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Genetics, Geography, and Human Variation

About the Authors

Dr. Dennis O'Rourke is the Interim Chair of the Anthropology Department at the University of Utah in Salt Lake City, Utah. He is also the Co-Chair of the International Review Board and Vice President of Research. Dr. O'Rourke received his BA (with Honors), MA and PhD in Anthropology  from the University of Kansas in 1973, 1976 and 1980. He then went on to his Post-Doctoral fellowship in the St. Louis School of Medicine at Washington University. There he focused on Psychiatry/Genetic Epidemiology.

As mentioned before in @rebeccaleon blog from week one of class;  Dr. O'Rourke work focuses on the sampling and analysis of ancient DNA,  quantitative methods, and population and evolutionary genetics. The areas and populations that he focuses on are native America, and the North American and Siberian Arctic.

Picture of Jake Enk cutting a Mammoth tooth.

Jake Enk is a Doctoral student at McMatser University in Hamilton, Ontario. He received his Masters at The University of Utah.

In reading this chapter of Human Biology (Chapter 4), the authors bring into focus the effects of genetics on human populations in reference to geography and how understanding these topics can be, at best, informative to the history of our species and, at worst, complicated or even convoluted. The equations aside (which are, admittedly,  of great importance in the derivation of these data), in this chapter we delve deeply into understanding the mechanics of mutation , the interplay between human migration and genetic information, and how these factors shape our, often debated, view of how human populations came into existence and continued to move around the globe.

The chapter begins with explanations of basic evolutionary factors, specifically how mutations come to prominence in populations and how not only natural selection, but population size directly affects how readily mutations take hold. In somewhat simpler terms, the smaller the population, the more readily the frequency of an advantageous allele will increase within that population (There’s a convenient figure on p.108 that lays it out for you). As interesting as data acquisition and these formulas are, the real meat of Chapter 4 lies within the application of these methods to understanding our species’ history and how this data can better serve that purpose.

To begin, we can challenge, disprove, or validate many earlier assumptions of this field through the use of genetics as evidence. One great example Chapter 4 illustrates is Hrdlička’s hypothesis of a replacement population in the Aleutian Islands roughly a thousand years ago. His 1945 work, based mainly on differences in cranial forms, hypothesized an existing Aleutian population being replaced by a wave of newcomers who would become the ancestors of modern Aleuts. While his assumptions weren’t definitive beyond a shadow of a doubt, modern genetic testing (after a few upsets with unfortunately small sample sizes) has proven his work correct, contributing even more validity to our understanding of human history.

Sometimes, however, this availability of new data can obscure what was previously seen as a clean-cut understanding of that history. The authors bring up the issue of the presence of Neanderthal genetic information in modern human and the debate as to how it got there, if it is Neanderthal DNA in the first place. Sequencing of Mitochondrial DNA from Neanderthals show incredible variation from modern humans and ancient anatomically modern humans (supporting that long-held belief that humans and Neanderthals never interbred), yet nuclear genome sequences suggest that one to four percent of our genome may be made up from Neanderthal DNA, possibly from admixture. So did early humans and Neanderthals mate, or could these traits be the result of latent genetic traits that we both received form an ancient ancestor? The answer: we don’t know. Maybe both. Perhaps you’ll be the ones to figure it out.

Finally, this chapter shows how even our understanding of genetic data in relation to prehistoric human events can be far more complicated than that data would initially suggest. In the example of the populating of Europe, differences in mitochondrial DNA and nuclear molecular DNA, which seem incompatible, illustrated a far richer story. In this case, the mitochondrial DNA suggests a movement of humans into Europe from the Middle East, roughly 10,000 years ago, coinciding with the advent of agriculture. The nuclear molecular DNA evidence, however, suggests something much different. Rather than 10,000 years ago, this evidence suggests a divergence in populations from 46,000 to 130,000 years ago. So which evidence is correct? Both. By comparing and combining these data, human biologist now surmise that while a clinal migration from the Middle East into Europe did take place around 10,000 years ago, Europe had already been populated by Homo Sapiens long before that event.

All in all, this chapter is helpful in illustrating the importance of genetic information and its contribution to our understanding of the human story. If you want to know the specifics of how DNA evidence is used to formulate new hypotheses and support or refute existing ones: this is the chapter for you... But don’t take our word for it! (Insert Reading Rainbow theme here)

1) What do you believe the current genetic distance is? Is this a good thing or bad thing?

2) What are some current examples of migration? And how have they influenced the current gene population?

8 thoughts on “Genetics, Geography, and Human Variation

  1. jrbelanich

    1) I think that the current genetic distance is probably quite small due to migrations and cross cultural matings. This is probably not a good thing, as the closer we are, the less genetic diversity between populations. And the less genetic diversity, the less genetic and phenotypic variation present in a species.

    2) Some current examples of migrations exist in war-torn regions, when populations leave due to political stresses. The populations then usually resettle in other areas potentially bringing new genetic diversity. Movements of populations will result in the mixing of genes and increases genetic diversity if the new area has never had those genes before.

    1. peadams

      2. Migration is occurring between human groups more than ever right now. There are very few places of the world that are completely isolated, and prejudice against people "different" from you is also decreasing. We are more mobile than ever due to cars and planes. In general, people are moving more than before. When people move more, they are more likely to have children with someone from a different area than they are from. In the past 500 years we saw the migration of Europeans to the Americas, Africa, and Asia. This was a drastic migration movement that has worked to homogenize the entire human population.

  2. Christopher Lynn

    This is incomplete. Where's the rest of your post? I think I gave a big speech in class about doing a narrative summary & not an outline, & this isn't even a complete outline. And where is the commentary? There are only 2 comments on here? You didn't send the link around to remind your classmates.

  3. Emily Barron

    Migration is happening on a huge scale, from war torn regions as refugees, between countries for economic reasons, and from rural to urban in periphery and semi periphery nations. I do think that cross cultural are becoming more common, however I'm not sure how strong their influence is when so much of the time people are sticking to mating within their own cultural groups. For example, immigrants from Mexico seem to rarely change the genetic diversity in communities that they move into because there are cultural (and linguistic) barriers to cross cultural mating.

  4. Andrea

    2) Migration is more prevalent today than ever before due to the fact that traveling has become more popular, it's not as common to live in one place your whole life, and then there are people who leave their homeland in search of a better lifestyle. An example of this is a family that migrates from Iran to America in order to escape warfare and in order to prevent their son from being drafted into the military. Due to the fact that migration this is decreasing genetic variation because it's more common for interracial couples to have children.

  5. Katie Coward

    1) I think the genetic distance is smaller now because of globalization. Because people can more easily and more quickly travel accross continents and different genetic populations, it is now easier for different gene populations to interbreed.

    2) Currently, migration is much more common due to the global market. People who work for international companies are more likely to move to different parts of the world more frequently. This has caused more interbreeding betweeen different populations.

  6. rebeccaleon

    1) Comparatively speaking the current genetic distance between populations today is much smaller than it has probably ever been in human history. Once people had the ability to travel around the world (as well as travel faster) they began to come into contact with many different groups that had never come in contact with one another before. This new contact led to interbreeding and now thanks to industrialization and globalization humans inhabit most areas of the planet. Not everyone, but a lot of people today are coming into contact with different groups on a regular basis, especially those of us who live in highly industrial/global societies such as our own. Even though the genetic distance today is smaller than it once was there is still a lot of genetic diversity. So, for now I don’t think it is a bad thing, because new groups coming in contact with each other introduces new genes into each group if there is interbreeding going on.

    2) One current example of migration is the recent influx into the United States of people of Hispanic descent. The Hispanic population in the U.S. is now the largest minority. I don’t know for certain how this has affected the gene pool in America, but just based on my own observations I doesn’t seem to have a huge effect yet. No doubt there is some admixture going on between Hispanic immigrants and the American population, but for the most part the Hispanic population seems to live/work/interact with other Hispanics. We talked about why this happens the other day in class, because immigrants who don’t speak English and/or don’t know the cultural rules (or even could be in the U.S. illegally) feel safer amongst those that are like themselves. Over the course of a few generations this might change a little bit and we could see more Hispanics integrating into the rest of American society (i.e., also interbreeding with Americans).

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