 Population genetics is intimately bound up with the study of evolution and natural selection, and is often regarded as the theoretical cornerstone of evolutionary biology. This is because “evolution” has traditionally been defined as any change in a population’s genetic composition (though this definition has been criticized); and natural selection is one important factor (though not the only one) that can lead to such a change. Natural selection occurs when some variants in a population out-reproduce other variants as a result of being better adapted to the environment, or ‘fitter’. Presuming the fitness differences are at least partly due to genetic differences, this will cause the population’s genetic makeup to change, as the genetic variants associated with higher fitness increase in frequency. By devising models of gene frequency change, population geneticists are thus able to shed light on the genetic basis of evolutionary change, and to permit the consequences of different evolutionary hypotheses to be explored in a quantitatively precise way.