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Evolution in the fast lane?
February 2008

human chromosomes
Human chromosomes
The question of whether we, as a species, are still evolving, sometimes inspires visions of a new-and-improved Homo sapiens, complete with super-sized brain, disease-resistance, and the ability to withstand the pollutants and toxins common in a techno-centric future. While science fiction writers have come up with imaginative and entertaining answers to the question of how humans might be evolving, the responses of the scientific community have been more staid. Perhaps, they've suggested, some genes for withstanding epidemic disease are currently on the rise. However, with the improved genetic sequencing technologies that have come online in the last decade, many biologists are now prepared to offer more specific hypotheses as to how species are changing. Recently, a team of researchers led by scientists at the University of Utah announced that they'd scanned the genomes of 270 people and found evidence that humans are not only evolving — but that we've been adapting at an unusually rapid pace — at least on evolutionary timescales. The group has also made the controversial suggestion that human populations on different continents are evolving away from one another. However, a look at the evolutionary biology behind the headlines highlights some limits of this research.

Where's the evolution?
How do scientists look at the DNA of people alive today and figure out how recently natural selection acted on their ancestors? The answer relies on an evolutionary phenomenon called genetic hitchhiking (or a selective sweep). To understand, imagine that a new advantageous mutation (X) occurs on Chromosome 4, in the middle of gene versions P, Q, and R. In genetic terms, we would say that the mutation and those genes are linked — that is, they are close together on the same chromosome. The new mutation is so beneficial that its carrier leaves lots of offspring — many of whom also carry the mutation and the other linked genes. Over many generations, natural selection increases the frequency of mutation X, and because they are physically attached to X, gene versions P, Q, and R come along for the ride (i.e., "hitchhike" to high frequency). Of course, as X spreads, recombination occasionally occurs between it and its neighboring genes, breaking down this tight association somewhat. We begin to see X in association with different combinations of gene versions (e.g., with r instead of R). If we examine the population at the end of this process of natural selection, we will see mutation X at high frequency, often occurring alongside the same set of gene versions (P, Q, and R), and less frequently alongside other gene versions (p, q, and r).

genetic hitchhiking

When geneticists observe this — a stretch of DNA that often shows up in the same genetic background (e.g., X usually appearing alongside P, Q, and R) — they begin to suspect that somewhere in that DNA segment lies a favorable mutation. And by studying how long the hitchhiking genetic sequence is — for example, whether X usually drags along just P and Q or P, Q, R, S, T, and U — among other factors, we can estimate how long ago the favorable mutation arose and began to spread. Younger favorable mutations tend to retain more hitchhikers than older ones, since the longer a mutant and its hitchhikers have been around, the more opportunities recombination and mutation have had to break them up.

When Henry Harpending of the University of Utah and his colleagues applied this technique to the genomes of people who trace their ancestry to different geographic regions (Europe, Africa, China, and Japan), what they found surprised them — lots of evidence for favorable mutations! Natural selection seems to have acted on these mutants in many different areas of our genome. In fact, the team identified more than 10,000 selection events (i.e., stretches of DNA bearing the marks of natural selection) that seem to have taken place in the past 80,000 years of human history. Interestingly, the researchers found that most of these selection events traced to the recent past, with the largest numbers having arisen in the last 10,000 years. Judging by these results, human evolution seems to have sped up: small numbers of beneficial mutations spread through human populations for most of our history, but since the end of the last ice age, we've experienced a renaissance of evolutionary innovation in which many new advantageous mutations arose and began to spread. This ratcheting up of our evolution seems to correspond with the timing of major lifestyle changes — a period when many groups of humans began relying on agriculture, as opposed to hunting and gathering, and started living in denser populations. Furthermore, according to the Harpending team's evidence, people from different geographic regions seem to have experienced different selection events. Only a small percentage of these positively selected genome regions were found in more than one of the four populations studied. This could mean that people on different continents were evolving in different directions.

SNP judgments
Many current techniques for analyzing the genome — including those discussed here and those used by genealogists to trace ancestry — rely on a form of genetic evidence called a SNP. SNP stands for single (S) nucleotide (N) polymorphism (P), and they're everywhere — all over the human genome. SNPs are places where, in different people, the genetic sequence varies by a single nucleotide letter: A, T, G, or C, the alphabet of the genetic code. This genetic difference need not translate to a physical difference. For example, the fact that one person carries an A at a particular site while another person carries a G at that location may not affect them at all — or, depending on the SNP, it might cause a change that gives one of them a survival advantage. Whatever the effect on their carriers, SNPs can be used like archaeological artifacts to help reconstruct the history of the chromosomes in which they are embedded. The Harpending team used SNPs as markers to try to figure out which stretches of DNA had been traveling together as genetic hitchhikers. In the hitchhiking example given above, P, Q, and R are SNP sites, meaning that P (the gene version strongly associated with the advantageous mutation) differs from p (the alternate gene version) because of a single difference in their genetic sequence — a SNP.

These results are intriguing (and controversial — they've already generated much discussion within the scientific community), but they do have limitations. The technique that the researchers used (looking for genomic evidence of past hitchhiking events) is reliable, but it is not particularly good at detecting very old or very recent episodes of selection. That's because old advantageous mutants have been around for so long that recombination and mutation may have already wiped out the evidence of their selective sweep. Recent advantageous mutants, on the other hand, may not have spread enough to be identified at all by this technique. And this highlights a problem with the portrayal of these results in some media outlets. The scientists' techniques focused on the time period between 5000 and 80,000 years ago, but some media sources extend this window to today — suggesting that rates of human evolution are now peaking and that human racial groups are currently evolving away from one another. In fact, this sort of genomic evidence of hitchhiking can't tell us much about how humans are evolving right now — or even how our evolution might have shifted in the past 200 years as the result of the industrial revolution or more frequent global travel. Sure, over the past 10,000 years, we've come along way — but figuring out the current trajectory of human evolution is a topic that this technique can't directly address.

Nevertheless, the new results are suggestive. Humans are now able to mediate our environments with technology — to keep ourselves warm, to treat diabetes with insulin, and to provide food for those without farming, hunting, or gathering skills, amongst a myriad of other cultural innovations. So, for example, in many developed countries, the gene versions that contribute to juvenile diabetes are no longer strongly selected against. Some have argued that such technological advances mean that we've opted out of the evolutionary game and set ourselves beyond the reach of natural selection — essentially, that we've stopped evolving. However, if they are correct, the Harpending teams' results imply that technological and cultural advancement does not necessarily halt natural selection, but may rather change its direction. After all, if the cultural innovations of the late Pleistocene spurred human evolution in new ways 10,000 years ago, perhaps the technological innovations of the past 200 years are simply changing the rules of the evolutionary game we humans are playing today.


Read more about it

Primary literature:

  • Hawks, J., Wang, E. T., Cochran, G. M., Harpending, H. C., and Moyzis, R. K. (2007). Recent acceleration of human adaptive evolution. Proceedings of the National Academy of Sciences 104(52):20753-20758.
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News articles:

Understanding Evolution resources:

Discussion and extension questions

  1. Review some background information on natural selection. Imagine that mutation X arises in a population of humans living in one of the first dense human settlements. The mutation helps its carrier survive a common disease. Explain how mutation X would spread through that population. Make sure to include the concepts of variation, selection, and inheritance in your explanation.

  2. How might a neutral gene version (one that is neither advantageous nor harmful) reach a high frequency in a population? Give at least one possible explanation.

  3. Why do some gene versions hitchhike along with an advantageous mutant while other gene versions don't? In other words, what allows a gene version to become a genetic hitchhiker?

  4. If you were a geneticist studying the genomes of a population living in Northern Canada and wanted to detect genetic hitchhiking, what sort of patterns would you be looking for in your genomic data?

  5. Consider two advantageous mutations (X and Z) in different parts of the genome. Mutation X is located in a part of the genome with an unusually high recombination rate. Mutation Z is located in a part of the genome with a lower recombination rate. All other things being equal, which of these mutations would you expect to exhibit a stronger hitchhiking effect? Explain your reasoning.

  6. Recall that natural selection favors genetically-based traits that allow their bearers to reproduce more successfully than others in the same population. What sorts of traits might you expect to be selected for in a society where humans are hunters and gatherers? What sorts of traits might you expect to be selected for in a population of humans living in a modern American city? Explain your reasoning.


Related lessons and teaching resources

  • Teach about natural selection: In this activity for grades 9-12, students build and evolve paper-and-straw "birds" to simulate natural selection acting on random mutations.

  • Teach about recent human evolution: This article for grades 9-12 explains how the ability to digest milk recently evolved and has spread through some human populations.

  • Teach about human evolution at a molecular level: In this lesson for grades 9-12, students investigate variation in the beta globin gene by identifying base changes that do and do not alter function and by using several internet-based resources to consider the significance in different environments of the base change associated with sickle cell disease.

  • Teach about detecting natural selection: This article for grades 9-12 describes the recent finding that, in some ways, chimpanzees might have evolved more than humans, explains another method that scientists use to detect natural selection, and, in the process, debunks common misperceptions of human evolution.


References

  • Hawks, J., Wang, E. T., Cochran, G. M., Harpending, H. C., and Moyzis, R. K. (2007). Recent acceleration of human adaptive evolution. Proceedings of the National Academy of Sciences 104(52):20753-20758.

  • Wade, N. (2007, December 11). Selection spurred recent evolution, researchers say. The New York Times.
    Retrieved December 21, 2007, from The New York Times

  • University of Utah News Center. (2007, December 10). Are humans evolving faster? The University of Utah.
    Retrieved December 21, 2007 from The University of Utah


Chromosome image by Robert Moyzis, University of California, Irvine, CA; U.S. Department of Energy Human Genome Program



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