The differences between individuals of a plant or animal species. Variation may be the result of environmental conditions; for example, water supply and light intensity affect the height and leaf size of a plant. Differences of this kind, acquired during the lifetime of an individual, are generally not transmitted to succeeding generations since the genes are not affected (see phenotypic plasticity). However, environmental factors may lead to epigenetic changes arising during the life of an organism, and these can sometimes be transmitted to offspring. Although these alter the phenotype, they do not affect the genotype and may be reversed subsequently. Genetic variation, due to differences in genetic constitution, is inherited (see continuous variation; discontinuous variation). The most important sources of genetic variation are mutation and recombination (see also crossing over). It is also increased by outbreeding. Wide genetic variation improves the ability of a species to survive in a changing environment, since the chances that some individuals will tolerate a particular change are increased. Such individuals will survive and transmit the advantageous genes to their offspring.