First Person: Jim Smith
An interview with Jim Smith about his work as an evolutionary biology researcher and evolution educator.
For all the thousands of words in his classic work On the Origin of Species, Charles Darwin never thought he would have the last word on speciation, the means by which distinct new species arise from existing ones. Indeed, in the first paragraph of his introductory chapter, he calls speciation “the mystery of mysteries.” Darwin’s theory of natural selection has borne the test of time, but to this day researchers are continuing to puzzle out the exact mechanisms whereby new species form. Geographic isolation as the primary means of reproductive isolation has been the prevailing view, but more recently another possible mechanism has gained traction: ecological speciation, in which one or several populations within a species carve out their own ecological niche, neither displacing the original species nor maintaining a significant genetic exchange with it. Such a process is thought to have led to the emergence of many new species in the past and continues to do so today, closely observed by scientists such as Jim Smith, professor of biology at the Lyman Briggs College and the department of entomology at Michigan State University. In a recent interview with Senior Editor Sandra Ackerman, Smith talked about his work as an evolutionary biology researcher and evolution educator.
What is distinct about the particular kind of fruit fly that you study?
My research focuses on Rhagoletis fruit flies, which belong to the family Tephritidae. These are true fruit flies, by the way, defined as flies that lay their eggs in living tissue, typically while it’s still attached to the host plant. The insects of the family Drosophilidae, which go by the name of fruit fly in everyday conversation, lay their eggs into decaying fruit, so strictly speaking they are not true fruit flies.
Rhagoletis pomonella, the apple maggot, has been a leading example of ecological speciation—that is, giving rise to distinct species on the basis of a switch to a new host plant, rather than by geographic isolation. In North America before the 1600s, R. pomonella had been happily living on the fruit of wild hawthorn plants (Crataegus spp.), but when the European settlers began cultivating apples here, some of the R. pomonella populations switched to the apple as their host plant instead. Adaptation to a new host included the use of the plant as a rendezvous point for courtship and mating, because these flies mated with others that they found nearby and then laid their eggs within the host plant. This appears to have led to the genetic isolation of the apple-infesting Rhagoletis in a relatively short time, evolutionarily speaking. Our thinking is that the apple- and hawthorn-infesting flies are now on independent evolutionary trajectories.
Does every host plant support a different species of Rhagoletis?
Well, the picture’s a little more complicated than that. In a paper published last year with my colleagues at MSU and the University of Notre Dame, we showed that a different species —Rhagoletis cingulata—infests three different kinds of cherry tree: black (Prunus serotina), sweet (P. avium), and tart (P. cerasus) cherry trees. The same flies attack all of them.
We put a lot of time into the Rhagoletis fieldwork: Not only do you have to find trees that are infested, but you have to come at exactly the right season to collect larvae (maggots), bring them back to the lab, rear pupae and adults, and then sequence their DNA to look for population- or species-level differences. And after all that, we didn’t find enough genetic difference to indicate host-associated differentiation, which might allow us to consider them different species! But that’s okay; you have to know what’s out there.
After studying the cherry fruit fly, where did you go next?
We’re now studying the phylogenetics of species that feed on the seed cones of the eastern red cedar (Juniperus virginiana) and other juniper species. R. juniperina have been very difficult to place phylogenetically within the genus. They’re not intermediate between two other species but simply unplaced within Rhagoletis, and we think they may have been involved in an adaptive radiation. So we’d like to know which ancestral hosts were being used when this adaptive radiation was initiated. The first part of this story was published in a paper this year in The Great Lakes Entomologist. We’re seeing more genetic variation in Rhagoletis on junipers in North America than we expected. We were surprised and excited when we saw how much genetic variation there was, both within a small geographic area and between juniper host plant species. Rhagoletis biodiversity, and insect biodiversity in general, is extremely undersampled, so we want to focus our efforts in that direction.
As an evolution educator, how do you present evolutionary concepts to students?
My colleagues and I have developed online materials for exploring biological systems in which all the components of the evolutionary pathway are known, from soup to nuts. Examples of such systems are powerful, but they’re not all that common; traditionally, evolution has been taught from a natural-history or whole-organism ecological perspective. But molecular and cell biological evidence of evolutionary processes is compelling. Once students see the entire pathway of evolution, from variation at the level of the DNA to the cell biology consequences of this variation, and the subsequent differential adaptive fitness of organisms, it’s very hard to interpret the observations any other way than as exemplars of ongoing evolutionary processes.
How do you lead students to think about questions of evolution?
I often find that one way to make evolution relevant to undergraduate students is exploring evolutionary principles in a medical context. This is the exciting new field of evolutionary medicine. A large number of the biology majors in my classes are pre-med, but many of them have never taken a course in evolution. They haven’t had occasion to think about the relevance of evolution to their future practice of medicine—and yet, when presented in the right way, it’s intrinsically interesting to students who are heading into healthcare professions.
So a lot of odds and ends of biology that they had in the back of their minds now start to fall into place?
That’s right. The context of human health and disease really brings the whole idea of evolution home to students, not as an abstract theory but as a whole framework for thinking about connections among various life forms, individual development, and the practice of medicine. And there are so many great examples from the health professions that we can use that people care about.
One of the first examples that comes to mind, of course, is cancer. Cancer is basically somatic cell evolution, so it makes sense that evolutionary thinking will allow us to better understand it. How does a cell or group of cells within a tumor become metastatic? Is it necessary for tumors to be well developed, or can metastasis evolve early on? Through the lens of evolutionary medicine, we can also do a much more fine-grained genetic analysis, often involving whole genome sequences of individual tumor cells. This lets us identify the genetic mutations responsible for different tumor types and look at the variation within and between individual patients, and then we have a more specific idea of what we’re dealing with in each case. We know that some treatments work better for certain cancer cell types than others, so patients can receive treatment that has a higher probability of being effective.
Another instance where the principles of evolution may be starting to shed light on our understanding of disease (and perhaps eventually medical practice) is in how we think about mental health. Bernard Crespi, at Simon Fraser University, has put forward the concept of autism as the opposite of schizophrenia, at the other end of a single spectrum. Crespi and his colleagues have characterized phenotypic differences in groups of people with autism and schizophrenia and have used genomic approaches to identify reciprocal copy-number variants that may mediate the risk of autism versus schizophrenia. Though the results are not conclusive, the point is they’re looking at some of these problems in a different way, trying to understand how they work and how people might best approach treatment.
Then there’s the question of why so many allergies seem to be more prevalent today than, say, a century ago. One possible explanation is evolutionary, that when people grew up in environments that were less hygienically controlled, they faced more immune challenges as children and thus their immune systems had a higher threshold of reaction—in other words, it took more of an irritant to trigger an immune reaction. This “hygiene hypothesis” is still being tested, and it’s worth considering this evolutionary explanation to see how well it explains observed data. As one of the leaders in evolutionary medicine—Randolph Nesse, whose Center for Evolution and Medicine at Arizona State University recently hosted the first meeting of the International Society for Evolution, Medicine and Public Health—likes to point out, our bodies are the products of evolution in an environment that doesn’t exist today. So it’s not surprising that we are susceptible to disease.
There are many more cases where evolutionary medicine may be able to shed new light on medical puzzles. I’m sure we’ll continue to discover more and more of them.
You’ve taught a senior seminar on evolutionary medicine for a number of years now, using this topic as a way to introduce core concepts of evolution to students who might otherwise never learn about them. How would you rate this approach so far?
I think it’s been remarkably effective, even more than I had hoped. In 2011, when I co-organized a symposium on evolution in humans and their close relatives at the annual American Association for the Advancement of Science meeting in Washington, DC, I took a busload of undergraduate students from MSU along with me. And they did me proud! To see my 20-year-old students—many of them in Washington for the first time in their lives—packing a seminar room at 8:30 on a Sunday morning, absorbed in a talk on glucose transport differences between chimpanzee and human brains, was one of the highlights of my teaching career.