Enviromental conditions change so the phenotypes needed for survival also change
Some individuals will be to the left or the right of the mean, they are known as having extreme phenotypes
These individuals will posses phenotypes more suited to the new conditions
More likely to survive and reproduce
Contribute more offspring to the population
Mean will shift in the direction of the offsprings phenotype
The phenotypes at one extreme will be favoured for whilst the opposite extreme is favoured against
In a popualation where ther is a range of fur lengths...
The original temperature of the enviroment is 10 degrees. The mean fur length is 15mm.
If the temperature falls, individuals with longer fur (to the right of the mean) will be favoured for, whilst those with shorter fur (to the left of the mean) are favoured against.
Over many generations, the mean fur length increases as individuals who posses long fur phenotypes are surviving and reproducing. Those with shorter fur do not reproduce as successfully as they either die or are not healthy enough.
The mean fur length continues to shift till the optimum is reached for the lower temperature.