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.
What a learner can do afterwards
- 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.
1 · Read
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.
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.
Natural selection favours inherited traits through unequal reproductive success. It is one cause of evolution, which changes inherited variation across generations.
More to read
Learn: Darwin, evolution, & natural selection
Natural selection acts on differences between individuals. When inherited traits help individuals leave more offspring on average, those traits can become more common. Competition for resources is one cause of unequal success. Predators, disease, and other pressures can also favour some traits. Longer life alone is not enough: passing traits to offspring matters. Selection is one mechanism of evolution.
19.3 Adaptive Evolution
Dark and pale peppered moths differed in how well birds could spot them. On soot-darkened trees, dark moths survived better and left more offspring. Bacteria with inherited antibiotic resistance can also survive treatment and multiply. Neither moths nor bacteria choose a needed mutation. Mutations can happen before or after environmental change; their usefulness depends on the conditions.
11.2 Mechanisms of Evolution
Evolution is change in inherited variation in a population across generations. It can occur between parent and offspring generations, without waiting for many rounds of selection. Natural selection, mutation, gene flow, and genetic drift can all contribute. If a trait affects neither survival nor reproductive success, selection does not favour it, but its frequency can still change through drift.
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