Is Human Population Growth Accelerating Evolution? (Today's Most Popular)
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June 23, 2011

Is Human Population Growth Accelerating Evolution? (Today's Most Popular)

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"We are more different genetically from people living 5,000 years ago than they were different from Neanderthals."
              John Hawks -- University of Wisconsin anthropologist

In a fascinating discovery that counters a common theory that human evolution has slowed to a crawl or even stopped in modern humans, a study examining data from an international genomics project describes the past 40,000 years as a time of supercharged evolutionary change, driven by exponential population growth and cultural shifts.

Thanks to stunning advances in sequencing and deciphering DNA in recent years, scientists had begun uncovering, one by one, genes that boost evolutionary fitness. These variants, which emerged after the Stone Age, seemed to help populations better combat infectious organisms, survive frigid temperatures, or otherwise adapt to local conditions.

The findings may lead to a very broad rethinking of human evolution, especially in the view that modern culture has essentially relaxed the need for physical genetic changes in humans to improve survival.

A team led by University of Wisconsin-Madison anthropologist John Hawks estimated that positive selection just in the past 5,000 years alone -- dating back to the Stone Age -- has occurred at a rate roughly 100 times higher than any other period of human evolution. Many of the new genetic adjustments are occurring around changes in the human diet brought on by the advent of agriculture, and resistance to epidemic diseases that became major killers after the growth of human civilizations.

"In evolutionary terms, cultures that grow slowly are at a disadvantage, but the massive growth of human populations has led to far more genetic mutations," says Hawks. "And every mutation that is advantageous to people has a chance of being selected and driven toward fixation. What we are catching is an exceptional time."

While the correlation between population size and natural selection is nothing new -- it was a core premise of Charles Darwin, Hawks says -- the ability to bring quantifiable evidence to the table is a new and exciting outgrowth of the Human Genome Project.

In the hunt for recent genetic variation in the genome map the project has cataloged the individual differences in DNA called single nucleotide polymorphisms (SNPs). The project has mapped roughly 4 million of the estimated 10 million SNPs in the human genome.
John Hawks' studies include trying to make sense of genetic fragments from different populations, and anthropological bone and tooth specimens, to show how humans have evolved during the past 30,000 years. And he attempts to integrate that knowledge with data from archeology and the historical record. Hawks' research focuses on a phenomenon called linkage disequilibrium (LD). These are places on the genome where genetic variations are occurring more often than can be accounted for by chance, usually because these changes are affording some kind of selection advantage.

The researchers identify recent genetic change by finding long blocks of DNA base pairs that are connected. Because human DNA is constantly being reshuffled through recombination, a long, uninterrupted segment of LD is usually evidence of positive selection. Linkage disequilibrium decays quickly as recombination occurs across many generations, so finding these uninterrupted segments is strong evidence of recent adaptation, Hawks says.

Employing this test, the researchers found evidence of recent selection on approximately 1,800 genes, or 7 percent of all human genes.

This finding runs counter to conventional wisdom in many ways, Hawks says. For example, there's a strong record of skeletal changes that clearly show people became physically smaller, and their brains and teeth are also smaller. This is generally seen as a sign of relaxed selection --that size and strength are no longer key to survival.

But other pathways for evolution have opened, Hawks says, and genetic changes are now being driven by major changes in human culture. One good example is lactase, the gene that helps people digest milk. This gene normally declines and stops activity about the time one becomes a teenager, Hawks says. But northern Europeans developed a variation of the gene that allowed them to drink milk their whole lives -- a relatively new adaptation that is directly tied to the advance of domestic farming and use of milk as an agricultural product.

The biggest new pathway for selection relates to disease resistance, Hawks says. As people starting living in much larger groups and settling in one place roughly 10,000 years ago, epidemic diseases such as malaria, smallpox and cholera began to dramatically shift mortality patterns in people. Malaria is one of the clearest examples, Hawks says, given that there are now more than two dozen identified genetic adaptations that relate to malaria resistance, including an entirely new blood type known as the Duffy blood type.

Another recently discovered gene, CCR5, originated about 4,000 years ago and now exists in about 10 percent of the European population. It was discovered recently because it makes people resistant to HIV/AIDS. But its original value might have come from obstructing the pathway for smallpox.

"There are many things under selection that are making it harder for pathogens to kill us," Hawks says.

Population growth is making all of this change occur much faster, Hawks says, giving a tribute to Charles Darwin. When Darwin wrote in Origin of the Species about challenges in animal breeding, he always emphasized that herd size "is of the highest importance for success" because large populations have more genetic variation, Hawks says.

The parallel to humans is obvious: The human population has grown from a few million people 10,000 years ago to about 200 million people at A.D. 0, to 600 million people in the year 1700, to more than 6.5 billion today. Prior to these times, the population was so small for so long that positive selection occurred at a glacial pace, Hawks says.

"What's really amazing about humans," Hawks continued, "that is not true with most other species, is that for a long time we were just a little ape species in one corner of Africa, and weren't genetically sampling anything like the potential we have now."

"Five thousand years is such a small sliver of time -- it's 100 to 200 generations ago. That's how long it's been since some of these genes originated, and today they are in 30 or 40 percent of people because they've had such an advantage. It's like 'invasion of the body snatchers.'"

The Wisconsin study was published in the Proceedings of the National Academy of Sciences.

The Daily Galaxy via a University of Wisconsin release, http://johnhawks.net/weblob

Comments

Don't forget the availabity of enhanced sources of food has enabled this expansion of the population. However, I remember a study as a undergraduate using rats.
1. This population had finate space (plant earth for us)
2. Unlimited food sources.
3. No restructions on breeding.
4. Free health care

They came a point of saturation where there were so many rats that the drive to breed started to fall of to the point where the population collapsed - even the quantity and quality of food remained the same.
Eventually all the rats died.

At what point do we reach 'saturation'? You see that now in Europe where the birth rate is less than the death rate.

What good is evolution without natural selection?

One can wonder what effect the recent progress in medicine will have on the evolution of the human genome. Mutated genes with a detrimental effect on the health of the individual which previously would have disappeared with the death of the bearer before his reproductive years will now be transmitted to future generations.
Over time, humans may become less and less resistant to diseases.

This tells us I think that the clock is ticking. We need to get off this world and branch out into the beyond.


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