Analogy With Marbles in A Jar
The process of genetic drift can be illustrated using 20 marbles in a jar to represent 20 organisms in a population. Consider this jar of marbles as the starting population. Half of the marbles in the jar are red and half blue, and both colors correspond to two different alleles of one gene in the population. In each new generation the organisms reproduce at random. To represent this reproduction, randomly select a marble from the original jar and deposit a new marble with the same color as its "parent" into a new jar. (The selected marble remains in the original jar.) Repeat this process until there are 20 new marbles in the second jar. The second jar then contains a second generation of "offspring", consisting of 20 marbles of various colors. Unless the second jar contains exactly 10 red and 10 blue marbles, a random shift occurred in the allele frequencies.
Repeat this process a number of times, randomly reproducing each generation of marbles to form the next. The numbers of red and blue marbles picked each generation fluctuates: sometimes more red, sometimes more blue. This fluctuation is genetic drift – a change in the population's allele frequency resulting from a random variation in the distribution of alleles from one generation to the next.
It is even possible that in any one generation no marbles of a particular color are chosen, meaning they have no offspring. In this example, if no red marbles are selected the jar representing the new generation contains only blue offspring. If this happens, the red allele has been lost permanently in the population, while the remaining blue allele has become fixed: all future generations are entirely blue. In small populations, fixation can occur in just a few generations.
Read more about this topic: Genetic Drift
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