on the importance attributed to, and quantity of, the traits considered. A skin-lightening mutation, estimated to have occurred 20,000 to 50,000 years ago, partially accounts for the appearance of light skin in people who migrated out of Africa northward into what is now Europe. East Asians owe their relatively light skin to different mutations. On the other hand, the greater the number of traits (or alleles) considered, the more subdivisions of humanity are detected, since traits and gene frequencies do not always correspond to the same geographical location. Or as put it: Genetically differentiated populations Another way to look at differences between populations is to measure genetic differences rather than physical differences between groups. The mid-20th-century anthropologist William C. Boyd defined race as: "A population which differs significantly from other populations in regard to the frequency of one or more of the genes it possesses. It is an arbitrary matter which, and how many, gene loci we choose to consider as a significant 'constellation'". Leonard Lieberman and Rodney Kirk have pointed out that "the paramount weakness of this statement is that if one gene can distinguish races then the number of races is as numerous as the number of human couples reproducing." Moreover, the anthropologist Stephen Molnar has suggested that the discordance of clines inevitably results in a multiplication of races that renders the concept itself useless. The Human Genome Project states "People who have lived in the same geographic region for many generations may have some alleles in common, but no allele will be found in all members of one population and in no members of any other." Massimo Pigliucci and Jonathan Kaplan argue that human races do exist, and that they correspond to the genetic classification of ecotypes, but that real human races do not correspond very much, if at all, to folk racial categories. In contrast, Walsh & Yun reviewed the literature in 2011 and reported that "Genetic studies using very few chromosomal loci find that genetic polymorphisms divide human populations into clusters with almost 100 percent accuracy and that they correspond to the traditional anthropological categories." Some biologists argue that racial categories correlate with biological traits (e.g. phenotype), and that certain genetic markers have varying frequencies among human populations, some of which correspond more or less to traditional racial groupings. Distribution of genetic variation The distribution of genetic variants within and among human populations are impossible