18.3 Selection

2026 Syllabus Objectives

  1. Describe natural selection with reference to:

    • (a) genetic variation within populations
    • (b) production of many offspring
    • (c) struggle for survival, including competition for resources
    • (d) a greater chance of reproduction by individuals that are better adapted to the environment than others
    • (e) these individuals pass on their alleles to the next generation

  2. Describe selective breeding with reference to:

    • (a) selection by humans of individuals with desirable features
    • (b) crossing these individuals to produce the next generation
    • (c) selection of offspring showing the desirable features

  3. Outline how selective breeding by artificial selection is carried out over many generations to improve crop plants and domesticated animals and apply this to given contexts

  4. Describe adaptation as the process, resulting from natural selection, by which populations become more suited to their environment over many generations

  5. Describe the development of strains of antibiotic resistant bacteria as an example of natural selection

  6. Outline the differences between natural and artificial selection

Natural Selection 🧬

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Natural selection is a fundamental process in evolution where individuals with advantageous features are more likely to survive, reproduce, and pass on their alleles to the next generation.

Key Definition 🔑

Natural selection: A process in which individuals with advantageous features are more likely to survive, reproduce and pass on their alleles to the next generation.

The Mechanism of Natural Selection

Adaptive features are caused by genes. Individuals whose alleles give them slightly better adaptive features are more likely to survive and reproduce than other individuals in the population. So, their alleles are more likely to be passed on to the next generation. Over time, this results in more and more of the population having these alleles.

The better-adapted individuals survive and reproduce, while those that have alleles that do not provide quite as good adaptive features die. We say that the better-adapted individuals – those that have alleles that give them slightly better adaptive features – are selected.

Four Key Points of Natural Selection

Natural selection can be summarized in four essential points:

1. Variation 📊

  • There is variation within a population of organisms
  • Some of these variations are caused by having different alleles of genes
  • Some of the variation affects the adaptive features of the organisms

2. Overproduction 🌱

  • Within a wild population, many more offspring are produced than will survive to become adults and reproduce
  • This leads to competition for limited resources

3. Best-adapted individuals more likely to survive and reproduce 💪

  • Within the population, individuals that have the best adaptive features have the best chance of surviving and reproducing
  • There is a 'struggle for survival' (this does not mean that the organisms fight each other)
  • It includes competition for resources such as food, water, space, and mates

4. Alleles that confer useful adaptations more likely to be passed on 🧬

  • Better-adapted individuals are therefore the ones that pass on their alleles to the next generation
  • The alleles that confer the best chance of survival therefore increase in the population
  • Alleles that are not advantageous become less common

Causes of Change Through Natural Selection

Natural selection can cause change in a population for two main reasons:

  1. A new allele could arise by mutation - creating new genetic variation
  2. The environment could change - altering which features are advantageous

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