Although global warming may not yet have reached catastrophic proportions, its subtle effects can already be seen in the natural world. Butterflies, for example, are shifting their home ranges toward cooler areas (ScienceNOW, 10 June 1999). Now, a new study in this month's Evolutionary Ecology Research shows that fruit flies are also feeling the heat--in their genes.
Back in the 1940s, geneticist Max Levitan of the Mount Sinai School of Medicine in New York City did his Ph.D. on the chromosomes of the eastern North American woods fly Drosophila robusta. Like many organisms, it sometimes has genetic variants called "inversions," where entire chunks of chromosome have been turned back-to-front. This unorthodox orientation means that these parts of the genome cannot be shuffled during the production of egg and sperm, and the genes on them are packages of permanently linked genes. As the young Levitan found out in lab experiments, certain of these gene packs, called 2L-1 and 3R-1, help the flies cope better with high temperatures. Although the exact causes for their heat-resistance are not clear, this feature probably explains why 2L-1 and 3R-1 are more common in the south.
But now all that has changed. Over the past 60 years, Levitan has been regularly putting out fly traps over the United States and parts of Canada, often returning to the same places. But lately he has been noticing something strange. "My first inkling came from the 1995 collections at Philadelphia," he says, when the frequency of the heat-resistant 2L-1 variant came in at 60%, whereas it used to be much rarer in Pennsylvania. Although he has become less mobile (his wife does not let him drive anymore), samples taken last year confirm his suspicions dramatically. In five localities, from St. Louis to Philadelphia, 2L-1 and 3R-1 have doubled or tripled in frequency since the 1970s, reaching levels previously only seen in southern Georgia and Alabama. Even in Central Park in New York City, the only place still easily within Levitan's radius of action, the trend could be detected. "Widespread climate change best explains [this]," Levitan says.
Other evolutionary geneticists are thrilled with the demonstration of a genome responding to global warming. On the other hand, they are also concerned. Bill Etges, who works on Drosophila at the University of Arkansas in Fayetteville, says: "Like canaries in a coal mine, [it is] another warning of the effects of human activities on the rest of the planet's living things."