 Genetic clines can be generated by adaptation to environment (natural selection), continuous gene flow between two initially different populations or a demographic expansion into a scarcely populated environment, with little initial admixture with existing populations.Cavalli-Sforza connected these gradients with postulated pre-historic population movements, based on archaeological and linguistic theories.However, given that the time depths of such patterns are not known, "associating them with particular demographic events is usually speculative".