Linanthus parryae is a tiny annual plant of the Mojave Desert. Like a number of other members of the family Polemoniaceae, individuals vary genetically in having either all-white or all-blue flowers. While genetic variation is abundant in all plant and animal species, it is rare for it to be so obviously visible. This property of Linanthus made it an ideal candidate with which to study the evolutionary forces responsible for maintaining genetic variation.
Over 50 years ago, evolutionary biologists studying Linanthus, including Th. Dobzhansky, C. Epling, H. Lewis and S. Wright, debated whether it was spatially varying natural selection that maintained the pattern of flower color variation, or whether it was genetic drift. Sewall Wright, a theoretician and one of the founders of modern evolutionary biology, maintained that genetic drift was the correct explanation. Harlan Lewis and other field biologists disagreed. The debate was never resolved.
In collaboration with Douglas Schemske of Michigan State University, I and many of my students studied Linanthus parryae over 12 years. We have collected strong evidence that genetic variation for flower color in Linanthus parryae is in fact maintained by spatially-variable natural selection.