Often taxonomists prefer to use phylogenetic analysis to determine whether a population can be considered a subspecies. Phylogenetic analysis relies on the concept of derived characteristics that are not shared between groups, usually applying to populations that are allopatric (geographically separated) and therefore discretely bounded. This would make a subspecies, evolutionarily speaking, a clade – a group with a common evolutionary ancestor population. The smooth gradation of human genetic variation in general tends to rule out any idea that human population groups can be considered monophyletic (cleanly divided), as there appears to always have been considerable gene flow between human populations. Rachel Caspari (2003) have argued that clades are by definition monophyletic groups (a taxon that includes all descendants of a given ancestor) and since no groups currently regarded as races are monophyletic, none of those groups can be clades.