Evo-Ed: Integrative Cases in Evolution Education

Cases for Evolution Education

Population Genetics

Within and between population genetic differences are used to characterize the distribution of variants in five genes associated with skin color across global populations.  This information helps us understand the evolution of skin color in humans.

global populations

Link to PowerPoint slides

PowerPoint Slides

Gene and allelesstrs

SNP (single nucleotide polymorphisms) and STR (short tandem repeats) data can be used to determine evolutionary relatedness among populations. These patterns of changes in DNA are the basis for the FST (fixation) statistic, an estimation of differences in populations based on genetic variation (π):

FST equation

An FST value of 0 indicates that two populations are interbreeding; there is insignificant variation in the alleles of a gene under study. No evolution is occurring.

An FST value of 1 indicates that two populations are very different from one another as evidenced from the alleles of a gene under study. Evolution is probably occurring.

FST scores can be calculated for genes known to be involved in skin color pigmentation. These scores indicate how conserved or different these genes and their alleles are among and between populations.

Norton et. al (2007) studied the global distribution (a total of 53 populations, of which 27 are represented here) of variations of six genes associated with skin color: MC1R, ASIP, TYR, OCA2, MATP aka SLC45A2, and SLC24A5 . They found that the melanocortin 1 receptor (MC1R) variation was greater within populations than between populations: the FST value was small. Thus, the remainder of their analysis was based on the other five genes.  

global pop gen map

The data presented here are the allele frequencies for the five genes under consideration. The frequencies reflect the occurrence of the variant associated with lighter skin. These data are represented by maps in the slide set.

Norton Data SkinColor Alleles

Summary of Norton data:

  • ASIP gene: The variant associated with lighter skin is common among populations across the Northern hemisphere and is found in New World populations as well. The variant associated with darker skin is found in African and a few Asian populations.
  • TYR gene: The variant associated with lighter skin is found predominantly in European populations and is absent in Asian populations with lighter skin.
  • OCA2 gene: A version of this gene is associated with lighter skin in Northern hemisphere populations in Europe and across Asia.
  • MATP = SLC45A2 gene: The variant associated with lighter skin is characteristic of western European populations, at a very high percentage.
  • SLC24A5gene: The variant associated with lighter skin is found throughout Europe and the Near East.

What do these data indicate?

Populations with lighter skin commonly share variants for two genes, ASIP and OCA2. These are important for global patterns in skin color.
The SLC24A5,andSLC45A2variants are common in Europeans, but not East Asians.
TYR is stable around the world, except for western Europe. The lightest skin color (eg. Northern England) is associated with the lighter versions of all genes under consideration here.