Natural Selection
Middle School
Definition
The process by which organisms with traits better suited to their environment tend to survive and reproduce more successfully, passing those traits to offspring. Over many generations, this gradually changes the characteristics of a population—the basis of evolution.
Worked examples
Peppered moths in England: before industrial pollution, light-colored moths blended with pale tree bark and survived better; after soot darkened trees, dark moths survived and became common.
The environment changed, so the trait that helped survival changed, shifting the population's characteristics.
Antibiotic-resistant bacteria: a few bacteria survive the drug, reproduce, and soon most of the population carries resistance genes.
Bacteria with the resistance trait survive treatment and pass it on, demonstrating selection in action.
Common mistakes
- Individual organisms evolve by natural selection during their lifetime. → Populations evolve over generations; individuals do not change their traits. Natural selection acts on variation already present; it does not alter an individual organism.
- Natural selection always produces the 'best' or most complex organism. → Natural selection favors traits that fit the current environment, not perfection. What is advantageous depends entirely on the specific environment and can change.
- Organisms try to adapt and pass those efforts to offspring. → Variation arises randomly; selection favors helpful traits already present. Traits are not acquired by effort or need—mutations and genetic shuffling create variation first.
Where you'll use it next
Natural selection is the foundation for studying evolution, speciation, and biodiversity in biology. You'll apply it to understand antibiotic resistance, conservation, genetics, and how populations adapt to climate change.
See also
Reviewed by Pat Cheng, M.Ed. — StudyPug Curriculum Lead · Last updated June 6, 2026