The genus Heliconius contains more than 40 species of brightly patterned butterflies. Researchers have investigated in the laboratory how one species, Heliconius heurippa, could have developed as a separate species. The phenotype of H. heurippa is intermediate between that of two other species, H. cydno and H. melpomene. Laboratory breeding experiments showed that: • matings between H. cydno and H. melpomene (parent species) produce fertile hybrid offspring • controlled matings of the hybrids produces individuals identical in appearance to H. heurippa within three generations • hybrid butterflies prefer to mate with each other, rather than with individuals of either of the parent species. The researchers concluded that the H. heurippa species could contain DNA from the two parent species as a result of hybridisation.

1. (a)(i)Suggest, with reasons, one prediction that can be made about the chromosome numbers of H. cydno and H. melpomene.[2]
2. (a)(ii)The researchers thought that, because the hybrid butterflies preferred to mate with each other, this could make speciation more likely to occur. Give reasons why the researchers thought that this made speciation more likely.[4]
3. (b)Heliconius butterflies taste unpleasant to predators such as birds. The bright colours on the wings of the butterflies act as warnings so that birds avoid eating them. Individual birds have to learn which patterns to avoid. If one Heliconius species is abundant, or if it has a pattern shared with another similar species, predators learn to avoid this pattern faster. Therefore this pattern provides a selective advantage. In the wild, Heliconius hybrids occur in small numbers and have patterns that do not resemble the established warning pattern of either parent species. These hybrids have a selective disadvantage. This is an example of a post-zygotic isolating mechanism. Explain how selection against hybrids can act as a post-zygotic isolating mechanism.[2]