5 Mimulus is a plant genus containing a diverse range of species that have colourful flowers to attract pollinators, such as bees and hummingbirds. Pollinators transfer pollen between flowers for plant sexual reproduction. Table 5.1 compares some features of two closely-related species of Mimulus that both grow in the same region of North America. The features in which they differ are: • the altitude at which the two species grow • their flower characteristics, including petal colour and the distance from the opening of the flower to the nectar on which the pollinators feed • the percentages of pollinator visits that they receive from bees or from hummingbirds. [Table 5.1] (a) With reference to the data in Table 5.1, explain the isolating mechanisms that prevent gene flow between M. lewisii and M. cardinalis populations. (b) Breeding experiments in the laboratory show that M. lewisii and M. cardinalis can breed together and produce offspring. The F1 hybrid offspring are fertile. (b)(i) Suggest, with reasons, what prediction can be made about the chromosome numbers of M. lewisii and M. cardinalis. (b)(ii) The F1 hybrids produce 50% fewer seeds than either of the two parent species. Explain how the reduced production of seeds by the inter-species (F1) hybrids can act as a post-zygotic isolating mechanism.