Course: BIS 105 Environmental Biology

Unit 3 Learning Module

Genetic Drift

Evolutionary Mechanisms

Natural selection is often the main cause of evolution in populations. However, there are other evolutionary mechanisms that can cause changes to occur in a population over time. One of these processes is referred to as genetic drift.

Genetic Drift

Genetic drift is a random change in allele frequencies that occurs in a population. The affects of genetic drift are usually most apparent in small population. When a small number of parents produce just a few offspring, allele frequencies in the offspring may differ, by chance, from allele frequencies in the parents. This is like tossing a coin. If you toss a coin just a few times, you may, by chance, get more or less than the expected 50 percent heads or tails. In a small population, you may also, by chance, get different allele frequencies than expected in the next generation. In this way, allele frequencies may drift over time. There are two special conditions under which genetic drift occurs. They are called bottleneck effect and founder effect.

1. Bottleneck effect occurs when a population suddenly gets much smaller. This might happen because of a natural disaster such as a forest fire. By chance, allele frequencies of the survivors may be different from those of the original population.

2. Founder effect occurs when a few individuals start, or found, a new population. By chance, allele frequencies of the founders may be different from allele frequencies of the population they left.