Evolution by Natural Selection: How Does it Work?

The evolutionary process has three basic components: variation, inheritance and differential survival or reproduction. If these components are present in populations, then adaptive evolution will occur. First, nearly every characteristic of organisms varies slightly among individuals within populations. This variation is clear if we take any group of people, dogs, fish, plants or any other group of organisms. Looking closely, we will see that aspects of size, shape, behavior and other traits vary among individuals. Some of this variation is due to the environment that an individual has experienced. For example, getting a bit more food might make an organism slightly larger than others in its population. This kind of variation, caused by differences in the environment experienced by organisms, is not passed on to offspring and does not contribute to the evolutionary process. However, many differences among individuals are due to genetic differences, and this does contribute to evolution. For example, taller than average parents tend to have taller than average offspring because they pass on their genes for large size. Finally, when the differences among individuals affect their ability to survive and reproduce, then the individuals with the favorable characters will leave more offspring than others. This differential survival or reproduction of individuals with different characteristics is natural selection. If the favorable trait is inherited, then more of the next generation will have that trait, having inherited it from their successful parents.

This is the basis of evolution by natural selection: variation that is inherited (genetic variability) confers an advantage in survival or reproduction (natural selection) causes change over time and descent with modification. The traits that confer success are called adaptations, and these spread through populations because the individuals that bear them leave more offspring than do individuals with other characteristics. When inherited variation affects survival or reproduction, then evolution is inevitable.

Contributed by: Dr. Sara Via