The genus Heliconius includes over 40 species of butterflies with vivid patterns. In laboratory studies, scientists examined how one species, Heliconius heurippa, might have arisen as a distinct species. The phenotype of H. heurippa is intermediate between that of H. cydno and H. melpomene. Breeding tests in the laboratory found that crosses between H. cydno and H. melpomene give fertile hybrid offspring; when these hybrids are crossed under controlled conditions, offspring matching the appearance of H. heurippa are produced within three generations; and the hybrid butterflies are more likely to mate with one another than with members of either parental species. The researchers therefore concluded that the H. heurippa species may contain DNA from both parent species as a result 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 mated mainly with one another, speciation could be more likely. Give reasons why they thought this increased the chance of speciation.
(b)[2]
Heliconius butterflies taste unpleasant to predators such as birds. Their bright wing colours act as warning signals, causing birds to avoid eating them.
Individual birds must learn which patterns to avoid. If one Heliconius species is abundant, 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 are found 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.
Worked solution & mark scheme
This 8-mark question has a full step-by-step worked solution and mark scheme. One marking point: “Their chromosome numbers are the same” …