A subspecies is a genetically distinct population of a species that has some phenotypic differences but is not yet reproductively isolated. 500000 years ago, the European house mouse, Mus musculus, evolved into two subspecies, Mus musculus domesticus and Mus musculus musculus.

1. (a)Suggest and explain how the two subspecies M. m. domesticus and M. m. musculus could have evolved from the original M. musculus population.[4]
2. (b)Today, M. m. domesticus populations are separated from M. m. musculus populations by a large hybrid zone. The hybrid zone formed 5000–1000 years ago when populations of the two subspecies overlapped and interbreeding occurred between the two subspecies, resulting in hybrids. Researchers investigated the populations in the hybrid zone. It was observed 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.[3]