10/22 Genetic Drift

 

In these story book pages there are blue mice and grey mice. The grey mice blend in very well with the street and sidewalk while the blue mice stick out. As a result of being so visible, a bird preys on a blue mouse. The loss of the blue mouse changes the allele frequency in the population of mice. With less blue mice available to reproduce, the subsequent generation has a higher frequency of grey mice and their alleles than the original population. This is  NOT genetic drift. The changes in allele frequency in this case were because of decreased fitness and were not random.

In these story book pages there is a population of grey and blue mice. Some of the grey mice are in the street when a car comes and runs them over. This reduced the population and changed the allele frequencies. The frequency of the allele leading to a blue coat is now much higher than it was. As a result, subsequent populations have a higher percentage of blue mice than the original population did. This IS due to genetic drift  because it occurred RANDOMLY! Neither phenotype was favored in this case and instead survival and changes in allele frequency were due to chance rather than fitness.

Genetic drift can alter the way evolution precedes by affecting the allele frequencies in a population. It can prolong the time a beneficial allele goes to fixation by increasing the frequency of the less fit allele. Depending on it's strength, drift can also allow the less fit allele to go to fixation.