Transcript of the Interview with Dr. Joseph Thornton on the Direction of Evolution
September 2, 2009
NIGMS: This is Stephanie Dutchen at the National Institute of General Medical Sciences at the National Institutes of Health. And I'm here with Dr. Joseph Thornton, who's an evolutionary biologist at the University of Oregon. Dr. Thornton, thanks for talking with me.
JT: Thank you for having me.
NIGMS: In your Nature paper this week, you found that evolution can only go forward in time - that it's not possible to return to an earlier state. What does this mean for people who have been studying evolution and wondering how it works?
JT: Well, it means a few things. One is that even if selection pressures change to favor something like the ancestor, you'll never get the same thing back. Adaptation will find some new way of performing the old function.
The other thing has to do with if natural selection can always find its way to the best possible solution to a given biological problem. And what our work suggests is that the pathways that are open to evolution depend on where the evolutionary process happened to start and where it wandered along the way.
So you can think of it this way: If we could rewind the history of life and let evolutionary history happen again, it's likely that different restrictive mutations would occur, some pathways that were actually followed during our history would become closed, others would open up, and we'd end up with a biology very different from the one we ended up with today. That's a way of saying that the bodies we have are just one of many possible roles of the evolutionary dice.
NIGMS: How confidently can we extrapolate from this one protein you were studying to human evolution or all of evolution?
JT: Yeah, this is a very good question. This is the first study of its kind. Whether our findings can be generalized to most other proteins and levels of biology, or if they're just sort of an odd one-off case, I can't answer for certain. But I can make a pretty rational prediction based on the reasons for irreversibility. And here's what I think:
Whenever the basis for the evolution of a new function is complex in that it involves multiple mutations that have to occur in combination, evolution will be irreversible. There will be some cases where the evolution of a new function has a very simple basis - where it just takes one mutation or it takes a couple mutations where each one gradually improves the function in a stepwise fashion. In those simple, linear cases, evolution should be quite reversible.
NIGMS: So the more complex a trait or an organism is, the less likely it is to be able to reverse evolution.
JT: That is what I expect we'll see as further studies are done.
NIGMS: And so what does come next?
JT: One important thing is for us and other groups to repeat this kind of study on other proteins and other traits to see how general it is. The other thing that would be worth figuring out is that if there are other possible histories, other trajectories that evolution could have taken, how many other pathways there could have been and what lies at the end of them.
NIGMS: Well, thanks for talking with me.
JT: That's great, thanks very much.