Introduction

Humans are a young species, showing surprisingly little genetic variation; we are capable of interbreeding across our entire range. Attempts to divide humanity into discrete groups on the basis of our genes are not very successful; some genetic elements may be more common in some regions of the world than others, but different genes follow different geographical patterns. For example, contours of similarity in blood protein polymorphisms tend to lie north-south, whereas contours of similarity in skin colour lie east-west, parallel to the equator (Boyd and Silk 2003).