 Regarding the dominant model: classically and summarily described (see Godfrey-Smith 2007), evolution occurs when individuals in a population accumulate a change to the degree that this change becomes a feature in the population. On this view, natural selection requires variation, fitness, and heritability (Lewontin 1970; though see Brandon 1990). When all three of these things coincide within populations, and change resultantly accumulates, evolution occurs. Proponents of species selection argue that these three conditions are met at the species level.