2.8 The Four Mechanisms of Evolution

How does evolution occur? There are four mechanisms, or processes, that can cause changes in frequencies of traits within populations over multiple generations. These four processes can occur on their own or in combination.

1. Mutation

Mutation is a change in an organism’s genetic code – its DNA. For example, a nucleotide may change from an A to a G, or a long stretch of DNA may be deleted. These changes happen through errors in DNA replication and can cause changes in an organism’s traits. Mutation produces variation within a population because mutation creates new versions of genes (also known as gene variants).

2. Natural selection

Natural selection occurs when a trait (a characteristic of an organism) increases in frequency in a population because the trait causes an organism to have more offspring. Natural selection results in adaptations, traits that are beneficial in specific environmental conditions. For example, having wider blood vessels is adaptive (beneficial) in high altitude environments, and therefore the trait of having wider blood vessels has increased in frequency over many generations within human populations in Tibet.

3. Migration

Migration is the movement of some individual organisms from one population to another population. Migration is also referred to as gene flow, because migration causes gene variants to leave one population and enter another population–you can imagine genes “flowing” from one population to another.

4. Genetic Drift

Genetic drift, often simply called drift, is the process of random changes in the frequency of a trait within a population. The frequency of a trait “drifts” along randomly, changing only because of random chance. For example, imagine a population of frogs that lives near a highway. Random individual frogs may be killed by passing cars. This could happen to change the frequency of green-colored frogs relative to brown-colored frogs – not because color matters for an organism’s health, but simply due to bad luck.

We will explore each process of evolution over the next four sections.