Stabilising Selection

Enviromental conditions remain stable

The phenotypes closer to the mean are favoured 

More likely to pass on their alleles as they survive & reproduce

Phenotypes at the extremes are favoured against 

Eliminates the phenotypes at extremes

In a population with a range of fur lengths...

In years with high temperatures, those with short fur are favoured

In years with lower temperatures, the opposite occurs

Therefore, when the temperature is fluctuating, individuals at both extremes survive.

However, if the enviromental temperature is constant, the extremes will never be favoured, they will be selected against in favour of those with a mean fur length

The mean will remain the same but there will be fewer individuals at either extreme