What is Evolution by Natural Selection?
Natural selection is the mechanism by which organisms with advantageous traits survive and reproduce more successfully than others. Over generations, helpful traits become more common and populations adapt to their environment. It's the foundation of modern evolutionary biology.
Natural selection occurs when individuals with traits better suited to their environment survive to reproduce more often, passing advantageous genes to offspring. Over many generations, beneficial traits become common and populations evolve—changing species composition without divine intervention.
- 1↓VariationIndividuals in a population have different traits (e.g., some beetles are darker, some lighter)
- 2↓Struggle for existenceResources (food, water, shelter) are limited; not all individuals survive
- 3↓Differential survivalIndividuals with advantageous traits are more likely to survive and reproduce (e.g., dark beetles hide from predators better)
- 4↓InheritanceSurvivors pass their advantageous traits to offspring via genes
- 5AdaptationOver many generations, advantageous traits become more common; population adapts to environment
Step-by-step worked examples
In a population of moths, dark moths survive predation better in polluted forests while light moths survive better in clean forests. Explain natural selection.
Variation: dark and light moths exist Limited resources: predators eat many moths Differential survival: in polluted forest, dark moths (camouflaged) are eaten less Inheritance: dark moths breed, passing alleles for darkness Adaptation: over generations, forest population becomes darker Conclusion: population color matches environment—natural selection
A disease kills some rabbits but a few have genetic resistance. What happens over time?
Variation: some rabbits carry disease-resistance genes Disease pressure: many rabbits die, but resistant rabbits survive Differential survival: resistant rabbits are more likely to breed Inheritance: survivors pass resistance alleles to offspring Adaptation: in subsequent generations, more rabbits are resistant Result: population evolves higher disease resistance through natural selection
Why does antibiotic resistance in bacteria spread so quickly?
Variation: some bacteria carry resistance genes Antibiotic exposure: selection pressure kills non-resistant bacteria Differential survival: resistant bacteria survive and divide rapidly Inheritance: each daughter cell inherits resistance genes Adaptation: resistant population dominates in days/weeks (fast because generation time is short) Conclusion: natural selection works on any timescale with selection pressure
Flashcards
Quick quiz
Q1.Which is NOT a requirement for natural selection to occur?
Q2.In a population of beetles, 90% are brown and 10% are red. A predator hunts by sight and eats red easily. What will happen?
Q3.A disease kills 50% of a population with genotype aa. What is this an example of?
Q4.If a beneficial mutation is rare, can natural selection spread it fast?
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Common mistakes
Natural selection 'needs' variation. If there's no variation, selection can't work. — Correct: Natural selection REQUIRES variation. Without variation, there's nothing to select. Mutations provide new variation.
An organism evolves during its lifetime to suit the environment. — Correct: An individual doesn't evolve; populations evolve over many generations. An individual either survives/reproduces or doesn't.
Natural selection is directional—always toward improvement. — Correct: Selection is environment-dependent. A trait 'good' in one environment is 'bad' in another. Evolution has no goal.
If a mutation is beneficial, it will definitely spread. — Correct: A beneficial allele CAN spread, but it's not guaranteed—especially if rare or if selection pressure is weak. Chance (genetic drift) also plays a role.
FAQ
How long does natural selection take to produce visible change?
It depends on selection pressure, generation time, and mutation rate. In bacteria, weeks. In humans, millennia. Faster reproduction = faster evolution.
Is natural selection the only mechanism of evolution?
No. Mutation introduces variation; natural selection acts on it. Genetic drift (random change) and gene flow (migration) also shape evolution.
Can natural selection eliminate bad alleles completely?
If an allele is recessive and hidden in heterozygotes, natural selection can reduce it slowly but may not eliminate it fully—heterozygotes survive.
What is the difference between adaptation and evolutionary fitness?
Adaptation is a trait that evolved because it was advantageous. Fitness is reproductive success—the number of viable offspring. An adaptation increases fitness.