They use clines (gene flow) in such a way make it impossible to say that a circle cannot be drawn around a cluster of like genes in one population can't include genes in another population. This is a matter of which genes are selected, such as only picking the genes that determine variation for fingers, toes, and belly buttons. This is a another fallacy (but not the lewontin-fallacy). Then they say the science says that all human differences are on continuum or spectrum of gene expression (because we all have eyeballs, toes, and fingers, and that no biological or physical expression is unique to any population. Then they say that since their is no physical appearance or biological difference between an expression in any one human population that can't be found in anther population, then all human differences are just the result of psycho-social differences or 'culture'.