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Cambridge IGCSE Biology · 0610

Chapter 18: Variation and selection (Part 2)

Natural selection

Natural selection is the process by which populations become more suited to their environment.

Mechanism
  1. Genetic variation exists within populations.
  2. Many offspring are produced.
  3. There is a struggle for survival, including competition for resources.
  4. Better-adapted individuals have a greater chance of reproduction.
  5. These individuals pass on their alleles to the next generation.

Exam Traps

  • Do not say organisms “choose” to evolve — selection is not purposeful.
  • Avoid confusing natural selection (environment selects) with selective breeding (humans select).

Adaptation and evolution

Adaptation
The process, resulting from natural selection, by which populations become more suited to their environment over many generations.
Example (antibiotic resistance)
Bacterial strains become resistant to antibiotics via natural selection. A mutation creates a resistant allele; when the antibiotic is used, the resistant cell survives and reproduces, passing on the allele.

Exam Traps

  • Do not say bacteria “develop” resistance to survive the antibiotic — resistant mutants already exist; antibiotics select them.

Selective breeding

Definition
Humans select animals or plants with desirable features and breed them together.
Process
Desirable individuals are crossed; offspring showing the traits are selected for further breeding over many generations to improve crop plants and domesticated animals.

Exam Traps

  • Do not say selective breeding creates new alleles by mutation — it selects existing variation.
  • Avoid claiming selective breeding always increases genetic diversity — it can reduce it.

Natural vs. artificial selection

Feature Natural selection Artificial selection (selective breeding)
Selection agent The environment. Humans.
Traits selected Advantageous for survival. Desirable to humans.
Timeframe Takes a long time. Takes less time.
Selective breeding in crops showing selection of largest corn and increased frequency in next generation
Diagram 1: Selective breeding in crops. A parent generation of corn with varied sizes; humans select only the largest to cross-pollinate; the resulting second generation shows a higher frequency of large corn.

Exam Traps

  • Do not say natural selection chooses traits useful to humans — that is artificial selection.
  • Avoid claiming artificial selection is always slower than natural selection — usually it is faster.

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