Both directional and disruptive selection are types of natural selection that alter the mean or variance of a phenotypic trait in a population over many generations. Directional selection occurs when an extreme phenotype at one end of a population distribution is favored, shifting the distribution curve. An example is the development of malaria resistance. Disruptive selection favors both extremes of a trait over intermediate phenotypes, such as with British land snails found in low vegetation areas that are favored if they are either very small or very large. Specialization of plants and pollinators can have both positive effects like increased food sources but also negative effects like reproductive isolation if dependence on a single pollinator species is too strong.