Genetic drift is change in allele frequencies in a population from generation to generation that occurs due to chance events. To be more exact, genetic drift is change due to "sampling error" in selecting the alleles for the next generation from the gene pool of the current generation. Although genetic drift happens in populations of all sizes, its effects tend to be stronger in small populations. Genetic drift is one of four factors (mutation, migration, genetic drift, and natural selection) causing gene pools to change over time, and genetic drift is at the heart of several recent theories of evolution. In the shifting-balance theory of evolution (Wright, 1931) genetic drift is part of a two-phase process of adaptation of a subdivided population. In the first phase genetic drift causes each subdivision to undergo a random walk in allele frequencies to explore new combinations of genes. In the second phase a new favorable combination of alleles is fixed in the subpopulation by natural selection and is exported to other demes by factors like migration between demes. Much of the basic theory of genetic drift was developed in the context of understanding the shifting balance theory of evolution.