A popular example of industrial melanism was peppered moth ( Biston betularia). There were white and black varieties one got preyed upon and the other was benefited by its dark colour,my question is did they (the white and the dark forms) belong to the same species (Biston betularia) or different ?
Yes, they belonged to the same species, Biston betularia. Only in this case the scientists could suggest the explanation, which refers to the microevolution event, which happens in the population of the same species. The representatives of both morphs could mate with each other and produce viable offspring. The black color is a dominant trait and the white color is a recessive one in this case.