 Because over time the effect of gene flow will tend to eventually swamp out any regional differences and cause one large homogenous population, for a stable cline to be maintained when two populations join there must usually be a selective pressure maintaining a degree of differentiation between the two populations.The mechanism of selection maintaining the clines in this scenario is often intrinsic.This means that the fitness of individuals is independent of the external environment, and selection is instead dependent on the genome of the individual.