Wild populations must continuously adapt to environmental changes or risk extinction. For more than fifty years, scientists have described instances of "rapid evolution" in specific populations as their traits (phenotypes) change in response to varying stressors. For example, Spanish clover has developed a tolerance for copper from the mine tailings in which it grows, and the horn size of Alberta bighorn sheep has decreased due to trophy hunting. But until now it hasn't been possible to reach any overarching conclusions about how different factors (such as harvesting, climate change, invasive species, or pollution) shape this rapid (now called "contemporary") evolution.