 However, the existence of particular types of clines, such as ring species, in which populations did not differentiate in allopatry but the terminal ends of the cline nonetheless do not interbreed, cast into doubt whether complete geographical isolation of populations is an absolute requirement for speciation.Because clines can exist in populations connected by some degree of gene flow, the generation of new species from a previously clinal population is termed parapatric speciation.Both extrinsic and intrinsic selection can serve to generate varying degrees of reproductive isolation and thereby instigate the process of speciation.