(h) the mechanism by which natural selection can affect the…

The idea

Natural selection changes a population because individuals vary genetically, and some variants leave more offspring. Within a population there is variation in characteristics, arising from mutation and, in sexual reproduction, from meiosis and random fertilisation. Predation, disease or competition create selection pressures. Individuals whose characteristics suit the environment are more likely to survive and reproduce, so they pass on the alleles responsible. Over generations the frequency of those advantageous alleles rises, and the frequency of disadvantageous alleles falls, so the population's characteristics shift. This is differential survival and reproduction acting on existing variation; it does not create variation on demand. A classic example is a shift in the frequency of a particular allele when a selection pressure such as a predator or a drug changes.

What earns the marks

• Populations show genetic variation in characteristics, arising from mutation and, in sexual reproduction, from meiosis and random fertilisation.
• Selection pressures such as predation, disease or competition act on the population.
• Individuals with characteristics better suited to the environment are more likely to survive and reproduce.
• Those individuals pass on the advantageous alleles, so allele frequencies change over generations.
• Over time the frequency of advantageous alleles rises and disadvantageous alleles fall, shifting the population's characteristics.
• Natural selection acts on existing variation; it does not create new variation in response to need.

Common mistakes and how to put them right

• Saying organisms 'adapt' during their lifetime to pass on useful features.

  Put it right: Selection acts on inherited variation already present; acquired changes are not passed on in this way.

• Claiming natural selection creates new mutations when needed.

  Put it right: Mutation generates variation randomly, and selection then changes allele frequencies.

• Describing change in an individual rather than a population.

  Put it right: Natural selection changes the allele frequencies and characteristics of a population over generations.

Exam tips

• Write the sequence in order: variation exists, selection pressure acts, some survive and reproduce more, advantageous alleles passed on, allele frequencies change over generations.
• Use the phrase 'frequency of alleles' rather than 'the organism adapts' to show population-level change.
• Link each step to a named selection pressure so the mechanism is explicit rather than vague.