Suggest and explain how the two subspecies $M.\,m.\,domesticus$ and $M.\,m.\,musculus$ may have arisen from the original $M.\,musculus$ population.
At present, $M.\,m.\,domesticus$ populations are divided from $M.\,m.\,musculus$ populations by a wide hybrid zone. This hybrid zone appeared 5000-1000 years ago when the two subspecies' ranges overlapped and interbreeding took place between the two subspecies, producing hybrids. Researchers studied the populations within the hybrid zone. It was noted that: - hybrid mice were infected by more intestinal worms than $M.\,m.\,domesticus$ and $M.\,m.\,musculus$ - $M.\,m.\,domesticus$ and $M.\,m.\,musculus$ individuals frequently mate together - hybrid male mice had a very low fertility score based on testis weight and total sperm production, whereas $M.\,m.\,domesticus$ and $M.\,m.\,musculus$ males had a very high fertility score - some female hybrids were sterile - crosses between a fertile female hybrid and a male from either subspecies produced a very low number of offspring $M.\,m.\,domesticus$ and $M.\,m.\,musculus$ usually have the same diploid number ($2n = 40$). Some individuals of $M.\,m.\,domesticus$ have a different diploid number ($2n = 34$). Discuss the extent to which pre-zygotic and post-zygotic isolating mechanisms maintain $M.\,m.\,domesticus$ and $M.\,m.\,musculus$ as two separate subspecies within the hybrid zone.