The peppered moth (Biston betularia) is frequently referred to as an example of evolution in action. This is because it occurs in two distinct forms, the usual light grey colouring peppered with black dots and a melanistic form that is predominantly sooty black. I am not convinced that this is a valid claim but that is another matter!
In the south of England, and that obviously includes Dorset, this standard grey version is the one we find. It is quite common and widespread across the area flying from May right through until the end of August.
A nocturnal species and that colour scheme is an extremely effective camouflage and so you rarely see this species by day.