The genus Heliconius includes over 40 species of butterflies with vivid patterns.
Scientists have used laboratory studies to examine how one species, Heliconius heurippa, may have arisen as a distinct species. The phenotype of H. heurippa is intermediate between that of two other species, H. cydno and H. melpomene.
Laboratory breeding investigations found that:
• crosses between H. cydno and H. melpomene (parent species) produce fertile hybrid offspring
• controlled crosses of the hybrids produce individuals that are visually identical to H. heurippa within three generations
• hybrid butterflies prefer to mate with one another, instead of with individuals from either parent species.
The researchers concluded that the H. heurippa species may include DNA from the two parent species as a consequence of hybridisation.
(a(i))[2]
Suggest, with reasons, one prediction about the chromosome numbers of H. cydno and H. melpomene.
(a(ii))[4]
The researchers believed that, because the hybrid butterflies preferred to mate with one another, speciation might be more likely to happen.
Give reasons why the researchers believed this increased the likelihood of speciation.
(b)[2]
Heliconius butterflies taste bad to predators such as birds. The bright wing colours serve as warning signals so that birds avoid eating them.
Individual birds need to learn which patterns to avoid. If one Heliconius species is plentiful, or if it shares a pattern with another similar species, predators learn to avoid that pattern more quickly. As a result, that pattern gives a selective advantage.
In the wild, Heliconius hybrids occur in small numbers and show patterns that do not look like 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 function as a post-zygotic isolating mechanism.
Worked solution & mark scheme
This 8-mark question has a full step-by-step worked solution and mark scheme. One marking point: “chromosome numbers match” …