According to Hardy-Weinberg principle, in infinitely large, randomly mating panmictic populations, gene frequency either remains constant (when no mutations and no selection) or exhibits directional change (under the influence of natural selection). But in small populations, the gene frequencies exhibit great nondirectional fluctuation generation after generation purely by chance. In one generation, the gene frequency of a particular gene may increase far beyond the normal range and may decrease extremely in the next generation. Some genes may be lost or reduced in frequency by sheer chance, others may increase. Some genes may be lost or reduced in frequency by sheer chance, others may increase.