At an average environmental temperature of 10OC 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.

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.

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.

Over further generations, the shift in mean fur length continues until it

reaches the optimum.