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Evolution of Evolution — Text-only | Flash Special Report
The Mythology of Natural Selection

Understanding how natural selection acts on DNA base pairs

By Hopi Hoekstra

More than any other scientist, Charles Darwin changed the way we view the world.  In providing a simple mechanistic explanation of how biological diversity is generated, he not only revolutionized the field of biology, but also the way we think about ourselves.  He argued that the apparent design we observe in nature—the close fit of organisms to their environment—could be explained by what he called natural selection, the differential success of genetic variants in a population.  It is remarkable that Darwin was able to formulate his ideas without any knowledge of the mechanism of inheritance because evolution by natural selection is an explicitly genetic theory. The complete story would not be told until a century later, with the discovery of the DNA double helix, for in this four-letter DNA code we can dissect out the mechanistic specifics—the genetic nuts and bolts—of how biological diversity arises.  Oh, wouldn’t Darwin be proud!

Natural selection can act on specific DNA base pairs to contribute to variation in form, physiology and behavior. Research in our lab is focused on understanding how organisms adapt to their environment by identifying the precise DNA base pairs responsible for these modifications. One of the traits we study is coloration.  Color and color pattern can be used to hide from predators, like the white winter coats of snowshoe hares, or to attract mates, like the brilliant blue-green tails of male peacocks—even small variations in color can have a large effect on an individual’s ability to survive and reproduce.

In the southeastern United States, oldfield mice (Peromyscus polionotus) typically occupy overgrown fields with dark soil, and accordingly, have a dark-brown coat, which serves to camouflage the mice from predators.  In the last few thousand years, these mice have also colonized the brilliant-white sand dunes of Florida’s coasts. Here, these beach-dwelling mice are almost completely white, blending perfectly into their new environment. Using a combination of field studies, classical genetics and modern molecular biology, we are working to understand how—through changes in pigmentation genes—these mice have adapted to their new environment.

Our work has revealed several interesting patterns.  First, we have found that most of the differences in mouse fur color are caused by changes in just a handful of genes; this means that adaptation can sometimes occur via a few large mutational steps. For example, we identified a single DNA base-pair mutation in a pigment receptor, the presence or absence of which accounts for about 30 percent of the color difference between dark mainland mice and light beach mice on Florida’s Gulf Coast.  To date, this is one of the few examples of how a single DNA change can have a profound effect on survival of individuals in nature.

Second, we have shown that the same adaptive solution can evolve by different genetic pathways.  Beach mice are not just restricted to Florida’s Gulf Coast, but are also found over 200 miles away on the Atlantic coast.  We have shown that mice on the eastern coastal dunes have also evolved light-colored fur, but through different mechanisms: the pigment-receptor mutation causing light color in Gulf Coast mice is absent from the eastern beach mice. Thus, similar evolutionary changes can sometimes follow a different path.

Using a variety of approaches—some, like molecular biology, way beyond Darwin’s ken, and others, like fieldwork, rooted firmly in Darwin’s own tradition of natural history,—we are discovering the genetic basis of what Darwin called “that perfection of structure and coadaptation which most justly excites our admiration,” and in so doing, provide molecular evidence for Darwin’s great theory.

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Hopi Hoekstra received her B.A. in Integrative Biology from the University of California, Berkeley. She completed her Ph.D. in 2000 as a Howard Hughes predoctoral fellow at the University of Washington.  For her dissertation work, she received the Ernst Mayr Award from the Society for Systematic Biology. She then moved to the University of Arizona as an NIH postdoctoral fellow, where she studied the genetic basis of adaptive melanism in pocket mice and was awarded the American Society of Naturalists Young Investigator Prize. In 2003, Hoekstra became an assistant professor at the University of California, San Diego, and was named a Beckman Young Investigator. In 2007, she moved to Harvard University, where she is a John L. Loeb associate professor of biology in the Department of Organismic and Evolutionary Biology and the curator of mammals at the Museum of Comparative Zoology. She serves as an associate editor of Evolution, a member of Faculty of 1000, on the council of the Society for the Study of Evolution and the American Genetics Association, and the scientific advisory board of the National Evolutionary Synthesis Center.

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Please see the Resources section for the Bibliography/Additional Reading list for this essay.