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  1. 1. At an average environmental temperature of 10 O C the optimum fur length for survival is 15 mm. this is the mean fur length of the population, with a number of individuals distributed either side of it.
  2. 3. If the average environmental temperature falls to 5oC, individuals with longer fur will be better insulated from the cold. These individuals are more likely to survive and so produce offspring. Those with shorted fur are less likely to survive and so produce fewer offspring.
  3. 5. Over many generations, the mean fur length of the population increases as more individuals with longer fur survive, and more individuals with shorter fur die from the cold. The proportion of alleles in the population for longer fur is increasing at the expense of those for shorter fur.
  4. 7. Over further generations, the shift in mean fur length continues until it reaches the optimum.