Have Humans Stopped Evolving?

Yesterday's Quirks & Quarks radio show had a segment on human evolution. Here's the description from their website:

January 6, 2007: Are Humans Still Evolving?

Evolution has made us what we are today, and we're increasingly learning what made modern humans different from our ancestors. But many scientists think that we have now removed nature's control over our genetic legacy. Our technology allows us to control our environment and survival to the degree that we may have stopped human evolution altogether. Is our growth and development as a species at a standstill? If not, what will we become in the future? Find out this week on Quirks & Quarks.

Listen to the podcast. The segment on human evolution starts about one third of the way through the show.

The idea that humans have stopped evolving is ridiculous. It reflects a deep and fundamental misunderstanding of evolutionary theory.

Fortunately, the blurb on the website doesn't reflect what was broadcast. The host, Bob McDonald actually does a very good job of sorting through the rhetoric and the show is an excellent summary of current scientific thinking. It's by far the best thing about the rate of human evolution that I've ever heard on public radio.

One of the people interviewed on the show is Steve Jones from University College, London. Jones claims that almost everyone is reproducing these days so natural selection isn't affecting humans any more. He contrasts the situation today with that in Shakespeare's time when 2 out of 3 babies didn't survive to adulthood.

This is one of the weaker parts of the show. The claim that natural selection isn't working on humans is false. It is refuted by Jones himself later on in the broadcast, and by Noell Boaz from Ross University in Dominica.

Let's deal with the increase in longevity that we've seen in some societies over the past 500 years. We'll dismiss the obvious bias in equating what happens in Caucasian societies with evolution of the entire species. What about the fact that people in London live longer today that they did in 1600? Does this have anything to do with evolution?

Jones thinks so. He says,

Now a lot of those deaths in the old days were due to genetic differences but if everybody stays alive, everybody gets through, no more natural selection.

I don't think so. It isn't obvious to me that people were surviving in 1600 because they had better genes. People died for all kinds of reasons that had nothing to do with genes. A famous example from the nineteenth century was the London cholera outbreak. In that case, you died if you were close to the contaminated Broad Street Pump and not because you had bad genes.

If you died of infection or malnutrition in 1600 it was probably due to bad luck and not bad genes. As living conditions improved, everyone benefited equally, not just those who might have been genetically susceptible. Thus, natural selection wasn't all that important back then and most of the improvements in health in developed countries have affected evolution directly. (The quibblers are waiting to pounce, so let me address two objections to that statement. First, there are other, more modern, medical advances that do affect selection—wait for them. Second, there are some examples of genetic effects on whether you survive disease. Some people might have been more resistant to the Black Plague, for example. Such examples are exceptions to the rule. The common assumption that most deaths in the past had something to do with natural selection is what I'm addressing here.)

So, let's be skeptical about the specific argument that Jones is making, namely that increased longevity, per se, is proof that the effect of natural selection is diminished in modern societies. A lot of negative selection—selection against less fit individuals—is still taking place in utero just as it always has. Lethal mutations result in spontaneous abortion or failure to produce viable sperm and eggs. This form of natural selection hasn't changed significantly. Also, even though severely handicapped children born today may survive longer, they probably won't reproduce.

On the other hand, there are medical advances that do affect natural selection. The most obvious one is the invention of eyeglasses. As Jones points out in the show, people with a genetic disposition for bad eyesight can now survive whereas back in the hunter-gatherer days it might have been much more difficult. Thus, natural selection in favor of good eyesight has been relaxed because of eyeglasses.

What does that mean for human evolution? To its credit, the Quirks & Quarks show doesn't jump to the false conclusion so common among the general public. Evolution hasn't stopped, it has increased! The removal of negative selection causes previously detrimental alleles to survive in the population; therefore, their frequency increases. Thus, evolution is happening today but was blocked by negative selection in the past.

The same argument applies to all medical advances that allow for previously handicapped individuals to survive in modern society. Human evolution is being accelerated. This is a point worth emphasizing because the opposite conclusion is so common. Most people think that removing strong negative selection means that evolution has stopped when, in fact, the exact opposite is true! The misconception arises because the general public thinks of evolution as a progressive improvement in the gene pool. Modern medicine is allowing "defective" individuals to survive. This can't be evolution according to that false understanding of evolution. (There are other things wrong with that false argument; namely, the concept that people with myopia or diabetes are somehow lesser citizens. This isn't the place to get into that discussion.)

Strong negative selection acts as a brake on evolution. It slows evolution down. Remove the brake, and evolution speeds up.

There's more to evolution than natural selection. Bob McDonald interviews Katherine Pollard from the University of California, Davis. She points out that much of evolution is due to random genetic drift. Drift has nothing to do with natural selection, so whether or not selection has decreased will play only a minor role in whether humans are evolving. You can't stop drift and you can't stop mutations. You can't stop human evolution. As McDonald puts it, "we still will evolve ... it's not the kind of evolution we imagine."

Evolution is not just the result of survival of the fittest. Furthermore, it is not progressive in spite of the fact that this misconception is widespread. As McDonald says in closing, "... this is an illusion about the way that evolution works. Evolution has never guaranteed improvement or progress, just change."

Change is good. It's good that humans are evolving. Things can only get better, right?