Of mice and mammoths
Details of a species' DNA can shed light on evolutionary adaptation.
To learn how an extinct mammoth managed its hair color, just ask a mouse. A simple mutation in a single gene enables some beach mice to adopt hair-color patterns that blend in with the sand. DNA from a bone of a 43,000-year-old Siberian woolly mammoth shows the same mutation. This fact implies that mammoths and mice used the same genetic mechanisms to color their hair to fit their lifestyles.
This news from genetic explorers, reported last Friday in Science, illustrates how details of a species' DNA can shed light on evolutionary adaptation. If those explorers can get enough of the stuff from extinct species, it would also give us a more accurate look back in time.
Holger Römpler at the University of Leipzig in Germany and colleagues, who studied the mammoth bone, warn against making too much of their discovery. They note that "it is currently impossible" to know the adaptive value of hair color for mammoths. But their work does show that scientists can discover the function of ancient genes.
"This opens up the possibility of studying a wide range of extinct species' features invisible in the fossil record," they say.
The gene in question provides codes for a protein that is a key factor in determining hair color in animals, including humans. Like all proteins, it consists of subunits made from amino acids. Hopi Hoekstra at the University of California at San Diego and colleagues show how a small change in just one of these subunits correlates with hair-color change in beach mice that live on Florida's barrier islands. Mice that live on light sand are lighter colored than their mainland cousins.
Geneticists have known that changes in how genes are regulated account for many differences between organisms. Now, the mouse and mammoth studies provide "striking examples of how amino acid changes in structural proteins can also be important," Dr. Hoekstra says. This gives an organism the potential to adapt quickly to environmental changes. The barrier islands are less than 6,000 years old. "So these color mutations may have evolved rapidly," she adds.
While this genetic mutation isn't the only factor allowing beach mice to camouflage themselves, it does account for up to 36 percent of the color variation in some of their populations. That is "a large mutation," Hoekstra says. It shows "that adaptation does not always occur gradually, but may happen in these relatively large jumps."
Getting at that kind of information for an extinct species is tricky. Old bones and other remains don't usually yield clean or complete DNA samples. Advances in chemistry enable scientists to make the most of what they find, especially by allowing them to make multiple copies of the samples. New techniques to analyze DNA rapidly also help. Using such techniques, Michael Hofreiter at the Max Planck Institute for Evolutionary Anthropology in Leipzig showed that mammoths are slightly more closely related to Asian elephants than to African elephants. They split from a common ancestor that lived about 440,000 years ago.
All of this work represents just the first steps in a new way to explore evolutionary change. Minute details of DNA structure not only give insight to the adaptations we see around us, they also give clues to the past that no paleontologist has had before.