Mimulus is a plant genus made up of a wide variety of species with bright flowers that attract pollinators such as bees and hummingbirds. Pollinators move pollen from one flower to another during plant sexual reproduction.
Table 5.1 sets out a comparison of some characteristics of two closely related Mimulus species that both occur in the same part of North America. The differences between them are the altitude at which each species grows; their flower traits, including petal colour and the distance from the flower opening to the nectar that pollinators feed on; and the proportions of pollinator visits they receive from bees or from hummingbirds.
(a)[5]
Using the information in Table 5.1, explain the isolating mechanisms that stop gene flow between M. lewisii and M. cardinalis populations.
(b(i))[2]
Breeding experiments carried out in the laboratory show that M. lewisii and M. cardinalis can interbreed and produce offspring. The F1 hybrid offspring are fertile. Suggest, with reasons, what can be predicted about the chromosome numbers of M. lewisii and M. cardinalis.
(b(ii))[2]
The F1 hybrids make $50\%$ fewer seeds than either of the two parent species. Explain how the reduced seed production of the inter-species (F1) hybrids can function as a post-zygotic isolating mechanism.
Worked solution & mark scheme
This 9-mark question has a full step-by-step worked solution and mark scheme. One marking point: “Unable to exchange pollen / cross-pollinate” …