Genetic drift (also called random genetic drift) means a change in the gene pool strictly by chance fixation of alleles. The effects of genetic drift can be acute in small populations and for infrequently occurring alleles, which can suddenly increase in frequency in the population or be totally wiped out. The alleles thus fixed by chance (genetic sampling error) may be neutral—that is, they may not confer any survival or reproductive advantage. Therefore, for small populations, genetic drift can result in a significant change in gene frequency in a short period of time. Genetic drift can be caused by a number of chance phenomena, such as differential number of offspring left by different members of a population so that certain genes increase or decrease in number over generations independent of selection, sudden immigration or emigration of individuals in a population changing gene frequency in the resulting population, or population bottleneck. Of these, population bottleneck can cause a radical change in allele frequencies in a very short time.