A cline describes the gradient differences between the prevalence of genotypes and phenotypes in different geographical regions. As one progresses from one region to another the prevalence of genotypes and phenotypes gradually changes as well. This observation has largely been taken as evidence of natural selection and gene flow. As the environment in which a organism lives gradually changes, so too do the traits that are beneficial to survival in that region. This speaks volumes towards the adaptability of organisms to their environment. Clines for humans have been observed to be largely continuous throughout regions demonstrating genetic flow in human variation.