- There are four forces of evolution: mutation, gene flow, genetic drift, and natural selection.
- Mutation creates new genetic variation in a gene pool.
- Gene flow and genetic drift alter allele frequencies in a gene pool.
In biology, evolution is the change in the characteristics of a species over several generations and relies on the process of natural selection. The forces that cause these characteristic changes are mutation, gene flow and genetic drift.
Mutation creates new genetic variation in a gene pool. It is how all new alleles first arise. In sexually reproducing species, the mutations that matter for evolution are those that occur in gametes. I can pass only these mutations to offspring. For any given gene, the chance of a mutation occurring in each gamete is exceptionally low. Thus, mutations alone do not have much effect on allele frequencies. However, mutations provide the genetic variation needed for other forces of evolution to act.
Gene flow occurs when individuals move into or out of a population. If the rate of migration is high, this can have a significant effect on allele frequencies. The allele frequencies of both the population they leave and the population they enter may change.
During the Vietnam War in the 1960s and 1970s, many American servicemen had children with Vietnamese women. Most of the servicemen returned to the United States after the war. However, they left copies of their genes behind in their offspring. In this way, they changed the allele frequencies in the Vietnamese gene pool. Was the gene pool of the American population also affected? Why or why not?
Genetic drift is a random change in allele frequencies that occurs in a 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.
- 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.
- Founder effect occurs when a few individuals start, or found, a new population. By chance, the allele frequencies of the founders may be different from allele frequencies of the population they left.