 The cline of heterozygotes that is created when these respective populations come into contact is then shaped by the opposing forces of selection and gene flow; even if selection against heterozygotes is great, if there is some degree of gene flow between the two populations, then a steep cline may be able to be maintained.Because instrinsic selection is independent of the external environment, clines generated by selection against hybrids are not fixed to any given geographical area and can move around the geographic landscape.Such hybrid zones where hybrids are a disadvantage relative to their parental lines (but which are nonetheless maintained through selection being counteracted by gene flow) are known as “tension zones”.