Students should be able to describe evolution as a change in the inherited…

The idea

Evolution is a change in the inherited characteristics of a population over time. It happens through natural selection: individuals in a population vary because of differences in their genes, and some variants are better suited to the environment than others. Those individuals are more likely to survive, reproduce and pass on the alleles that produce the advantageous characteristics. Over many generations, the frequency of those alleles in the population increases, so the inherited characteristics of the population change. If populations become so different that they can no longer interbreed to produce fertile offspring, they are different species, so natural selection may result in the formation of a new species.

What earns the marks

• Evolution is defined as a change in the inherited characteristics of a population over time, not a change in an individual during its lifetime.
• Variation within a population arises from differences in alleles, including mutations, so individuals have different phenotypes.
• Natural selection occurs because individuals with characteristics better suited to the environment are more likely to survive and reproduce.
• The alleles responsible for advantageous characteristics are passed to offspring, so their frequency in the population increases over generations.
• Over long periods, accumulated changes in allele frequencies can make populations so different that they can no longer interbreed to produce fertile offspring, forming a new species.
• Answers should use the sequence: variation, selection pressure, survival, reproduction, inheritance, changed allele frequency over time.

Common mistakes and how to put them right

• Saying that an individual evolves.

  Put it right by explaining that evolution is a change in the inherited characteristics of a population over time.

• Confusing natural selection with Lamarckian inheritance of acquired characteristics.

  Put it right by stating that only inherited alleles, not characteristics acquired during life, are passed on.

• Thinking that a new species forms whenever a population changes.

  Put it right by explaining that new species form when populations become reproductively isolated and can no longer produce fertile offspring.

Exam tips

• Define evolution precisely as a change in the inherited characteristics of a population over time.
• When describing natural selection, always link variation to survival, reproduction and the inheritance of advantageous alleles.
• Use the term 'population' rather than 'species' or 'organism' when describing changes in allele frequency.