The random changes in gene frequencies occurring by chance and not under the control of natural selection are called genetic drift or random genetic drift or sampling error. This theory was advocated by geneticist Sewall Wright in 1930. It explains the change in gene frequency. It is caused due to sampling error. A new generation arises from a sample of sex cells, randomly taken from the gene pool of the preceding generation. When a sample is drawn in a large population randomly, chances that the sample will contain nearly the same gene frequency that is present in the gene pool are high. But in a small population, chances that the sample will contain an altered gene frequency are very high.