There are a lot of legal and political barriers
to doing population genetics studies of American Indian tribes at present, but fewer upon their distant cousins in Canada and Mexico. Here’s the abstract
of a new genome study of Indians in Mexico:
Mexico harbors great cultural and ethnic diversity, yet fine-scale patterns of human genome-wide variation from this region remain largely uncharacterized. We studied genomic variation within Mexico from over 1000 individuals representing 20 indigenous and 11 mestizo populations. We found striking genetic stratification among indigenous populations within Mexico at varying degrees of geographic isolation. Some groups were as differentiated as Europeans are from East Asians. Pre-Columbian genetic substructure is recapitulated in the indigenous ancestry of admixed mestizo individuals across the country. Furthermore, two independently phenotyped cohorts of Mexicans and Mexican Americans showed a significant association between subcontinental ancestry and lung function. Thus, accounting for fine-scale ancestry patterns is critical for medical and population genetic studies within Mexico, in Mexican-descent populations, and likely in many other populations worldwide.
The article in Science
is a little excited but still interesting:
People from Mexico show stunning amount of genetic diversityBy Lizzie Wade Thursday, June 12, 2014 – 2:00pm… Mexico contains 65 different indigenous ethnic groups, 20 of which are represented in the study, says Andrés Moreno-Estrada, a population geneticist at Stanford University in Palo Alto, California, and the study’s lead author. Working with Carlos Bustamante, another Stanford population geneticist, the team sampled the genomes of indigenous populations all over Mexico, from the northern desert of Sonora to the jungles of Chiapas in the south. Over centuries of living so far apart—and often in isolation because of mountain ranges, vast deserts, or other geographic barriers—these populations developed genetic differences from one another, Bustamante explains. Many of these variants are what he calls “globally rare but locally common.” That is, a genetic variant that’s widespread in one ethnic group, like the Maya, may hardly ever show up in people of different ancestry, like people of European descent. If you study the genomes of only the Europeans, you’d never catch the Maya variant. And that’s a big problem for people with Maya ancestry if that variant increases their risk of disease or changes the way they react to different kinds of medication. “All politics is local, right? What we’re starting to find is that lots of genetics is local, too,” Bustamante says.When the team analyzed the genomes of 511 indigenous individuals from all over Mexico, they found a striking amount of genetic diversity. The most divergent indigenous groups in Mexico are as different from each other as Europeans are from East Asians, they report online today in Science. This diversity maps onto the geography of Mexico itself. The farther away ethnic groups live from each other, the more different their genomes turn out to be.
To some extent there’s a selection mechanism going on here that, while it doesn’t invalidate this result, does suggest why it’s somewhat inevitable.
Native populations within Mexico are pretty much the people who stayed home and didn’t out-marry. The working definition of an Indian in Mexico has been somebody who wears the distinctive clothes of his or her Indian people. If somebody leaves home and makes a new life in a different area, they typically wear the European-style clothes of the mestizo majority. If you put on European clothes, you are assumed to have chosen to belong to mestizo, because even if you are still 100% Indian, your descendants will likely be mestizo.
But most people in Mexico or of Mexican descent these days are not indigenous but rather mestizo, meaning they have a mixture of indigenous, European, and African ancestry. Do their genomes also vary by what region of Mexico they come from, or has all that local variation been smoothed out by centuries of different groups meeting, mixing, and having babies?To answer that question, the team collaborated with Mexico’s National Institute of Genomic Medicine, which has been collecting genetic data from mestizos for many years. Somewhat surprisingly, they found that mestizos in a given part of Mexico tended to have the same “rare” genetic variants as their indigenous neighbors.
Outside of the shallow lake of the Valley of Mexico, Mexico had difficult internal transportation until the coming of the railroads in the Porfirio Diaz era in the late 19th Century, which slows down blending of family trees. When Americans got over the Appalachians, they could raft down the Ohio River, then go up the Mississippi and Missouri, first being pulled by mules walking along the bank, later by paddle wheel steamship. So, Americans moved rapidly. And most of the 19th Century immigrants were from northwest Europe, which had long enjoyed good water transport, so northwest Europeans tend to be relatively related to each other before they got here.
In contrast, most of Mexico is quite elevated and lacks easy river transport. The railroad from the port of Veracruz on the Gulf of Mexico to the capital at 7000+ feet in Mexico City wasn’t completed until 1873. But by the time the dictator had to flee to Paris with the coming of the Mexican Revolution after 1910, there were over 15,000 miles.
The mestizo genomes “track so well with the indigenous groups that we could use the genetic diversity in mestizos to make inferences about [their native] ancestors,” Pasaniuc says. Strong genetic markers of Maya ancestry, for example, show up in the genomes of modern people living in the Yucatán Peninsula and the northern part of Mexico’s Gulf Coast in the modern state of Veracruz, which likely reflects a pre-Columbian Maya trade or migration route. “It gives us a historical understanding of what these populations have been up to,” says Christopher Gignoux, a postdoc in Bustamante’s group at Stanford.Even more important are the study’s clinical implications. … He found that people with genetic variants common in the east of the country had different results on the lung function test than did people with variants from the west. That means doctors probably shouldn’t be using the same criteria to diagnose lung disease in both populations, he says. “What we demonstrated is that depending upon the type of Native American ancestry you have, it can dramatically influence the diagnosis of lung disease, in a good or a bad way,” Burchard explains.
I’m guessing that there was some selection for altitude. Mexico doesn’t have a lot of extreme altitude populations like Tibet or Bolivia do. Still, much of the rainfall, healthy climates, and richer soil, such as at the base of volcanos, is at up to, say, 9,000 feet. The Mexico City 1968 Olympics were contested at 7,300 feet, which is why the 10 second barrier
in the 100m dash (electronically timed) was broken there.
An interesting question is whether there are differences in behavioral tendencies between the Indians in different parts of Mexico. I would not be surprised if the Mexicans of Southern California haven’t changed somewhat over the course of my lifetime as the average location of ancestry inside Mexico has trended southward.
Former Mexican foreign minister Jorge Castaneda’s 2011 book Manana Forever?
observes that the Mayan regions of southern Mexico tend to be poorer but more civilized and orderly, while the “north is industrious, modernizing, violent, lighter-skinned, and devoid of charm …” In short, the north sounds a lot like Los Angeles.
A lot of this has to do with the European admixture levels, but it also could be that northern Mexican Indians tended to be wilder hunter-gatherers, while southern Mexican Indians tended to be more settled agriculturalists. But that’s all speculation and of course we shouldn’t allow speculative thoughts into our heads.