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Natural selection and adaptation

Variation and selection · Selection · note 1 of 3

Spec 18.3.1, 18.3.4Download all Selection notes (PDF)Download all notes (PDF)4 pages

Natural selection and adaptationSpec 18.3.1, 18.3.4

Natural selection is the process by which individuals that are better adapted to their environment survive and pass on their alleles. It can be described in five steps.

  1. There is genetic variation within populations: individuals have different alleles.
  2. Organisms produce many offspring, more than the environment can support.
  3. There is a struggle for survival, including competition for resources such as food, water and mates, and pressure from predators and disease.
  4. Individuals that are better adapted to the environment have a greater chance of surviving and reproducing than others.
  5. These individuals pass on their alleles to the next generation, so the alleles for helpful features become more common.
Three generations of beetles in which yellow beetles are eaten by birds more often than green beetles, so the proportion of green beetles rises (opens full size in a new tab)
Over generations the proportion of the better-adapted (green) beetles rises.
Supplement (what this means)Supplement: only for the Extended papers (2 and 4). Core students can skip it. What the labels mean

Adaptation is the process, resulting from natural selection, by which populations become more suited to their environment over many generations.

Supplement (what this means)Supplement: only for the Extended papers (2 and 4). Core students can skip it. What the labels mean

Notice that individuals do not change. It is the population that changes, because the proportion of individuals with helpful alleles increases from one generation to the next.

Common mistake:

Do not write that organisms 'adapt because they need to' or that a mutation 'happens because it is needed'. Mutations are random. The environment only selects which individuals survive.