Evolution
Natural Selection
Differential survival and reproduction — Darwin's mechanism of evolution
Natural selection is the differential survival and reproduction of individuals due to heritable variation in traits. Proposed by Charles Darwin and Alfred Russel Wallace (1858). Mechanism of evolution. Requires: (1) Variation — individuals differ. (2) Heritability — variation passed to offspring. (3) Fitness differences — some variants more reproductively successful. Result: traits that improve fitness become more common. Examples: peppered moths during Industrial Revolution, beak adaptations in Galápagos finches, antibiotic resistance, lactose tolerance evolution. Key: not directed; not "progress"; not individuals selecting themselves.
- Proposed byDarwin and Wallace, 1858
- RequiresVariation, heritability, fitness differences
- ResultTraits improving reproduction become more common
- Famous exampleGalápagos finches; peppered moth
- Modern proofAntibiotic resistance; bacteria experiments
- MisconceptionDoesn't lead to "perfection" or "progress"
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Why natural selection matters
- Evolution. Mechanism of biological change.
- Medicine. Antibiotic resistance, virus evolution.
- Agriculture. Breeding programs (artificial selection).
- Ecology. Species interactions and adaptation.
- Cancer. Tumor cells evolve resistance.
- Conservation. Population genetics.
- Public health. Disease evolution.
Common misconceptions
- Selection is "survival of the fittest." Fitness = reproduction, not strength.
- Selection has direction. No goal; just immediate fitness.
- Individuals evolve. Populations evolve.
- Need drives mutations. Mutations random; selection sorts.
- Selection makes perfect. Just better than alternatives.
- Slow process. Can be fast (antibiotic resistance, peppered moths).
Frequently asked questions
How does natural selection work?
Three requirements. (1) Individuals vary in traits. (2) Some of that variation is heritable (passed via genes). (3) Some variants survive/reproduce better than others (fitness differences). Result: more successful variants leave more offspring; their traits become more common over generations. Eventually: population changes; new species may form (speciation).
What's fitness?
Reproductive success. Number of viable offspring left in next generation. Not "survival of the fittest" in fighting sense. Examples: organism that survives long but doesn't reproduce: low fitness. Organism that survives only briefly but reproduces a lot: high fitness. Average fitness of trait determines whether it spreads.
What's the peppered moth story?
Classic example. Before Industrial Revolution: light-colored moths common (camouflage on lichen-covered tree bark). Industrial pollution killed lichens; tree bark became dark from soot. Dark moths now camouflaged; light moths visible to predators. Dark form became dominant. After pollution control: light form recovered. Demonstrated rapid evolutionary response (within decades).
How does antibiotic resistance evolve?
Bacterial population includes diverse genetic variants. Antibiotic kills susceptible bacteria; resistant variants survive (rare initially). Resistant bacteria reproduce; population becomes mostly resistant. Antibiotic no longer effective. Demonstrates: natural selection happens fast in bacteria. Resistance can spread between bacteria via plasmids — accelerates. Major medical concern.
What are types of natural selection?
(1) Stabilizing: average phenotype favored; reduces variation (e.g., birth weight). (2) Directional: extreme phenotype favored; population shifts (e.g., evolution of dark moths). (3) Disruptive: both extremes favored; intermediate disfavored (e.g., divergent beaks). (4) Sexual selection: traits attractive to mates favored (e.g., peacock tail). Different patterns shape evolution.
Is selection deterministic?
No. (1) Random factors: who reproduces, who dies. (2) Genetic drift: random changes in allele frequencies, especially in small populations. (3) Bottlenecks: dramatic events reduce population, randomly select genes. (4) Founder effects: small group establishing new population has limited gene pool. Selection + drift together drive evolution; selection dominates in large populations.
What about misconceptions?
Common errors. (1) "Survival of the fittest" — fitness = reproductive success, not strength. (2) "Evolution has direction/progress" — no goal; just response to environment. (3) "Individuals evolve" — populations evolve; individuals don't. (4) "Need" creates traits — selection acts on existing variation; doesn't create what's needed. (5) "Perfectly adapted" — adapted enough to survive, not optimally.