The Theory of Population Genetics 97 



Table 6.4 | Breeding Size — 400 



Genetic Drift 



Consider a barrel containing 10,000 black marbles and 10,000 white 

 marbles representing the gametes of a population with a gene fre- 

 quency of p = cj = .50 at some locus. A random sample of 2,000 

 marbles from this barrel represents the 1,000 diploid individuals that 

 will make up the breeding population of the next generation. Perhaps 

 the first sample consists of 979 white marbles and 1,021 black mar- 

 bles (p = .49 ) . The "gene pool" barrel is then reconstituted witli 

 9,790 white marbles and 10,210 black marbles, and the sampling of 

 2,000 marbles is repeated. This time assume that a sample of 

 1,033 white and 967 black marbles represents the gene frequency 

 (p = .52) of the breeding population of the next generation. Once 

 again the barrel is reconstituted, with 10,350 white and 9,670 black 

 marbles. A continuation of this process would, under most circum- 

 stances, produce a very slight fluctuation of gene frequency around 

 the original figure of p = .50. Note that this model is constructed so 

 that the population size remains constant. 



Now consider another barrel containing 500 white marbles and 

 500 black marbles to represent the gametic gene pool of a smaller 

 population with gene frequency of p = .50. Suppose 10 marbles 

 representing 5 indixiduals are withdrawn at random and that 6 are 

 black and 4 are white ( p = .60 ) . The original gametic population 

 is reconstituted with 600 black and 400 white marbles and the pro- 

 cedure repeated. Now 8 black and 2 white marbles are drawn ( gene 

 frequency p = .80) and the barrel reconstituted with 800 black and 

 200 white. On the third sampling one might get 7 black and 3 white 

 ( gene frequency p = .10), and thus reconstitute the barrel with 700 

 black and 300 white. It is easy to see that, in carrying through this 

 procedure, much more violent fluctuations in gene frequency have 

 been caused than arose with the larger population. This random 



