Human Variation, Race and Adaptation
Human variation is the study of differences and similarities among populations in body form, physiology, genetics and disease resistance. The exam expects you to explain adaptation, clines and plasticity, and to show why biological race is not a valid classification.
Core concepts
- Cline: a gradual change in allele or trait frequency across geography, e.g. skin pigmentation.
- Polymorphism: two or more common forms in a population, e.g. ABO blood groups.
- Adaptation: inherited change favoured by selection over generations.
- Acclimatization: reversible physiological adjustment within a lifetime, e.g. increased red cell production at altitude.
- Developmental plasticity: permanent adjustment during growth.
- Bergmann's and Allen's rules: body mass tends to increase and limb length to decrease in colder climates.
Race
Nineteenth-century typological races were based on arbitrary traits and ignored continuous variation. Human genetic variation is mostly within populations, not between them. UNESCO statements (1950, 1951) rejected race as a biological explanation of cultural differences; race is better analysed as a social category with real effects through racism.
Worked example
In malarial regions the sickle-cell allele (HbS) reaches high frequencies. HbS homozygotes often die of sickle-cell disease, yet heterozygotes have some protection against Plasmodium falciparum malaria. The allele is maintained by balancing selection. This is not a 'racial' trait: the same allele occurs across linguistically and culturally diverse populations, and its distribution follows the malaria map.
Traps
Do not treat skin colour as a race marker; do not confuse acclimatization with genetic adaptation; do not use Lamarckian explanations; do not say variation is absent-clinically important differences such as HbS, G6PD deficiency and thalassaemia exist and matter for health screening.