When a lot of the squirrel population was killed off by Lyme Disease, there was a random change in how the genes are portrayed, because the gene pool was dramatically drained. Now, squirrels are smaller, and have a darker brown coat color.
The squirrel population was forced to migrate towards the sea because of changing weather. Because of this, they lost alleles for their thick winter coat, and their alleles also changed to allow them to be better swimmers to hunt for food.
The sex cells of the parents in the population have started to mutate, and through meiosis, the population is changing. This is rare, but it happens. Because of this random mutation that was successful, the squirrels start turning green.
The squirrels have realized that their population will survive longer if they are stronger and bigger. This causes the females to look for the strongest, largest males to mate with, causing a larger and stronger population.