Head size, blood type, the ability to cope with large amounts of alcohol, and sickle cell anemia have what in common?

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All of these traits were originally distributed in easy-to-see geographic clines, meaning they varied in frequency according to geography or region. For example, sickle cell anemia is more common in areas with high rates of malaria, as the genetic mutation that causes sickle cell also offers some protection against the disease. Blood type frequencies also vary geographically, with some types being more common in certain regions. Head size has been linked to climate, with larger heads found in colder areas to help retain heat, while smaller heads are found in warmer areas to help dissipate heat. Alcohol tolerance also varies by region, with some groups having higher rates of alcohol dehydrogenase, an enzyme that helps break down alcohol, than others.

Head Size, Blood Type, the Ability to Cope with Large