(based on similarity), rather than cladistic (based on monophyletic descent, that is from a series of unique ancestors)." Evolutionary biologist Alan Templeton (2013) argued that multiple lines of evidence falsify the idea of a phylogenetic tree structure to human genetic diversity, and confirm the presence of gene flow among populations. Marks, Templeton, and Cavalli-Sforza all conclude that genetics does not provide evidence of human races. Previously, anthropologists Lieberman and Jackson (1995) had also critiqued the use of cladistics to support concepts of race. They argued that "the molecular and biochemical proponents of this model explicitly use racial categories in their initial grouping of samples". For example, the large and highly diverse macroethnic groups of East Indians, North Africans, and Europeans are presumptively grouped as Caucasians prior to the analysis of their DNA variation. They argued that this a priori grouping limits and skews interpretations, obscures other lineage relationships, deemphasizes the impact of more immediate clinal environmental factors on genomic diversity, and can cloud our understanding of the true patterns of affinity. 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 diversity among non-African populations is the result of a serial founder effect process, with non-African populations as a whole nested among 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. Clines One crucial innovation in reconceptualizing genotypic and phenotypic variation was the anthropologist C. Loring Brace's observation that such variations, insofar as it is affected by natural selection, slow migration, or genetic drift, are distributed along geographic gradations or clines. For example, with respect to skin color in Europe and Africa, Brace writes:To this day, skin color grades by imperceptible means from Europe southward around the eastern end of the Mediterranean and up the Nile into Africa. From one end of this range to the other, there is no hint of a skin color boundary, and yet the spectrum