GENETIC DRIFTGenetic drift refers to random fluctuations in allele frequencies due to chanceevents (see figure 6.4, pg. 142). The previous lectures have all dealt with deterministic (predictable) evolutionary forces often referred to as linear pressures. Genetic drift is a stochastic (random) force that can scramble the predictable effects of selection, mutation, and gene flow. While it might seem that a random force would be of little significance to evolutionary "progress" (we'' confront this loaded term later), genetic drift is an extremely important force in evolution. However, its strength depends on the size of the population, as a simple exercise in coin tossing will illustrate. In ten tosses you might easily get seven heads; in 1000 tosses, however, you would never get 700 heads with a "fair" coin. The same sort of randomfluctuation in allele frequencies can occur in small populations: consider a bag full of red and green marbles each in equal frequency; pull out a small handful and the frequency in your hand will probably not equal the frequency in the original bag. Let that handful
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