 For example, through environmental selection acting on populations and favouring particular allele frequencies, large genetic differences between populations may accumulate (this would be reflected in clinal structure by the presence of numerous very steep clines).If the local genetic differences are great enough, it may lead unfavourable combinations of genotypes and therefore to hybrids being at a decreased fitness relative to the parental lines.When this hybrid disadvantage is great enough, natural selection will select for pre-zygotic traits in the homozygous parental lines that reduce the likelihood of disadvantageous hybridisation - in other words, natural selection will favour traits that promote assortative mating in the parental lines.