ANT1 19 Genetic Variation
Why is skin color related to distance from the equator?
The Rickets Hypothesis: sunlight provides Vitamin D • Our bodies need Vitamin D to process Calcium • We get Vitamin D from the sun • When humans migrated north, days were shorter and more overcast = less sunlight • Those with less melanin in their skin tended to be healthier because they got enough Vitamin D for strong bones. • Over many generations of directional selection, average skin color of temperate populations lightened (less melanin).
Total genetic diversity in humans vs. other apes
The amount of genetic diversity among ALL humans around the globe is remarkably small compared to other mammals, including the great apes.
Where solar radiation is weak,
light skin allows for better Vitamin D absorption
Genetic variation within a population is negatively correlated with geographic distance from Africa
the farther the population is from Africa, the less genetic variation within that population
If everyone on the planet were wiped out, except for one local group, we would still have...
we would still have 85 to 95% of the total genetic variation in our species. -The average individual carries about 30% of the total species variation.
Causes of variation between groups:
— Natural Selection • Different environments favor different traits o Example: lactose tolerance, skin color, body shape — Genetic Drift • Founder Effect o Example: Comparison of Mormon and Amish populations
Causes of variation within groups:
— Selection-mutation balance • Low frequencies of harmful alleles — Adaptive lag • Some genes used to be beneficial, but environment has changed and the old genes are still around o Example: non-insulin dependent diabetes — Balanced polymorphism • Heterozygote advantage o Example: Sickle-cell anemia
Why was the population small recently?
Small original population in Africa Everyone outside of Africa is descended from the subset of original population that left Africa (group of H. sapiens in 2nd migration ~60 kya).
Each subsequent founder effect further reduces variation
Original populations contain the most variation. Populations furthest away from the original source contain the least variation.
Why are humans so genetically similar to each other, compared to chimps?
- humans live almost everywhere (huge population size) -But chimps live only in a small region of Africa (small population size) -The human population must have been very small quite recently in the past. • Not enough generations have passed to increase our genetic variation (chimps are a much older species than humans)
Molecular data imply that all modern human populations outside of Africa are descendants of people that moved out of Africa about 60 kya
-60 kya: people from one African population migrate to Eurasia and Australasia -60 kya - present: Dispersed human populations diverge
primary cause of skin color variation
-The amount of melanin in the skin is the primary cause of skin color variation. • More melanin = darker skin
The sources of variation within and between groups can be different!
-Variation within each lawn = genetic -Variation between lawns = environmental
compute variation within and between populations
3 chimpanzee populations: • East Africa (Pan troglodytes schweinfurthi) • Central Africa (P. t. troglodytes) • West Africa (P. t. verus)
Other sources of variation between groups: Genetic Drift
Founder Effects • Amish: Very small founding population (200 people), therefore quite distinct from other European populations. • American Mormons: Very large founding population (200,000 people), therefore genetically quite similar to other European populations.
• And yet, despite this small amount of genetic diversity, the human species exhibits a remarkable amount of variation.
Human morphology varies • Height and body proportions • Skin color • Hair color and texture • Facial features Human physiology varies • Lactose tolerance • Hemoglobin types • Susceptibility to diseases Human behavior varies • Subsistence strategies • Social organization • Beliefs, values, etc.
"The fact that variation within populations has a genetic component does NOT mean that differences between groups are caused solely by genetic differences." (Boyd & Silk)
It is important to avoid jumping to genetic conclusions - Environment is a powerful force.
`Variation in Lactose Tolerance
Most humans cannot digest the sugar in milk (lactose) after they the age of five. >> A regulating gene shuts off the gene that produces the necessary enzyme. • The ability to digest fresh milk is more common among pastoral populations - human groups that have domesticated animals (like cattle)
The lesson to be learned is this:
The system that produces us - in all our variety - is very complex. It is silly to try to reduce differences between human groups to mere alleles - especially when there isn't much genetic diversity to begin with.
Sickle-cell Anemia Balanced polymorphism:
Two or more alleles are maintained in a population due to the fact that heterozygotes have higher fitness than either type of homozygote
Variation in Body Shape
Variation in body proportions is related to heat loss. • Recall: o Neanderthals were stocky>>cold-adapted o Early Homo sapiens were lanky>>arm-adapted • Surface to volume ratio describes the simple physics of retaining heat. o Greater body bulk in cold climates o Shorter limbs in cold climates
A series of founder effects
When populations expand, local groups become isolated from each other and genetic differences begin to accumulate
Genetic variation within and between groups
Within group = 93.7% Between groups = 6.3% -Most human genetic variation is contained within groups. -Very little variation exists between groups.
The Folate Hypothesis: sunlight depletes folate levels
• Folate deficits in pregnant women are linked to neural defects, like spina bifida • Folate is essential for DNA synthesis, new cell production, and sperm production. ***Where solar radiation is strong, dark skin protects folate levels.
Variation within vs. variation between
• Forces leading to each • Patterns in humans
Clinical Variation
• Gradual variation over geographic distances >>Called "clinal" variation or "clines" • Those groups who live closest to each other are more similar than those who live farther away. >>More gene flow between nearby groups + adaptation to similar environments
Explanation Jamaica sprinting
• In Jamaica, there is a cultural value placed on sprinting that is absent in many places, such as the U.S., where talented athletes pursue the sports that are prestigious here (e.g., NFL) • The annual high school Track & Field Championships are a major national event. As important to Jamaicans as the Super Bowl is to Americans!
Kenya long distance running explanation
• In Kenya, many runners grow up and train at high altitudes, which increases the number of Red Blood Cells and the efficiency of oxygen delivery to muscles. • Great for distance running!
Example: Track & Field
• Many great sprinters are Jamaican • Many great distance runners are Kenyan • Must be due to genetic differences, right? Wrong!
Mitochondrial DNA tells a similar story: We can use the global variation in mtDNA to reconstruct a phylogeny
• Shows that the deepest branches in the phylogenetic tree are in Africa. • One African branch links Africans with the rest of the world. • Implies that humans originated in Africa.
Variation in Skin Color
• Skin color variation is distributed geographically according to solar radiation. • Equatorial people have darker skin color than people from more temperate climates do. • Skin pigmentation is significantly correlated with distance from the equator.
Summary
• Teaches us that genes are not deterministic. Their expression is sensitive to external factors. • The possibilities are limited, of course, but the system is extremely flexible. • Potential for multiple phenotypic outcomes from the same genetic starting point (or, the same phenotypic outcome from multiple genetic starting points). • This flexibility has enabled humans to flourish in nearly any environment on earth, largely with the help of cultural adaptations.
The Rickets Hypothesis
• This hypothesis explains variation in Africa, India, Asia, and most of the Americas. • But Native Alaskans have more melanin than would be expected >>Possibly due to dietary source of Vitamin D
• That is not to say there are no genetic differences, but that genetic differences are not necessarily the reason for differences in performance.
• To be able to attribute differences entirely to genes, we would need to expose people to identical environments and see what happens. It's practically impossible to do this!
