In 2015, Keith Hunley, Graciela Cabana, and Jeffrey Long analyzed the Human Genome Diversity Project sample of 1,037 individuals in 52 populations, finding that non-African populations are a taxonomic subgroup of African populations, that "some African populations are equally related to other African populations and to non-African populations," and that "outside of Africa, regional groupings of populations are nested inside one another, and many of them are not monophyletic." Earlier research had also suggested that there has always been considerable gene flow between human populations, meaning that human population groups are not monophyletic. Rachel Caspari has argued that, since no groups currently regarded as races are monophyletic, by definition none of these groups can be clades.