Clines and ecotypes are variants of a particular species adapted to a specific locale or set of environmental conditions. Charles Darwin (1809-1882) put forth his concept of evolution by natural selection to explain patterns of within species variation in 1859. Early-twentieth-century plant ecologists and systematists such as Frederic Clements (1874-1945) and Gote Turesson (1892-1970) recognized the usefulness of Darwin's theory and built on it. These plant biologists reasoned that variation within species reflects adaptations to specific environmental conditions.

Different populations of the same species often grow across a range of environmental conditions, encompassing variation in moisture levels, soil composition, length of growing season, types and amounts of herbivores, and, for animal-pollinated species, even variation in the composition of the pollinators. These differences in environmental conditions may generate different selection pressures across the species range, resulting in genetic divergence among populations. For example, studies of Northern...