Natural Selection
Grade 8 · Science · Study Guide
Learn how organisms with favorable traits survive and reproduce, driving change in populations over time.
Natural selection is the process by which organisms with traits better suited to their environment survive, reproduce, and pass those traits to offspring. Charles Darwin proposed this mechanism of evolution after observing variation in species across the Galapagos Islands.
Four conditions are required for natural selection: variation (individuals differ in traits), heritability (traits are passed to offspring), overproduction (more offspring are born than can survive), and differential survival (individuals with favorable traits survive and reproduce at higher rates).
Over many generations, natural selection can cause a population to change significantly. Adaptations — inherited traits that increase survival or reproduction — accumulate. Organisms that are poorly adapted leave fewer offspring, so those traits become less common over time.
| Requirement | Meaning | Example |
|---|---|---|
| Variation | Individuals differ in traits | Some moths are light, some are dark |
| Heritability | Traits pass to offspring | Dark moths have dark offspring |
| Overproduction | More offspring than can survive | Hundreds of eggs, few survive |
| Differential survival | Better-adapted survive more | Camouflaged moths avoid predators |
| Natural Selection | Artificial Selection |
|---|---|
| Environment selects traits | Humans select traits |
| Slow — many generations | Can be relatively fast |
| Example: peppered moth | Example: dog breeding |
Key terms
- Natural Selection
- The process by which organisms with favorable traits survive and reproduce more successfully.
- Adaptation
- An inherited trait that increases an organism's fitness in its environment.
- Fitness
- An organism's ability to survive and reproduce in its environment.
- Variation
- Differences in traits among individuals in a population.
Practice questions
Which of the following is NOT a requirement for natural selection to occur?
- Variation in traits
- Traits that are heritable
- A stable environment with no change
- Differential reproduction rates
Show answer
Natural selection requires variation, heritability, overproduction, and differential survival — not a stable environment. In fact, environmental pressure drives selection.
A population of beetles lives on green leaves. Green beetles survive better than brown ones. After many generations, most beetles are green. This is an example of:
- Artificial selection
- Natural selection
- Genetic mutation
- Sexual reproduction
Show answer
The environment (predators on green leaves) selects for green coloring — beetles with better camouflage survive and reproduce more.
Natural selection acts on individual organisms' genes directly, changing their DNA during their lifetime.
Show answer
Natural selection acts on phenotypes (expressed traits), not individual genes directly. Changes occur at the population level across generations, not within an organism's lifetime.
An inherited trait that increases an organism's survival or reproductive success is called an ___.
Show answer
Adaptations build up over many generations through natural selection favoring beneficial traits.
Explain how antibiotic resistance in bacteria is an example of natural selection.
Show answer
This shows all four requirements: variation exists, resistance is heritable, bacteria overproduce, and resistant ones survive differentially.
Fun facts
- The peppered moth became a classic example of natural selection — during the Industrial Revolution, dark moths survived better on soot-covered trees, rapidly increasing in population.
- Darwin spent five years on the HMS Beagle voyage gathering evidence, but waited over 20 years to publish his theory of evolution by natural selection.
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