What is natural selection, with an example?
Natural selection is the process by which traits that improve an organism's chances of survival and reproduction become more common in a population over successive generations. It is the primary mechanism that drives evolution.
Think of it like a basketball tryout where only the tallest players make the team. Over time, the team's average height increases. In nature, the "tryout" is the struggle to find food and avoid predators, and the "coach" selecting the players is the environment itself.
What it is
For natural selection to happen, a population must have three things. First, there must be variation: individuals in a group must have different traits, like different fur colors. Second, these traits must be heritable, meaning they are passed from parents to offspring through genetics. Third, there must be differential reproductive success. This means some traits give individuals a better chance to survive and have babies than others.
Why it works
Natural selection works because environments have limited resources. Organisms face constant pressures like predators, climate changes, and food scarcity. These environmental pressures act as a filter. If an individual has a mutation that makes them slightly faster or better camouflaged, they are more likely to survive this filter. When they reproduce, they pass that advantageous trait to the next generation. Over time, the frequency of this advantageous allele, often represented mathematically as in the equation , increases in the population.
Where students slip up
The most common mistake is thinking that individual organisms evolve during their lifetime. An individual cannot change its genetics just because the environment changes. Populations evolve over generations, not individuals. Another common trap is thinking that organisms "try" to adapt. Evolution has no goal or intention; it is simply the automatic result of some individuals surviving better than others.
Worked through
Explain natural selection using the classic example of the peppered moth during the Industrial Revolution.
Before the Industrial Revolution in England, most peppered moths had light-colored wings. This was an advantage because they blended in perfectly with the light-colored bark and lichens on the trees, hiding them from hungry birds. A small number of moths had a genetic mutation that made them dark-colored. These dark moths were easily seen and eaten, so they were rare in the population.
When factories began burning coal, the soot covered the trees and killed the lichens, turning the tree trunks black. Suddenly, the light-colored moths stood out and were heavily preyed upon by birds. The dark-colored moths were now camouflaged.
Because the dark moths survived longer, they reproduced more, passing the gene for dark wings to their offspring. Over several generations, the population of peppered moths shifted from mostly light-colored to mostly dark-colored. The environment changed, and natural selection favored the dark trait.
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Where this comes from: OpenStax Biology 2e, Chapter 11: Evolution and Its Processes · Khan Academy, Unit: Natural Selection · Campbell Biology, 12th Edition, Chapter 22: Descent with Modification
See also