Barrett 10: Contemporary Human Diversity

Species vs. Race

• Species

  • Members of the same species can mate and produce viable offspring

  • All humans (Homo sapiens) belong to the same species

• Race

  • Members of the same species that are “distinctive” in some way

  • Racial classifications are often arbitrary and non-functional

Race as a taxonomic level

• Classification of animals and plants into races has often been ill-defined and idiosyncratic

  • Often overlaps with ecotype or subspecies

Human racial classifications

• In humans, while certain groups may exhibit similarities in traits, there traits are rarely coherent in the same way

  • Ex: ABO blood group and skin colour both show patterns in geographical distribution, but do not line up at all

• Additionally, biological and cultural characteristics are often conflated

• Race is not a scientifically useful scheme for categorizing human diversity

Human genetic variation

• Relatively low compared to other primates

• No large differences between geographic areas

• Majority of variation (~85%) is within populations, not between them

Founder effect

• Caused by random sampling of large populations that start new smaller populations

• Increases power of genetic drift

Genetic bottleneck

• Loss in allelic diversity during sharp reductions in population size

• Rare alleles most likely to be lost

What is a “population”

• Potentially interbreeding group of individuals of the same species that live within a restricted geographic area

• Isolation can reduce gene flow, leading to independent evolution and “population structure”

Population structure

• With complete isolation, some alleles increase in frequency through random change and “fix”

• Different alleles may fix in different populations, leading to divergence

• Migration allows gene flow and reduces differentiation

• Isolation correlates with distance

Genetic evidence for “Out of Africa”

• Human populations structure strongly supports a small number of founders leaving Africa to colonize the whole planet

• Geographic distance from Africa shows high negative correlation with measures of population-level genetic diversity

• Genetic differences between randomly chosen African individuals are greater than between European and Asian individuals

• Genetic differences between African populations are on average greater than between European and Asian individuals

• Alleles found outside of Africa are often a subset of the African gene pool

• Continent-specific alleles are far more common in Africa than any other continent

• However, genetic data suggests multiple migrations out of Africa

• All present-day non-African peopl are descended from Homo sapiens ancestors who left Africa within the last few hundred thousand years

HIV resistance and the CCR5 locus

• HIV enters white blood cells by binding to the CCR5 protein

• Individuals who are homozygous for the d32 allele are (mostly) immune to infection

• Example of potential selection in contemporary humans

• d32 is already at relatively high frequency in Europeans because it confers some resistance to smallpox and bubonic plaque

Is height being selected for?

• Possibly, increased reproductive success in men

• But most of the height increases in the last century are likely down to better health and nutrition