The question reads as: "The lens of many vertebrate eyes is a crystallized form of a protein that also functions in digestion as a metabolic enzyme. This shows that

A. Vision and digestion co-evolved

B. Digestion necessarily evolved prior to vision since it is a more basic function

C. Evolution is opportunistic

D. Vision and digestion evolved around the same time"

My approach: Based on the given information it appears the protein might have given rise to analogous organs. I don't feel B has to be true in all cases. C is a good candidate ​but its meaning isn't clear. I'm not sure about D. To me the answer appears to be A but I would be happy to be have this confirmed.

  • $\begingroup$ If you think your question has been answered, you can click on the check sign next to your favourite answer. $\endgroup$
    – Remi.b
    Jun 10, 2017 at 4:13

2 Answers 2


I think the question you have to answer is a bit badly phrased. Overall, I mainly agree with you, except for point A. Here are my comments for each points:


co-evolution is defined for the evolutionary dynamic between two lineages. It is typically not used to talk about two body parts. Consider for example the definition from Understanding evolution:

The term coevolution is used to describe cases where two (or more) species reciprocally affect each other's evolution.

, wikipedia

In biology, coevolution occurs when two or more species reciprocally ("co-") affect each other's evolution.

or dictionnary.com

evolution involving a series of reciprocal changes in two or more noninterbreeding populations that have a close ecological relationship and act as agents of natural selection for each other, as the succession of adaptations of a predator for pursuing and of its prey for fleeing or evading.

I would note that the term coevolution can sometimes be used in its border line case as in this paper for example. It is particularly confusing as the term co-adaptation is sometimes used to talk about the interaction of two phenotypic traits. But stil, I would NOT check A


I agree with you! The conclusion is not valid from the premise (which means that it is not necessarily true, so even if the conclusion may be true, it cannot be concluded from the premise)


I agree with you! The statement is a bit unclear.

Typically, evolution is opportunistic would mean that throughout evolution history, a lineage may well evolve to reuse an existing gene for another purpose (neofunctionalization for example) which is very likely what happened here.


I agree with you! The conclusion is not valid from the premise.

In short

I would check C.


I would say C is the right answer. Here, "Evolution is opportunistic" means something like "Evolution is a process that re-uses structures and mechanisms".

The question is about what you can conclude from the information given. The other three answers (A, B, D) are about the relative timing of when these functions appeared. Only C talks in more general terms about re-use of an existing protein for another purpose.

Of course, it seems reasonable to suppose that digestion evolved before vision - however, we can't really conclude that from this one example.

As a side note, I did a project a while ago about eta-crystallin, which is thought to have been an aldehyde dehydrogenase that is 'moonlighting' as a structural protein in the eye lens.

See : Crystal structure of eta-crystallin: adaptation of a class 1 aldehyde dehydrogenase for a new role in the eye lens


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