Why should we care about skin color from a clinical perspective?
- Inadequate protection from sunlight has a major impact on human health. There is controversy over the extent to which different types of melanin can influence susceptibility to UV radiation.
- Vitamin D deficiency may result from inadequate exposure to sunlight.
- In both cases, understanding the genetic architecture of human skin color is likely to provide a greater appreciation of underlying biological mechanisms.
How do we quantify skin color?
Measurement of skin reflectance. The percent skin reflectance gives an indication of skin color.
How is “race” justified through skin color?
- Discontinuity in skin reflectance of different populations is used to justify the concept of race. However, when you look at the whole world instead of 3-4 populations, interpopulation variation in skin reflectance has a continuous distribution.
- What is the fallacy of racial classification?
- Skin color, frequency of blood type, frequency of lactose intolerance, height are all continuous traits. These genes aren’t linked to each other. Thus, skin color doesn’t affect any of these. How can you group people based on a single trait, when there are many continuous traits that have independent distributions.
What are the factors influencing human skin pigmentation through evolutionary time?
- Environment: UV radiation, precipitation
- Migration: Length of migration, recency of migration
- Other cultural adaptations: clothes wearing, shelter, shade seeking, timing of daily activities.
- Diet: Vitamin D content, folute content.
What selective advantage can be hypothesized for the observed variation in human skin color / pigmentation?
- Populations that live in the tropics receive enough UV light from the sun to synthesize vitamin D all year long.
- But those that live at northern or southern latitudes do not.
- In temperate areas, UV supply is variable.
Explain human skin and how skin color works
- Human skin contains specialized cells and tissues that support many vital metabolic systems.
- These include cells that protect the vascular system and sweat glands from UV radiation damage, and support vitamin D synthesis.
- Produce melanin, which protects against (UV) DNA damage and folate breakdown.
- UVB rays penetrate epidermis and prompt melanocytes to produce melanin in melanosomes packaging. Keratinocytes take up the melanosomes to protect against DNA damage.
Penetrate blood vessels in the dermis and destroy folate.
UVB rays that reach keratinocytes converts cholesterol into basic Vitamin D, which the liver and the kidney turn into active vitamin D.
Hypotheses to explain worldwide distribution of skin pigmentation?
- Light skin allows for the synthesis of provitamin-D in the upper latitudes.
- Dark skin protects against degradation folate in the tropics.
- Dark skin protects against skin cancer in the tropics. (not an important factor for evolution. This would occur after reproduction. Evolution only cares to maximize fitness.)
- Dark skin protects the sweat glands and vascular bed from UVA damage in the tropics.
- Light skin protects against frostbite. (differentiate between association and causality)
Genetics of skin color evolution:
The primary determinant of variability in human skin color is the amount, density and distribution of the pigment melanin. The metabolic pathway to melanin is extremely complicated, involving several intermediate steps. Melanin is a polygenic trait.
What is polygenic?
Multiple genes working together produce a continuous distribution in degrees.
How do we know that human skin color variation meets the criteria that it resulted brom natural selection?
- There is substantial phenotypic variation associated with geographic and environmental variation.
- There are several plausible hypotheses for natural selection
- There is a known genetic basis for pigmentation variation.
Independent selective events for human skin color variation examples:
- Significant genetic variation among populations from Africa, Europe and Asia.
- Data suggests that light skin color is a derived state and is of independent origin in Europeans and Asians.
- Dark skin color seems of unique origin, reflecting the ancestral state in humans.
- Neanderthals were a source of adaptive variation for loci involved in skin phenotypes. (Neanderthals moved out of Africa much earlier, and thus were more adapted to low UV areas)
What is the biological race concept?
- A group of populations sharing certain characteristics that distinguishes them from other populations.
- This relates to the subspecies concept which divides species into distinct and distinguishable groups in the early stages of speciation.
What are the problems with biological race?
- 1. Continuous variation of traits deems group formation arbitrary.
- 2. There is more variation within populations than between populations.
- 3. The term race is purely descriptive and doesn't reveal anything about our evolutionary history. There are better ways to describe our geographical variation.
Why are women lighter skinned than men?
- Pregnancy and lactation require more calcium than men.
- Calcium uptake is facilitated by vitamin D.
- Lighter skin allows for higher vitamin D production through UVB.
Why hasn't Arabs' skin color changed?
Instead of biological evolution, cultural evolution in the form of clothes and residing in tents has developed.
Why is melanin referred to as natural sunscreen?
Melanin neutralizes free radicals in the skin formed by UV rays and protects against the harmful effects of UV (just like sunscreen).