B.7F-Evolutionary Mechanisms_B.10D Approaching – 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 even mix of red and white varieties. Over time, you notice fewer and fewer white flowers. What is causing the changes in this population? The simple answer is that the changes are due to chance. There are several evolutionary mechanisms, or processes, that can cause the random changes.

Chance can cause big changes in small populations. By chance, a population of ten rabbits loses five members that had never reproduced. The five rabbits may have died before reaching the reproductive age. Or the rabbits may have lived an entire lifetime without reproducing. The rabbit population would lose 5/10 or 50% of its gene pool. The gene pool is made up of all the alleles in a population. Alleles are different versions of the same gene.

Look at the diagram of the rabbit population. See how random chance changed the gene pool from the first generation to the second? This change due to chance is known as genetic drift.

Look at the rabbit diagram again. Observe what happens to the gene pool between the second and third generations. The recessive gene was erased due to chance. The number of gene versions decreased from two to one. This means the genetic variation, or diversity, has decreased. Genetic drift can result in a loss of diversity.

Genetic drift can apply to a large population that loses a lot of members. This can happen after a natural disaster. A fire can wipe out a large portion of a forest. Members survive by chance. The gene pool of the survivors may be very different from the original. There may be a higher percentage of some alleles. There may be a lower percentage of others. Some may be completely erased.