For example, a species of fern might exist simultaneously on what is now the eastern coast of South America and the western coast of Africa. Clines - Some genetic variation among populations of many species is correlated with geographic distribution. Clines are widespread among most groups of organisms. Results in preservation of variation in large populations as differences between subpopulations. Much of the variation in traits in a population is neutral and so is not affected by natural selection.
Neutral variation is especially common at the molecular level.
Polymorphism - genetic differences within local populations. Polymorphisms preserve genetic variation when success of a genotype or phenotype depends on its frequency relative to other genotypes or phenotypes in a process. Called Frequency-dependent Selection. The changes related to clines are changes in phenotype and genotype. Prevalent in or specific to a particular locale.