(e) the factors that can affect the evolution of a species

The idea

Evolution is a change in allele frequencies in a population over generations. Several factors drive or constrain it. Mutation creates new alleles and so provides the genetic variation on which selection acts. Meiosis, including crossing over and independent assortment, and random fertilisation reshuffle alleles into new combinations. Natural selection then changes allele frequencies when heritable variation affects survival and reproduction: individuals with advantageous alleles reproduce more, passing those alleles on. Directional selection shifts a phenotype in one direction; stabilising selection favours the mean and reduces extremes. Genetic drift, especially in small populations, changes frequencies by chance rather than by fitness, and the founder effect and bottlenecks reduce variation. Gene flow through migration between populations can introduce or remove alleles.

What earns the marks

• Mutation introduces new alleles, increasing genetic variation that selection can act on.
• Meiosis (crossing over and independent assortment) and random fertilisation produce new allele combinations in offspring.
• Natural selection changes allele frequencies when heritable variation affects survival and reproductive success.
• Directional selection shifts the phenotype in one direction; stabilising selection favours the mean phenotype.
• Genetic drift, founder effect and bottlenecks change allele frequencies by chance, especially in small populations.
• Gene flow via migration between populations can introduce or remove alleles.
• Reproductive isolation prevents interbreeding and allows populations to diverge into separate species.

Common mistakes and how to put them right

• Thinking evolution happens to an individual during its lifetime.

  Put it right: Evolution is a change in allele frequencies in a population across generations.

• Confusing genetic drift with natural selection.

  Put it right: Drift is a chance change in allele frequency, not driven by fitness differences.

• Assuming mutation is always harmful.

  Put it right: Mutations may be neutral, harmful or advantageous, and are the ultimate source of new alleles.

• Believing selection acts directly on genotype.

  Put it right: Selection acts on the phenotype, which reflects the genotype and environment.

Exam tips

• Name the specific factor (mutation, selection, drift, gene flow, isolation) rather than writing 'evolution' vaguely.
• State the direction of effect on allele frequency, for example 'increases the frequency of the advantageous allele'.
• Link each factor to variation and to reproductive success to show the mechanism.