Directional selection

• Directional selection - Selection that favours individuals that vary from the mean of the population

  • Changes the characteristics of the population
• If the environmental conditions change, the phenotypes that are best suited to the new conditions are most likely to survive

  • Some individuals, which fall to either the left or right of the mean, will possess a phenotype more suited to the new conditions
  • More likely to survive and breed
  • Contribute more offspring (and the alleles they possess) to the next generation and other individuals
• Directional selection:

  • Individuals with alleles for a single extreme phenotype are more likely to survive and reproduce
  • Occurs in response to environmental change
• Example of Directional selection: ANTIBIOTIC RESISTANCE IN BACTERIA

  • Some mutated bacteria survive being treated with antibiotics and are able to divide and build up a population of resistant bacteria
  • Members of this population were more able to survive, and so multiply in the presence of the antibiotic
  • This population increases so the frequency of the allele increases in the population
  • The population's normal distribution curve shifts
• Directional selection results in phenotypes at one extreme of the population being selected for and those at the other extreme being selected against