Variation in science refers to the differences or deviations from the recognized norm or standard. In biology, variation can be any difference between cells, individual organisms, or groups of organisms of any species caused either by genetic differences or by the effect of environmental factors on the expression of the genetic potentials. Variation may be shown in physical appearance, metabolism, fertility, mode of reproduction, behavior, learning and mental ability, and other obvious or measurable characters. Environmentally caused variations may result from one factor or the combined effects of several factors, such as climate, food supply, and actions of other organisms. Variations are classified either as continuous, or quantitative (smoothly grading between two extremes, with the majority of individuals at the center, as height varies in human populations); or as discontinuous, or qualitative (composed of well-defined classes, as blood groups vary in humans) . In genetics, variation refers to an individual that possesses characteristics different from the others of the same kind. Genetic variation usually arises as a mutation in a gene that encodes a protein or an RNA. Variations may be reflected in the phenotype of the individual, e.g. a difference in size, color, or pattern, or may be detectable only by DNA or protein sequencing.