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Voles as Model Organisms in Evolutionary Biology

Chris Conroy
Biological Science Department
Stanford University
May 2001

Voles are among the most abundant mammals today, as they have been throughout the last two million years, and are widely studied by biologists and paleontologists. Because their existence is so widespread in time and space and they display a broad variation in tooth and bone morphology and ecology, there is a lot of knowledge about evolution in general that we might gain by studying voles from both modern and fossil perspectives.

After investigating voles from a modern, molecular perspective, I am looking at ways to use their fossils to learn more about animals and evolution in general. My interest is in using this group as a model to determine the factors involved in speciation and the accumulation of genetic variation. Understanding these processes is tantamount to understanding the preservation and creation of biological diversity, a major goal of biologists today, in light of its rapid loss in recent years.

Evolution is a unifying theme in biology. Everything alive originated from somewhere and something else. This includes everything below the level of the organism, such as cell structure and chromosomes, which come from a complex history of population movement, extinction, speciation, and natural selection. Therefore, one might suggest that investigations into the raw processes of evolution, that is, who and what survives or succumbs over evolutionary time scales, can be relevant to all aspects of biology. All sorts of biologists can gain better understanding, and perhaps appreciation, of their interests if they place them into an evolutionary perspective.

Although there are countless model animal systems that one might study, I was drawn to mammals because they can tell us much about the Earth's history since the end of the age of dinosaurs and, since we are also mammals, studying mammal evolution might lead to a few insights about ourselves. Where do we, as humans and mammals, come from? From what kind of diversity did we get our intelligence, our physiology, our eyes, ears and mouths?

Voles and lemmings are very common mammals at higher latitudes around the world. They are small rodents that generally live in colder, wetter parts of the planet and usually have small ears and short tails. They are often known as meadow mice, and include some highly specialized species like the muskrat, a semi-aquatic rodent. They have only been around for a few million years, but they are highly abundant today and in the animal fossil record. They are, in fact, so abundant in the fossil record that the appearance of certain kinds of voles helps to establish time patterns for other mammals.

I have investigated molecular evolution within vole species, among species, and in relation to other kinds of rodents. What I have uncovered is that relationships inferred from morphology, such as skull shape or teeth patterns, often don’t correspond to those one might uncover with molecular data. In my case, the molecular data has been DNA from the cell's mitochondria, the energy-creating organelle inside cells. Interestingly, one species I studied has more genetic variation than other pairs of species. Conversely, some pairs of "good" species are not as genetically different as some populations within species. Most importantly, it appears that the genetic variation one sees today did not accumulate gradually, as some might predict. Rather, it appears to have been created, or at least preserved, in rapid leaps and bounds, followed by periods of stability. There are many reasons why molecular variation might show such a pattern, and I am interested in the processes that have led to this particular one.

In the case of voles, we know much about their biology and fossil history and are thus in a good position to better understand their DNA. I am leaning towards responses to climate changes as a major factor in vole evolution, but there are many alternatives. This is the hard part in evolutionary biology; other than working with very fast living organisms like bacteria and viruses, it is generally impossible to study evolutionary processes in the lab. Research must all be inferred from pattern, hypothesizing possible culprits and testing for congruence.

Understanding the processes that have led to the genetic and morphologic variation in voles is one of my long-term goals, which I plan to approach in a few different ways. One approach involves a project with which I am involved in as a postdoctoral student at Stanford University. Other researchers and I are trying to estimate dates of species divergence from a combined molecular and fossil approach. The work involves constructing phylogenies (branching patterns which look like trees) among species of voles and lemmings, then placing fossil dates, like fruits grow, along the branches.

It's not as simple as it might sound. First, there is the laboratory work of extracting and sequencing DNA. Next there is analytical work, such as looking at mutations in DNA as steps in the history of the species; the more closely related two species are, the more similar the DNA sequences, and vice versa. Many of the methods of constructing phylogenies from molecular data (DNA sequences) are highly complex, often requiring days or weeks of computer time to construct a single phylogeny or tree.

Then there is the fossil end of things. What fossils go where on a phylogenetic tree is complicated. Methods for placing fossil dates on trees are only beginning to be invented because the theory and the computer methods of applying the theory are relatively new. Animal fossils represent animals that at some point in time were alive, but were they direct ancestors of relatives today? If so, is it a recent ancestor or an ancient ancestor? That is, even if it looks like something we see living today, it may actually be very ancient and just not have changed over millions of years. Our approach is iterative. As new genes are sequenced we can check our phylogeny for robustness and as new fossils are described we can test their age against what we have predicted from the DNA.

This work is truly collaborative in that it involves people with backgrounds in such diverse disciplines as molecular evolution, paleontology, biostratigraphy, and geology. Without such combined effort, I would have had a difficult time gathering and interpreting the paleontological data and, likewise, I was of use to my collaborators who don't have much experience with molecular data and phylogenies. Together, we are learning that fossils, which might be relatively well-dated themselves, are often difficult to ascribe to a particular time point in the evolution of a group. But, perhaps with enough data points, we can narrow our estimates and finally get to our goal of correlating climate or other variables with evolutionary events. This will potentially uncover what factors are most influential in vole evolution.

This, of course, is just the beginning. So what if some climatic variable does or does not correlate with speciation or other events in their history? Just because they happen at the same time doesn't necessarily mean they are causally related. There are numerous other factors to bring in, including behavior of single individuals, cyclic behavior of populations, and developmental biology. Voles and lemmings are one of the best-studied groups of mammals on the planet and thus show the most promise for integrating these ideas into a succinct hypothesis for how animals evolve. Our work on tying their evolution to time is an important step in this integration and will be a positive example for others.