Types of Selection

• What is selection pressure?

  An environmental factor affecting survival
• Give two examples of selection pressures.

  Temperature and antibiotic availability
• What is a dominant allele?

  Allele expressed even with one copy
• How might antibiotic resistance develop in a hospital?

  Increased antibiotic use creates selection pressure
• What are the two forms of selection?

  Stabilising and directional selection
• What does directional selection do to phenotypes?

  Acts against one extreme phenotype
• What happens to the frequency of phenotypes in directional selection?

  One phenotype becomes more common
• How does natural selection affect genetic change in a population?

  It favors characteristics towards one end
• What happens to fur length in mammals during directional selection?

  Mean fur length increases over generations
• What does stabilising selection do to variation in a population?

  Reduces variation around the mean
• What is the effect of stabilising selection on extreme phenotypes?

  Acts against both extremes in phenotypes
• What is an example of stabilising selection in rodents?

  Mean fur length remains constant
• What happened to swallows during the cold spell in 1996?

  Larger body mass swallows survived better
• What type of selection occurred with swallows during the cold spell?

  Directional selection favoring larger body mass
• What type of selection is described when agouti rabbits thrive in good weather?

  Stabilising selection favoring agouti rabbits
• What happens to rabbit populations during a climate change to snow?

  Directional selection favors white rabbits
• What are the three types of adaptations in species?

  Anatomical, physiological, and behavioral
• How does sickle cell anaemia affect gene distribution in malarial regions?

  High frequency due to malaria resistance
• What happens to the gene frequency of sickle cell anaemia in non-malarial regions?

  Low frequency due to selection against it