 The genetic or phenotypic trait in question always shows a steeper gradient between groups than within groups, as in continuous clines.Discontinuous clines follow the same principles as continuous clines by displaying either

It was originally assumed that geographic isolation was a necessary precursor to speciation (allopatric speciation).The possibility that clines may be a precursor to speciation was therefore ignored, as they were assumed to be evidence of the fact that in contiguous populations gene flow was too strong a force of homogenisation, and selection too weak a force of differentiation, for speciation to take place.