Give an example of convergent evolution and suggest why it has occurred.

This is an example of a question a Biological Sciences candidate could expect at an Oxford interview.
The interviewer might start by asking for a definition of convergent evolution ("convergent evolution is the independent evolution of similar features in species of different lineages. Convergent evolution creates analogous structures that have similar form or function but were not present in the last common ancestor of those groups." - Wikipedia). A strong candidate would then be expected to describe an example of convergent evolution and explain why it has occurred - fundamentally this is due to organisms occupying similar niches. They could also mention that some traits are constrained by the environment and can only take a few forms (e.g. eyes). There are a number of different examples the candidate could choose from (e.g. echolocation in dolphins/bats, vision in cephalopod/vertebrates, flight in birds/insects, carnivory in plants, C4 photosynthesis in plants). A good candidate would reach this with some help, perhaps after being told the definition of convergent evolution.
An interviewer might then extend the question by picking up on certain aspects of the candidates answer.