Natural Selection Works On Humans, Too

Mining data from the Framingham Heart Study, scientists say they've been able to tease out the effects of natural selection on humans. Evolutionary biologist Stephen Stearns explains how evolutionary forces may produce shorter, rounder, more fertile women in the future.

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Evolution through natural selection is pretty straightforward in theory. But it's been tricky to document in humans. For one, it takes years to observe and it's especially hard to tease out what may be an actual shift in inherited traits from what's just background noise. To see a change, you would need to have a lot of data collected over a long period of time. Well, lucky for evolutionary biologists, just such a database exists, the data collected as part of the Framingham Heart Study. Biologists have been crunching some numbers from the study and they say there's evidence they can see natural selection at work.

Joining me to talk more about it is my guest Stephen Stearns. He's the Edward P. Bass professor of Ecology and Evolutionary Biology at Yale University in New Haven, Connecticut. Thanks for joining us today.

Professor STEPHEN STEARNS (Ecology and Evolutionary Biology, Yale University): Thank you for having me.

PALCA: And you're welcome to join our conversation by dialing 800-989-8255. That's 800-989-TALK or if you're on Twitter, you can tweet us your question by writing the @ sign followed by SciFri. And if you want more information about what we'll be talking about this hour, go to our Web site at www.sciencefriday.com, where you'll find links to our topic. So, to start with, Dr. Stearns, maybe you can tell us a little bit about the Framingham Study because it wasn't intended to show evolutionary changes. There was a different idea, I think.

Prof. STEARNS: The Framingham Heart Study is the gold standard of long-term medical studies. It was started in 1948, with the aim of trying to understand what are the environmental causes of heart disease? So, they enrolled, initially, about 5,000 people and tracked them throughout their lives. They measured them 29 times. Every time they measured them, they measured them for 80 different traits. And then they enrolled their children and their grandchildren in the study. So, it continues today. We are now more than 60 years into it and more than 14,500 people are part of it.

So, it is a wonderful long-term database. It's given us too extremely important messages for the medical community. The first was the cholesterol levels are associated with heart disease; and the second is that smoking is associated with heart and lung disease. And as such, the Framingham Heart Study actually is responsible for changing the living habits of people all around the world. It's an extremely important ongoing study.

PALCA: So, okay, that's a Framingham Study. Now, say, just a basic primer on evolution by natural selection and how you might be able to see that in action with the Framingham Study.

Prof. STEARNS: Well, if you go back to basic principles, then natural selection will recur whenever you have a population of individuals that vary in their reproductive success, that's one condition. And for humans, that means they vary in family size. Some people have a family of one children. Some have two children and so forth. There has to be variation in the number of children. So, that will be the first condition. Then if you're interested in some particular trait like, say, cholesterol or blood pressure or something like that, people would be measured for that and you would see that varied in that trait. Some would have high cholesterols, some would have low.

And then you would ask the question, does their cholesterol level actually correlate with their reproductive success? Does it have anything to do with number of children they have? And at that point if it does, then you've established the natural selection is operating. But you haven't really yet determined whether there will be any reaction to it. Because for there to be a response to natural selection, those traits have to be inherited. The children have to be like their parents. And if that's the case, then evolution will occur.

PALCA: I see. And so, what where are you able to see that hints that evolution is occurring in humans from the Framingham Study?

Prof. STEARNS: Well, we pretty much followed what I just laid out for you. We first went through and we determined how many children the women had in the Framingham study in the first two generations. And then we took the traits of medical significance - like cholesterol level and blood pressure and so forth. And we also added in age at first birth and menopause, because those are very interesting and important traits. And we asked how they were co-related with reproductive success.

However in doing so, we first had to try to take into account all of the other things that could be influencing that had been measured in Framingham. So, what we were really looking at was the independent co-relation, say, of cholesterol level with reproductive success after taking account of things like education level and so forth to the degree that we could. The Framingham Heart Study wasn't really set up to study income and ethnicity and things like that.

So, there were certain factors we couldn't build into it. But to the extent that we could, we tried to measure the biological effects independent of the cultural effects.

PALCA: Right. Now, if humans are animals and natural selection was involved in the appearance of every species that we see on earth, if that theory is correct - and I know most biologists and scientists around the world think it is - why wouldn't natural selections still be at work on humans?

Prof. STEARNS: Oh, that's a very good question. I think that the result that we have here is no surprise to any evolutionary biologist. It's exactly what we expected to see. I think the reason that the study has some impact is that not everybody is an evolutionary biologist. And, in particular, it's interesting that there has been kind of a long standing view in the medical community, that because medicine has been so good at dealing with childhood disease - there are very few deaths now, relatively few deaths from infant mortality, few people die as teenagers, most people survive until they start having children - that evolution therefore had stopped. But that was due to the misapprehension that evolution is the survival of the fittest. It's not. It is the survival of the reproductively most successful or the increased representation of the reproductively most successful. So, in a sense, this study is aimed at simply trying to lift the level of discussion and get everybody on the same page.

PALCA: And, maybe, I guess, we haven't exactly spelled this out, but what features did you see and why would you expect, you know, as we said in the beginning of this that women might turn out to be shorter and rounder in the future based on what you've seen.

Prof. STEARNS: Well, we did find that we predict they will be slightly shorter and slightly plumper. But I think the broad take home message is, natural selection seems to be broadening the reproductive options by increasing the breadth of the reproductive window. In other words, we predict that they will start having their first baby a little earlier and they will reach menopause a little bit later. We also predicted that they will have better cardiovascular health.

So, natural selection actually seems to be improving things like cholesterol and blood pressure in a positive way. And that's helpful, of course. But then on the issue of why might they be a little bit plumper, I'd like to put that in perspective. You have to realize the women who go into the Framingham database, include people who are born in the 19th century. So, we have some women who are born in the 1890s and they experienced conditions both in the early part of the 20th century and then in The Great Depression that were pretty stressful.

And then later on in the study - it continues on, of course, through the '70s. And in the '70s we had the jogging craze where, you know, women were doing -staying fit, so they lost weight there. And a woman has to have about 20 percent body fat before she will ovulate, menstruate and be able to conceive a child. And the reason that she needs that 20 percent body fat is that it's her security that she'll be able to make enough milk to actually take care of her baby once it's born.

So natural selection, actually evolution, has set up our physiology in such a way that we don't reproduce unless we'll be able to take care of the baby. So I really think that all our study is doing is picking up that effect. We have some women in it that are quite skinny, and we have others that are kind of normal or pleasingly plump or just, you know, attractively well-rounded, if you will. And our correlation is showing that it's the ones that have a little bit more body fat, kind of a normal body fat, that are having the babies.

PALCA: All right. Let's take a call now and go to Terry(ph) in Lewes, Delaware. Terry, you're on the air.

TERRY (Caller): Thank you. My question is, and I guess I should preface it by saying I'm a firm believer and supporter of evolutionary theory. I'm a science teacher myself. But my question is how can you determine human reproductive success in a study that involves just three generations or whatever, when humans reproduce by choice so much more now. And personally, you know, I didn't want more than two children, but I don't think that I would have had any reason - I mean problem having many more, you know.

PALCA: Interesting point. So in other words, you could have had 12, and you would have greater reproductive success as measured by this study, but you simply - I got it, okay.

TERRY: But I chose not to. So…

PALCA: So Dr. Stearns, how do you cope with that kind of an issue?

Dr. STEARNS: Well, I'm not surprised. This paper has caused some stir, and I've had questions all week long. I think it's important to realize that the amount of variation in reproductive success that we can account for with the stuff we can measure in Framingham is only about five percent.

So we're pulling out a small biological signal out of the background of a great deal of environmental and cultural noise. So that leaves 95 percent as the realm of other stuff, which can very well include free will and choice.

So I think that, you know, my response basically is because the study is so long, and it involves so many people, we were able to get this biological signal out of all of that cultural background. But it's a small signal, and it leaves great scope for individual choice.

PALCA: Okay. I think we have time for one more quick call. Let's go to Zach in Salt Lake City. Zach, we only have a minute or so. What's your question?

ZACH (Caller): I wanted to ask, I've heard it said before that sickle-cell anemia and malaria, they correlate in some respect with increased survival and therefore increased reproductive success. I'm just wondering: How does - have you compared that data with this study, and…

PALCA: Interesting. Zach, I'm going to ask Dr. Stearns to respond because we only have a second or a few seconds left.

Dr. STEARNS: Zach, you're absolutely right. Malaria does act as a selective agent on sickle-cell anemia in Africa. But when you take those people and move them to an area that doesn't have malaria, and it's not acting anymore, then sickle-cell anemia is something that's bad for the people who have it, and the trait tends to disappear. So that's a very dynamic evolutionary thing that is actually currently going on all over the world.

PALCA: And just very briefly, you intend to continue looking at these people in the Framingham study? Is there more you can get from this?

Dr. STEARNS: Well, we have another paper in press on the men. And that - well, not in press. We have a paper in preparation on the men, and they also are under selection for decreased age at the point where they have their first child.

PALCA: Okay.

Dr. STEARNS: Interestingly, they aren't under as much selection as the women are for improved cardiovascular health. So there are some interesting differences. But generally speaking, there's a lot of stuff there, and there will be more papers.

PALCA: Okay. Well, we'll have you back when those come out. Thank you very much for joining us. Steve Stearns is a professor of ecology and evolutionary biology at Yale University. We'll be right back after a short break.

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