ANTH FINAL: Part 5 - Modern Human Genetic Variation

How does the total amount of genetic variation in modern humans compare to the total amount in chimpanzees? What might this tell us about recent human evolutionary history and the dispersal of modern humans across the globe in the past 70,000 or so years?

Modern humans have less total genetic variation than chimpanzees, despite our global distribution. This suggests a recent common ancestry and a population bottleneck in human history, likely during our dispersal out of Africa, which reduced genetic diversity.

How does overall genetic variation in Africa compare to the overall amount among people whose ancestry was in other major geographic regions (e.g., East Asia, Europe, South America)? In what way does the answer to this question support the hypothesis that Homo sapiens originated in Africa?

Genetic variation is highest in Africa compared to all other regions. This supports the "Out of Africa" hypothesis because populations that remained in Africa retained more of the original genetic diversity, while others carried only a subset of that diversity as they migrated out.

What is the relationship between the overall genetic similarity between people with Subsaharan ancestry and populations with ancestry in other parts of the world, on the one hand, and the distance of those populations from Africa? How much (in general terms) of the variation that exists outside of Africa also exists in people whose recent ancestry is in other parts of the world?

Genetic similarity to Subsaharan Africans decreases with distance from Africa. Most of the variation outside of Africa is a subset of the variation found within Africa, due to the founder effect and bottlenecks during migration.

What do the answers to these questions tell us about the dispersal of modern humans throughout the world?

A: They show that humans dispersed from Africa in a stepwise, serial fashion, losing genetic diversity along the way. This supports the view that all modern humans share a recent common African origin.

Why are we particularly likely to find adaptive variation in genes related to diet, physiology, and energetics and signals of “positive” selection on variants of these genes?

These genes are closely tied to survival and reproduction in changing environments. As humans migrated and adapted to new diets and climates, natural selection favored variants that improved nutrient use, metabolism, and physiological function.

WhIn particular, why should we expect to find evidence of positive selection on aspects of physiology and energetics in populations with long histories of residence in “extreme” environments? Think of high-altitude populations and the problem of hypoxia, for example.

In extreme environments, like high altitudes (where oxygen is low), survival requires specialized adaptations (e.g., more efficient oxygen use). Positive selection would favor genetic variants that improve physiological performanceunder these conditions.

Understand the logic of using comparative data to analyze the “partitioning” of modern human genetic variation within and among groups, where people are allocated to groups at three levels. What is the most important conclusion that consistently results from such analysis, and what does that tell us about the concept of “race”?

The key finding is that most human genetic variation (~85–90%) exists within populations, not between them. This shows that the concept of race has little biological basis, as visible traits like skin color do not reflect deeper genetic differences.

In particular, what does it tell us about the validity of using a few obviously visible phenotypes like skin color to classify people into presumed biological groups?

It shows that using skin color and other visible traits to define "races" is scientifically invalid. These traits are influenced by a small number of genes and are adaptations to environment, not markers of deep genetic divisions.

As you think about this question and others above, also think of some examples of polymorphisms that vary in frequency among groups because some of the variants are adaptive in particular environments (where “environment” can include human niche construction, like farming or animal domestication). Obvious ones include hemoglobin variants, variants of genes that influence skin color, and variation in the numbers of copies of the salivary amylase gene (an example we encountered at the beginning of the semester and is described in the textbook; copy number variation is associated with variation in the ability to digest high-starch foods). Also, know something about variation in the ability to digest lactose.

• Hemoglobin variants (e.g., sickle cell trait) provide malaria resistance in some African and Mediterranean populations.

• Skin color genes vary by UV exposure—darker skin in high UV areas, lighter skin in low UV areas.

• Amylase gene copy number is higher in populations with high-starch diets, aiding digestion.

• Lactase persistence is more common in populations with a history of dairy farming, allowing digestion of lactose into adulthood.

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What is a SNP (Single Nucleotide Polymorphism)?

It's a single-base change in DNA that is common in a population and used to study genetic variation.

What is the founder's effect?

A reduction in genetic diversity that occurs when a new population is established by a small number of individuals.

What does a cline refer to in genetics?

A gradual change in a trait or allele frequency across geographic space, often adapting to the environment.

What are some examples of polymorphisms that vary in frequency due to adaptability?

Hemoglobin variants, skin color genes, amylase gene copy number, and lactose digestion ability.

Define polymorphism

A genetic trait with two or more common variants (alleles) in a population.