Scientific American (Aug. 31) -- To survive hostile environments, an organism often has to acquire new traits. But the rules of evolution appear to restrict how many such characteristics it can optimize at once. In a new study, researchers, led by Seppe Kuehn, a biological physicist at the University of Illinois at Urbana-Champaign, say they found that some bacteria make a genetic trade-off: the microbes involved were able to develop only one of two new traits and selected the one that best helped them thrive in a given setting.