Searching for Natural Selection in a Wildflower
Evening snow (Linanthus) is an amazing little wildflower that adorns desert areas of southern California. Its blossoms open in the evening, spreading fragrance across a harsh landscape. Two varieties have been noticed; one with white flowers, and one with blue flowers. Scientists noticed that the white ones sometimes grow on one side of a ravine, and the blue ones on the other; in other places, the two varieties grow in a blue-white mosaic. Is this pattern due to genetic drift (i.e., chance), or to natural selection?
Elisabeth Pennisi wrote about this in Science.1 Her opening line might open some eyes about the difficulty of deciding a question this simple: “Sixty years ago, studies of these patterns provided key support for a powerful evolutionary theory. Now, two evolutionary biologists have found that the theory doesn’t hold in this species.”
Two researchers decided to settle the debate with a long-term field study. Their decision was that natural selection was the winner, at least a little: “In the seed-transplant studies, each color flower typically did best on its own turf, indicating that selection played a role.” There may have been some environmental influences at work, in other words, that tended to make one color predominate in one environment and the other in different environments. But is anyone certain?
“The study shows the unimportance of drift in Linanthus,” says evolutionary biologist Masatoshi Nei of Pennsylvania State University in State College. “In this sense, [the] finding shakes the ground of the shifting balance theory.” But he is cautious about making generalizations, given that other studies suggest otherwise: “The relative importance of selection and drift depends on the genes and populations studied.”
1. Elisabeth Pennisi, “Natural Selection, Not Chance, Paints the Desert Landscape,” Science, 19 October 2007: Vol. 318. no. 5849, p. 376, DOI: 10.1126/science.318.5849.376.
So in a 13-year study, these scientists could only point to a little bit of natural selection that might have played a role in the color pattern of two varieties within the same species? And they expect us to believe that science has proved that humans have bacteria ancestors due to this wondrous mechanism of natural selection?