Gendrift biography

Genetic drift refers to random fluctuations in allele frequencies due to chance events see figure 6. 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 tosses, however, you would never get heads with a "fair" coin.

The same sort of random fluctuation 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 determine the frequency in a new population that grows back to the original population size.

A second small handful will randomly shift the frequency to yet another frequency. Genetic drift is not a potent evolutionary force in very large randomly mating populations. To illustrate the consequences of genetic drift we will consider what happens when drift alone is altering the frequencies of alleles among many small populations.

To illustrate this we need to understand Population structure , which describes how individuals or allele frequencies in breeding populations vary in time and space.

Evolution by natural selection

This structure is determined by the combined effect of deterministic and stochastic forces. We will introduce the idea of population structure by showing how genetic drift and inbreeding can change the frequencies of genotypes in populations. Consider a grid of small populations e.