The random changes in gene frequencies in a population occurring only by chance alone rather than by natural selection are called genetic drift or the Sewall Wright effect. It refers to the elimination of the genes of certain traits when a section of population migrates or dies of natural calamity. It alters the gene frequency of the remaining population. Hence genetic drift is a mechanism of evolution that acts in concert with natural selection and changes species characteristics over time. Although genetic drift occurs in all populations but its effects are more marked in very small' isolated populations