or so. Britain's cloudy skies may have played a role in that the genes for light hair could also cause light skin, reducing the chances of vitamin D deficiency. Sexual selection could also favor blond hair. The technique also enabled them to track the selection of polygenic traits—those affected by a multitude of genes, rather than just one—such as height, infant head circumferences, and female hip sizes (crucial for giving birth). They found that natural selection has been favoring increased height and larger head and female hip sizes among Britons. Moreover, lactase persistence showed signs of active selection during the same period. However, evidence for the selection of polygenic traits is weaker than those affected only by one gene. A 2012 paper studied the DNA sequence of around 6,500 Americans of European and African descent and confirmed earlier work indicating that the majority of changes to a single letter in the sequence (single nucleotide variants) were accumulated within the last 5,000-10,000 years. Almost three quarters arose in the last 5,000 years or so. About 14% of the variants are potentially harmful, and among those, 86% were 5,000 years old or younger. The researchers also found that European Americans had accumulated a much larger number of mutations than African Americans. This is likely a consequence of their ancestors' migration out of Africa, which resulted in a genetic bottleneck; there were few mates available. Despite the subsequent exponential growth in population, natural selection has not had enough time to eradicate the harmful mutations. While humans today carry far more mutations than their ancestors did 5,000 years ago, they are not necessarily more vulnerable to illnesses because these might be caused by multiple mutations. It does, however, confirm earlier research suggesting that common diseases are not caused by common gene variants. In any case, the fact that the human gene pool has accumulated so many mutations over such a short period of time—in evolutionary terms—and that the human population has exploded in that time mean that humanity is more evolvable than ever before. Natural selection might eventually catch up with the variations in the gene pool, as theoretical models suggest that evolutionary pressures increase as a function of population size. Early Modern Period to present A study published in 2021 states that the populations of the Cape Verde islands off the coast of West Africa have speedily evolved resistance to malaria within roughly