Natural selection and population change
Explain adaptation through heritable variation and differential reproduction.
Before you begin
- Genes and alleles
- Variation within a population
Connect variation to reproductive success
Individuals in a population vary, and some differences are heritable. In a particular environment, some variants may contribute to higher survival or reproductive success. If those individuals leave more offspring carrying the relevant alleles, allele frequencies can change over generations. Mutations do not arise because an organism consciously needs a particular improvement.
Distinguish populations from individuals
Selection acts through differences among individuals, but evolution is measured as change in populations over generations. Environmental change can alter which variants are advantageous. Genetic drift and gene flow can also change allele frequencies, so an observed frequency shift is not alone proof of natural selection. A convincing explanation links the trait, inheritance, environment and reproduction.
Worked example
Some bacteria already carry an antibiotic-resistance allele before treatment. Explain how the resistant proportion may increase.
Show the worked solution
- The initial population contains heritable variation in resistance.
- Treatment reduces susceptible bacteria more strongly under the stated conditions.
- Resistant survivors reproduce and pass on resistance, increasing their proportion.
Differential survival and reproduction can increase the resistant allele's frequency.
Try it yourself
Which statement describes evolution by natural selection most accurately?
Common mistakes
- 'Fittest' means reproductive success in a context, not necessarily greatest strength.
What can you explain now?
Why is a nonheritable improvement from practice not by itself evidence of genetic evolution?
Compare with the explanation
It need not change inherited allele frequencies across generations.
An individual can change during life without passing that acquired change genetically to offspring. Evolutionary evidence must address inheritance and population-level change.
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