John Willis loves figuring things out--specifically, how the wildflower Mimulus adapts to different environments. Colonies adapt to different elevations, or degrees of drought, or soil types. Some have even evolved to live on highly contaminated soil near a copper mine. Which of their genes change, and do they change in many little ways or one big way? Why do some separated groups lose the ability to reproduce with their neighbors? Are the genes that help them adapt the same ones that prevent living hybrid offspring? The Willis group tests this with "tricky crosses" between different varieties. If the copper mine variety mates with nearby types, the offspring all die. But when one parent is crossed to a third and their viable offspring to the other parent, it produces some living offspring and some that die. By analyzing which parts of the parents' chromosomes each type inherited, the lab can zero in on the killer gene. Scientists assumed that the copper-tolerant gene was the killer, but Willis recently showed that a near neighbor, which "hitchhiked" with copper tolerance into the population, was guilty. The lab also uses tricky crosses to study a genetic arms race fought inside the seed, between the parents. But that’s a different story.