---
title: "How Natural Selection Works"
description: "Explain how heritable variation and unequal reproductive success can change trait frequencies across generations through natural selection, one mechanism of evolution; competition is one possible sour"
canonical: https://lightmysky.com/learn/science/how-natural-selection-works-mt_FuVEZ1Ac9s
source: https://lightmysky.com/learn/science/how-natural-selection-works-mt_FuVEZ1Ac9s.md
retrieved: 2026-10-19
---

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# How Natural Selection Works

Explain how heritable variation and unequal reproductive success can change trait frequencies across generations through natural selection, one mechanism of evolution; competition is one possible source of selection pressure.

Subject: Science · Area: Ecosystems & Habitats · Ages 12 to 14
Page: https://lightmysky.com/learn/science/how-natural-selection-works-mt_FuVEZ1Ac9s

## Ready when they can

- Explains how heritable variation and unequal reproductive success allow evolution by natural selection, with competition as one possible selection pressure.
- Applies the concept to a specific example (e.g. antibiotic resistance in bacteria, peppered moth)
- Explains why individuals with advantageous traits leave more offspring
- Distinguishes natural selection, one mechanism, from evolution, a change in inherited variation across generations.

## Lesson: How Natural Selection Works

You met variation in height and blood group, and mutations that change DNA. Natural selection helps explain antibiotic resistance. Four ideas connect many examples: variation, heredity, competition, and differences in survival and reproduction. Selection favours inherited traits that help individuals leave more offspring. Competition often creates that advantage, but hazards such as antibiotics can too. If a trait changes neither survival nor reproductive success, selection does not favour it.

*(drawing: Competition can favour some inherited traits. What matters is whether a trait helps individuals leave more offspring.)*

**Example.** In one infection, a few bacteria already carry inherited antibiotic resistance. Treatment kills susceptible bacteria; resistant survivors reproduce and pass resistance on. After two divisions, each survivor has four descendants. No bacterium chooses resistance. A mutation can arise before or after treatment starts, without appearing because it is needed. Peppered moths show selection too: on soot-darkened trees, birds caught more pale moths, so dark moths left more offspring. Likewise, inherited speed can help rabbits escape predators and reproduce.

Natural selection is one mechanism of evolution. It acts through differences in survival and reproduction associated with inherited traits. Evolution means a change in inherited variation in a population across generations; it can be measured between parent and offspring generations. It need not wait for many rounds of selection. Mutation, gene flow, and random genetic drift can also cause evolution. With equal survival and reproductive success, selection does not favour a trait, though drift can still change its frequency.

**Recap.** Natural selection favours inherited traits through unequal reproductive success. It is one cause of evolution, which changes inherited variation across generations.

## Practice

8 questions on this page, each with its working shown.

## Needs first

- [Classifying Organisms](https://lightmysky.com/learn/science/classifying-organisms-mt_IY3KwGLZgk)
- [Variation in Species](https://lightmysky.com/learn/science/variation-in-species-mt_k1HXbEwG8f)
- [The Red Queen Hypothesis](https://lightmysky.com/learn/science/the-red-queen-hypothesis-mt_mKNmXqz_Oo)
- [Dinosaur-to-Bird Transition](https://lightmysky.com/learn/science/dinosaur-to-bird-transition-mt_mqQ-DtH5m-)
- [Genetic Mutation](https://lightmysky.com/learn/science/genetic-mutation-mt_rOqo-8GeKt)
- [Mates, Kin and Animal Strategies](https://lightmysky.com/learn/science/mates-kin-and-animal-strategies-mt_YNrrNE23dZ)

## Opens up

- [Extinction & Rapid Change](https://lightmysky.com/learn/science/extinction-and-rapid-change-mt_-r3B4FQyX3)
- [Speciation and the Rate of Evolution](https://lightmysky.com/learn/science/speciation-and-the-rate-of-evolution-mt_OdcdfeRyo1)
- [Evidence for Evolution](https://lightmysky.com/learn/science/evidence-for-evolution-mt_ohUnzoI_nx)