mosquitoes to evolve so that they enter their winter dormancy later, according to researchers at the University of Oregon. These findings provide the first example of a genetic shift for a seasonally related trait, they claim.

William Bradshaw and Christina Holzapfel studied the pitcher-plant mosquito, Wyeomyia smithii, under laboratory conditions. These insects survive the winter by entering their pupae and lying dormant until spring. Day-length (photoperiod), an indicator of seasonal change, drives mosquito behavior, the authors say. That is, shortening days trigger the onset of dormancy.

The team found that the mosquitoes now enter their dormant phase 8 to 10 days later than they did in the 1970s. The shift was detectable over a time interval as short as five years.

Bradshaw and Holzapfel believe that those insects that have adapted to longer summers by delaying dormancy until days are shorter, can stay active for longer. “This response to day length is a highly heritable trait that is subject to natural selection. Populations are responding genetically, that is, evolving, as a consequence of selection for shorter critical photoperiods due, presumably, to longer growing seasons generated by climate warming,” says Bradshaw.

According to the authors, variation in populations has two sources: genetic and environmental. They say that by controlling conditions in the lab, they factored out all possible environmental sources of variation revealing the genetic differences.
The insects that enter dormancy later have an advantage over the others and can pass this on to their offspring. They are able to continue to reproduce longer, leaving more offspring, and can accumulate more nutritional reserves. (Proc. Natl. Acad. Sci. U.S.A. 2001, 98, 14,509–14,511) —MARIA BURKE