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Linanthus parryae (pronounced “parry-eye”) is a tiny annual plant living in deserts and dry regions of California (U.S. A.). In years with lots of rain, so many L. parryae germinate that the ground is covered with the little white-flowered plants, whose common name is “desert snow.” But in some places, the “snow” is purplish-blue, because, though white-flowered plants are more common overall, L. parryae can also be blue-flowered. And there are some populations that are almost all blue-flowered. This has made them very interesting to evolutionary biologists who want to understand the forces behind genetic differences within and between populations. Since flower-color is a genetic difference we can easily see, L. parryae is a good organism for those kinds of studies.
Why are some populations of L. parryae almost all white, some almost all blue-purple, and some mixed? Does natural selection favor white-flowered plants in one place and blue-flowered plants in another? Or maybe flower-color (and any traits that might be linked to it) is neutral, i.e., it doesn’t affect the plant’s survival. If that’s the case, than it must be the randomness of genetic drift that has resulted in these L. parryae color mosaics. In the 1930’s the geneticist Sewall Wright developed mathematical models that showed that in small populations one type of a gene can become “fixed” as a result of randomness in who survives and reproduces. In the early 1940’s, Carl Epling and Theodosius Dobzhansky, the first to study L. parryae, concluded that genetic drift was responsible for the flower-color patterns they saw, and they cited Wright’s models (Epling & Dobzhansky 1942). Wright analyzed their data and agreed that genetic drift was the answer (Wright 1943).
But Epling and others returned to the desert and collected more data. Their conclusion this time was that it’s natural selection, not genetic drift acting to create and maintain the flower-color differences: “The conclusion seems warranted, therefore, that the frequencies of blue and white flowered plants are in the long run the product of selection operating at an intensity we have been unable to measure…” (Epling et al. 1960). However, Wright still believed that genetic drift was responsible (Wright 1978). Wright “won,” and Linanthus parryae became a textbook example of genetic drift acting in natural populations. But no one had yet collected any data actually measuring fitness, i.e., survival and reproductive success, in L. parryae to see if there were differences in how well white-flowered plants did vs. those with blue flowers. So beginning in the early 1990’s, evolutionary biologists Paulette Bierzychudek and Douglas Schemske began many years of data collection and experiments to figure out whether Wright was really right (Schemske & Bierzychudek 2001, Turelli et al. 2001, Pennisi 2007, Schemske & Bierzychudek 2007).
After over a decade of detailed, meticulous, (hot, dry, & dusty) work in the Mojave Desert and in their laboratories, they concluded that Wright was wrong. It’s actually natural selection, varying in space and time that has created the differences in flower-color among populations of L. parryae. In years with overall high seed production, white-flowered plants make more seeds. But in bad years, blue-flowered plants make more. That explains the mixed populations. Schemske and Bierzychudek also found that there are some places where ecological differences between different areas favor one flower-color over the other, and these areas can be separated by very short distances, explaining the all white and the all blue-flowered populations. So though genetic drift can still explain differences among populations of some species, we now know that flower-color in Linanthus parryae is not an example.