Human variation

The essentials

All living humans belong to one species, Homo sapiens, but populations differ in visible features and in genetic traits. Anthropologists study this variation to understand adaptation to environment, gene flow and history. Most genetic variation exists within populations rather than between them, which is why the old idea of fixed, separate "races" is rejected by modern science.

Key points

• Body size and shape: Bergmann's rule says that in colder climates, warm-blooded animals of a species tend to have larger, bulkier bodies (less surface area per unit volume, so less heat loss). Allen's rule says limbs and other extremities tend to be shorter in cold climates and longer in hot climates. Inuit (stocky) and many East African pastoral groups (tall, slender) are common examples.
• Skin colour depends mainly on the pigment melanin. Darker skin protects against strong ultraviolet (UV) radiation near the equator; lighter skin in low-UV regions helps the body make vitamin D. Skin colour changes gradually across regions (a cline).
• Nose shape: narrower noses are more frequent in cold, dry climates, helping warm and moisten air.
• Genetic traits: blood groups, the ability to taste PTC (phenylthiocarbamide), lactase persistence in adults (common among historically dairying populations), sickle-cell trait in malaria regions.
• Adaptation types: genetic adaptation (over generations), developmental (acclimatisation during growth, e.g. larger chest among high-altitude natives), and short-term physiological acclimatisation (e.g. sweating, tanning).

Exam tips

• Remember "Bergmann = bulk, Allen = appendages".
• Variation is mostly clinal (gradual), not in sharp boundaries.

Common mistakes

• Treating tanning as a genetic change; it is a short-term physiological response.
• Assuming visible traits like skin colour reveal overall genetic relatedness; they often reflect local selection.