This structure is determinedby the combined effect of deterministic and stochastic forces. To illustrate this we need to understand Populationstructure, which describes how individuals (or allele frequencies)in breeding populations vary in time and space. To illustrate the consequences of genetic drift we will consider whathappens when drift alone is altering the frequencies of alleles among manysmall populations. Genetic driftis not a potent evolutionary force in very large randomly mating populations. Ifyou pulled out all the marbles in the bag (= large population) then thefrequency would be maintained exactly in the next generation. A second smallhandful will randomly shift the frequency to yet another frequency. Let that handful determine the frequency in a newpopulation that grows back to the original population size. The same sort of random fluctuationin allele frequencies can occur in small populations: consider abag full of red and green marbles each in equal frequency pull out a smallhandful and the frequency in your hand will probably not equal the frequencyin the original bag. In ten tosses you might easilyget seven heads in 1000 tosses, however, you would never get 700 headswith a "fair" coin. While it might seem that a random force would be of littlesignificance to evolutionary "progress" (we'' confront this loadedterm later), genetic drift is an extremely important force in evolution.However, its strength depends on the size of the population, as a simpleexercise in coin tossing will illustrate. Genetic drift is a stochastic (random)force that can scramble the predictable effects of selection, mutation,and gene flow. The previous lectureshave all dealt with deterministic (predictable) evolutionary forces oftenreferred to as linear pressures. Genetic drift refers to random fluctuations in allele frequencies dueto chance events (see figure 6.4, pg.
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