Natural Selection

  1. How may some within a species show variation?
    Due to mutations forming different alleles
  2. What effects may the development of advantageous alleles have on an organism?
    • Allow them to survive longer
    • Allow them to reproduce more and pass more of their alleles to the next generation
  3. What will the increased reproduction of individuals with advantageous alleles result in?
    Over many generations the frequency of these alleles in the population will increase
  4. What is directional selection?
    • When a change in conditions favours one extreme or another of a normal distribution
    • Causes the mean value to move in the direction of these individuals
  5. What is stabilising selection and when does it occur?
    • Occurs in an unchanging environment where the mean value is the best adapted and is selected for 
    • Extremes are selected against
    • Mean valises are unaltered but standard deviation is reduced over many generations
  6. How may isolation be an issue for members of a species?
    One population may not be able to mate with another
  7. Give three examples of causes of isolation
    • Geographical isolation (lake Malawi)
    • Behavioural isolation (mutation causes different courtship display)
    • Temporal isolation (mutation causes breeding seasons to differ)
  8. How may a different environment affect a species?
    There will be different selective pressures
  9. How can one discern whether two animals are of the same species?
    • Breed them together 
    • If the offspring is fertile the animals are of the same species
  10. Give an example of a species that spectated and describe how it happened.
    • Cichlids in lake Malawi
    • Low rainfall caused the lake to separate into pools
    • Different environment in each pool with different selectional pressures 
    • Fish mutate over time gaining advantageous alleles 
    • These individuals survived longer and produced more offspring 
    • Over time the frequency of the alleles increased
    • The two populations became so genetically different that, when the lake returned to its former levels, the fish could no longer interbreed
Card Set
Natural Selection
AQA BIOL4 Natural Selection