Human Culture, an Evolutionary Force

[BridgeTheGap]

As with any other species, human populations are shaped by the usual forces of natural selection, like famine, disease or climate. A new force is now coming into focus. It is one with a surprising implication — that for the last 20,000 years or so, people have inadvertently been shaping their own evolution.

The force is human culture, broadly defined as any learned behavior, including technology. The evidence of its activity is the more surprising because culture has long seemed to play just the opposite role. Biologists have seen it as a shield that protects people from the full force of other selective pressures, since clothes and shelter dull the bite of cold and farming helps build surpluses to ride out famine.

Because of this buffering action, culture was thought to have blunted the rate of human evolution, or even brought it to a halt, in the distant past. Many biologists are now seeing the role of culture in a quite different light.

Although it does shield people from other forces, culture itself seems to be a powerful force of natural selection. People adapt genetically to sustained cultural changes, like new diets. And this interaction works more quickly than other selective forces, “leading some practitioners to argue that gene-culture co-evolution could be the dominant mode of human evolution,” Kevin N. Laland and colleagues wrote in the February issue of Nature Reviews Genetics. Dr. Laland is an evolutionary biologist at the University of St. Andrews in Scotland.

http://www.nytimes.com/2010/03/02/science/02evo.html?th&emc=th

[Jim__]

1. A question on lactose tolerance.

According to the article:

The best evidence available to Dr. Boyd and Dr. Richerson for culture being a selective force was the lactose tolerance found in many northern Europeans. Most people switch off the gene that digests the lactose in milk shortly after they are weaned, but in northern Europeans — the descendants of an ancient cattle-rearing culture that emerged in the region some 6,000 years ago — the gene is kept switched on in adulthood.

Lactose tolerance is now well recognized as a case in which a cultural practice — drinking raw milk — has caused an evolutionary change in the human genome. Presumably the extra nutrition was of such great advantage that adults able to digest milk left more surviving offspring, and the genetic change swept through the population.

This instance of gene-culture interaction turns out to be far from unique. In the last few years, biologists have been able to scan the whole human genome for the signatures of genes undergoing selection. Such a signature is formed when one version of a gene becomes more common than other versions because its owners are leaving more surviving offspring. From the evidence of the scans, up to 10 percent of the genome — some 2,000 genes — shows signs of being under selective pressure.

I assume that when people are lactose intolerant they essentially can't drink milk. So what is the scenario? There are some lactose tolerant people in the population and they just happen to drink milk and find that they are tolerant? Or, the entire population sometimes drinks milk, say in some type of emergency - e.g. famine, and the people who are lactose tolerant,thrive? Or, something else?

I don't understand exactly what the process is.

[Igel]

2. Milk is a great thing.

You start herding for meat. A cow can "harvest" grass that we couldn't eat and make it into food. Human children could drink the milk, but after about age 2 or 3 they start reacting badly to it.

If you have a cow you can use it to supplement a child's milk. If you don't, the mother in a pre-modern society will nurse her child for several years. Cows increase the likelihood of a child's surviving. Moreover, if the mother doesn't have to nurse her fertility is likely to go up, and she'll be in better shape when she does conceive. Lactation leaches calcium from bones.

Having cows is a good thing. It means that your tribe is likely to increase more quickly.

But being exposed to cow milk makes clear who, exactly, has the genetic mutation that enables them to digest lactose. Those kids will be able to drink milk for longer, and probably have a slightly increased survival rate or be healthier--more protein, more calcium. In other words, you'd expect the population of lactose-tolerant adults to gradually increase, and for kids who do become lactose intolerant to have the gene switch off later. Societies that have had cow milk for millennia have a higher average age when lactose intolerance, if it's going to kick in, kicks in.

Now, a who can prowl over a large area and eat stuff that we'd get no nutrition from can be used to feed us in a way that killing and eating them never could. You kill a cow and a few weeks later you have nothing but bones and skin. Or you can keep a cow and drink her milk for months. In times of low food, or when your tribe's expanded to tax food resources, adults that produce lactase are likely to survive better or be healthier. This means they're more likely to have kids and more likely to fend off predators. Tribes with adults who can drink milk will tend to be bigger and stronger and spread.

Combine the two processes: Cows help keep adult breeding populations up and healthy, and help make sure their offspring more often survive to adulthood. The development of lactose-tolerance has arisen 3-4 times because it's advantageous; a later age for the development of lactose-intolerance is also advantageous and also seems to occur in the presence of cow udders.

[qazplm]

4. I have gone back and forth on my personal lactose tolerance

I drank milk like a fish when I was a teenager.

Then in my 20s, I became lactose intolerant. Serious gas whenever I drank milk. Then in my early 30s, I became lactose tolerant again. Now at 40 I can drink milk no problem.

A bit weird.

[Hosnon]

3. It doesn't make sense to say that "culture has long seemed to play just the opposite role".

Or that "culture was thought to have blunted the rate of human evolution". The author seems to be equating selection (general) and natural selection (specific). Natural selection is not the only form of selection; other forms are artificial selection (e.g., dog breeding) and "technological" selection (e.g., bad eyesight: with my eyesight, my genes would not have made it as far as they have were I at the mercy of natural selection).

To imply that natural selection is the only form of selection is inaccurate.