For the first time, scientists believe they have found an instance of climate change causing a species to alter its genetic makeup — not just its behavior. To date, species changes stemming from climate change have all been behavioral, such as animals shifting their migration patterns. But after studying the DNA and mating habits of red squirrels in the Yukon for 10 years, scientists at the University of Alberta, Canada, have found that the squirrels give birth three weeks earlier in the year, corresponding to the earlier arrival of spring. More important, the scientists found that the change is partially genetic in origin — that squirrels inclined to breed earlier in the season benefit from better access to food and territory, then pass on their early-breeding preferences to the next generation.