 Evolutionary biologists often define ‘evolution’ as any change in a population’s genetic composition over time. The rationale for this definition is the idea that all other aspects of evolution, e.g., the spread of novel phenotypic traits and the formation of new species, stem ultimately from changes in gene frequencies within populations. The four factors that can bring about such a change are: natural selection, mutation, random genetic drift, and migration into or out of the population. (A fifth factor—changes to the mating pattern—can change the genotype but not the gene frequencies; many theorists would not count this as an evolutionary change.) A brief introduction to the standard population-genetic treatment of each of these factors is given below.