B.7F-Evolutionary Mechanisms_B.10D on level – 02

It is natural for populations to change from one generation to the next. Maybe you have seen a small patch of wildflowers with an equal mixture of red and white varieties. You notice over the course of several generations, that there are fewer and fewer white flowers. What is the driving force behind these generation-to-generation changes in populations? The simple answer is that random chance is behind these changes. There are several evolutionary mechanisms that can cause such random changes in the genetic structure of a population.

Small populations can be greatly impacted by chance events. For example, a population of ten rabbits randomly lost five members that had never reproduced. That population would lose 5/10, or 50%, of its gene pool. The gene pool consists of all the alleles, or versions of the same gene, in a population. This major change in the genetic population can cause a difference in the frequencies of the different alleles from one generation to the next. This random change in frequencies, due to chance events, is known as genetic drift. In the rabbit population, the frequency of the dominant allele drifted from 50% to 70% from the 1st generation to the 2nd generation; the recessive allele frequency drifted from 50% to 30%.

It is not necessary that a small population physically lose members. There may be random members that simply did not reproduce during their lifetimes. Either scenario may lead to genetic drifting of allele frequencies from one generation to the next. Genetic drift can result in a population that has lost much of its genetic variation. This could explain the frequency change from the second to the third generations of the rabbit population or in the small patch of red and white wildflowers. Genetic drift could ultimately eliminate the recessive allele in these populations.

The same principle of genetic drift can apply when a large population suffers great losses, such as the result of a natural disaster. Consider a fire wiping out a large portion of a forest. By chance among the survivors, some alleles are overrepresented, some alleles are underrepresented, and some alleles may be eliminated altogether. In turn, the survivors may generate a new population that has genetically drifted a great deal from the original population.