Answer: genetic drift
A mechanism of evolution that suggests changes in the gene pool of a population are due to chance is genetic drift. A genetic drift is a natural phenomena in which changes occur in allele frequencies in a population from one generation to the next generation and subsequent generations. The main reason for genetic drift is the selection of allele for the next generation a kind of sampling error. Although genetic drift can occur in populations of all sizes, it's effects are stronger in small populations. In a genetic drift a beneficial allele may be lost or a harmful allele can become fixed in a gene pool of the population purely by chance.