How does it work?
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