Evolution of Evolution — Text-only | Flash Special Report
Interview with Hopi Hoekstra
BOBBIE MIXON: Biologists say evolution in organisms is ongoing all the time. Some changes increase the chances of survival for a species, while others do not. Dr. Hopi Hoekstra is a John L. Loeb associate professor of biology in the Department of Organismic and Evolutionary Biology at Harvard University. She works to identify specific genetic markers that allow organisms to evolve and increase their rate of survival and she looks for clues in a most unusual place, the DNA base pairs responsible for fur color in mice. Dr. Hoekstra, what convinced you that researching mice would be a good idea?
DR. HOPI HOEKSTRA: That's a great question. My parents actually asked me the same question. I think the short answer is, after I graduated from college I fell prey to the lure of studying charismatic megafauna and I worked on grizzly bears, but I quickly learned that getting enough samples to say anything was statistically significant was very difficult. So, mice offered the opportunity to catch large sample sizes and, in fact, nobody minds if you go out and catch mice. They're easy to work with but at a much more practical level. There's lots of variation in mice. They vary a lot depending on what type of environment they live and for the types of questions we're interested in, there's a large number of genomic resources that make it possible to explore mice at the genetic level as well.
BOBBIE MIXON: And what kind of questions are you interested in?
DR. HOPI HOEKSTRA: So, we're very interested in trying to understand how organisms adapt to their environments, and to do this, we probe the genomes of these mice trying to identify DNA base pair changes that are responsible for differences in the way organisms either look, their morphology or the way they behave.
BOBBIE MIXON: Now, we know that Charles Darwin was a great observational naturalist. What are the observed differences between the old field mice and the beach mice that you study?
DR. HOPI HOEKSTRA: So, the most obvious difference that's quite striking is that beach--is in their color. So, beach mice, relative to their inland ancestors, are very pale and have a unique color pattern, making them camouflaged from predators on Florida's brilliant white sand beaches where they live.
BOBBIE MIXON: Are there differences we can't see?
DR. HOPI HOEKSTRA: Most certainly there are, for example, their physiology. They probably have--beach mice probably have quite a unique diet because the food that's available to them on these beaches is quite limited. Most likely there are also differences in, for example, behavior because they live in much more open habitat and are more prone to predation.
BOBBIE MIXON: Now, there's something that interests me. Would the chances for survival of a beach mouse be greater, less or about the same as a field mouse if it was transplanted to the southeastern portion of the country where field mice live?
DR. HOPI HOEKSTRA: That's a really interesting question and, in fact, it's a question that we're addressing experimentally right now. Because the beach mice are white in color and are well adapted to living in white sand beaches, if you were take that mouse and put it in an old field, for example, where the soil is dark in color, they would stand out as a big target for predators and we're testing this by making models of mice that vary in color, putting them out in different substrate colors and recording predation events. And, in fact, we're showing that white mice on a dark substrate are prone to much more predation than if that model mouse blended in with the environment.
BOBBIE MIXON: Let's see. I feel a schoolteacher asking you multiple-choice questions, but let me try one more.
DR. HOPI HOEKSTRA: OK.
BOBBIE MIXON: Are changes in mice color a result of natural selection acting on DNA or are they a response to environmental conditions, a response to threats from predators, a response to sexual selection or is it a combination or all of the above?
DR. HOPI HOEKSTRA: I guess in some ways you could say it's a combination. The natural selection is going to favor the best phenotype but this, of course, can have an affect on the patterns of genetic diversity at those genes that are responsible for phenotypes so, for example, on pigmentation chains. So, you'll see the signature of natural selection in the DNA variation. Of course, what the best phenotype is depends on the environmental condition and in the case of these beach mice, we're not sure about sexual selection. We don't know if differences in color affect mate choice, for example, but it's something that we're actively studying and so we may have an answer for you soon.
BOBBIE MIXON: Now you adaptive solutions can evolve via different genetic pathways. Do you find it curious--I find it curious--that the seemingly preferred evolutionary change in these mice is fur color? Why haven't we seen them develop teeth like a saber tooth tiger or a horn like a rhino to ward off predators?
DR. HOPI HOEKSTRA: That's a really interesting question. So, I think the first answer is that evolution really can only work with the variation that exists in a population, that variation that's available for natural selection to work on, and so if there's not variation in tooth size or presence or absence of a horn, for example, then evolution can't work on those traits. There are also constraints. So, for example, mice use their teeth for grinding up plant material. If all of a sudden they started to develop teeth that were longer or bigger, that may affect their ability to feed. But I think the most straightforward answer is that for mice to survive, it's better to hide from their predators than to try to fight them off.
BOBBIE MIXON: And that's Dr. Hopi Hoekstra, biologist, Harvard University.