What is Natural Selection?
Natural selection is the process by which organisms better suited to their environment survive and reproduce more successfully, passing their advantageous traits to the next generation. Over many generations, this drives evolution — the change in heritable traits of populations over time.
Natural selection is evolution's main mechanism: individuals with heritable traits that improve survival and reproduction leave more offspring, so those traits become more common in the population over generations.
- 1↓VariationIndividuals in a population differ in heritable traits.
- 2↓OverproductionMore offspring are born than the environment can support.
- 3↓SelectionIndividuals with favorable traits survive and reproduce more often.
- 4↓InheritanceFavorable traits are passed to offspring.
- 5AdaptationOver generations, the population becomes better suited to its environment.
Step-by-step worked examples
Peppered moths in industrial England: before pollution, light-colored moths were common; after soot darkened tree bark, dark moths became common. Explain using natural selection.
Variation: moths came in light and dark colors (a gene mutation). Selection pressure: soot-darkened bark made light moths visible to birds. Dark moths were camouflaged and survived/reproduced more. Over generations, dark allele frequency rose in the population.
A population of 1,000 beetles has 200 with a pesticide-resistance gene. After heavy pesticide use for several generations, resistant beetles now make up 800 of 1,000. What's the change in frequency?
Initial frequency = 200/1000 = 20% Final frequency = 800/1000 = 80% Change = 80% − 20% = 60 percentage-point increase This shows selection favoring the resistance allele.
A hospital treats an infection with an antibiotic that kills 99% of bacteria, but 1% carry a resistance mutation. Explain the outcome after repeated treatment.
Antibiotic acts as the selective pressure. Susceptible bacteria (99%) die; resistant bacteria (1%) survive. Survivors reproduce, passing on the resistance gene. Repeated treatment increases the resistant population's share each round.
Flashcards
Quick quiz
Q1.Which of the following is required for natural selection to occur?
Q2.The peppered moth example shows natural selection acting on…
Q3.In evolutionary biology, 'fitness' refers to…
Q4.Which statement about natural selection is TRUE?
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Common mistakes
Organisms evolve to get what they need (e.g., giraffes 'wanted' long necks). — Correct: Evolution has no intent — variation arises randomly, and selection favors traits already present that improve survival.
Natural selection acts on individuals, changing them during their lifetime. — Correct: Individuals don't evolve; populations do, as trait frequencies shift across generations.
'Survival of the fittest' means only the strongest survive. — Correct: Fitness means reproductive success — a small, camouflaged animal can be 'fitter' than a strong, visible one.
Evolution and natural selection are the same thing. — Correct: Natural selection is one mechanism of evolution; others include genetic drift, gene flow, and mutation.
FAQ
What is natural selection?
It's the process by which organisms with advantageous heritable traits survive and reproduce more successfully than others, shaping populations over time.
What is an example of natural selection?
The peppered moth's color shift during industrial pollution and pesticide/antibiotic resistance in insects and bacteria are classic examples.
What are the four principles of natural selection?
Variation, heritability, overproduction of offspring, and differential survival and reproduction (Darwin's postulates).
How does natural selection lead to evolution?
By repeatedly favoring beneficial heritable traits generation after generation, natural selection shifts allele frequencies, which is evolution.