user16866: Is it appropriate to update a question's tags given:

the tag is technically irrelevant to the content of the question itself
BUT is relevant to the answer (well voted or correct, of course)?

Two Examples:

Species ID questions will typically ask what kind of XXX something is and tag the question with XXX. However, the species is actually a YYY -- which gets pointed out in the answer -- so should it be tagged with YYY?
This question. It asks about pigments in food, but the answers are rather plant/animal focused. Should a botany or zoology tag be added to the question although it doesn't ask directly about either one?

My opinion is YES because people searching for similar questions later would benefit from coming across questions with tag-relevant answers. 
Thoughts?

user3624: To me, the first should definitely be retagged (since the current tag is incorrect), but I'm hestant to the second one. The question doesn't have anything directly to do with either botany or zoology, even though those are used as examples in the answers.
Generally, obviously missing or incorrect tags should be edited, but I think tags should have a clear connection to the question (which is why I find you second example problematic). However, if answers show that the question is framed in the wrong way (indicating missing tags) or based on factually incorrect information the tags should be changed.

user25568: I absolutely agree with user @theforestecologis 's proposal. 
Whatever our view to study is interdisciplinary or not; the nature is itself interdisciplinary... so there are connections or correlations among different areas and ways  of studies. 
So theoretically it is possible an asker asked a question focused within subject or topic X; but its answer lies within subject or topic Y. So it is obviously okay to add or replace tags based on answer. 

user27148: I mostly agree with @fileunderwater's answer but came across another example where I thought retagging was clearly appropriate so decided to make a separate answer:
What is this small tissue structure found in kidney?
I added the "endocrinology" tag to that question, because the answer is that the structure referenced was in fact an endocrine gland. Based on the question, it wasn't really possible for the asker to know that tag was relevant, or for someone else who doesn't know the answer to add the tag, but I still think it's appropriate to add a tag in this circumstance. It isn't that the tag isn't relevant to the question: it is.
For your example (2), the reason I would say not to tag those things is in part because those particular tags are incredibly broad, and not particularly relevant to the content. Good answers to that question need not address either or both of zoology and botany, even if the current answers do.

16866: <p>Is it appropriate to update a question's tags given:</p>

<ul>
<li><p>the tag is technically <em>irrelevant</em> to the content of the question itself</p></li>
<li><p>BUT <em>is relevant</em> to the answer (well voted or correct, of course)?</p></li>
</ul>

<p>Two Examples:</p>

<ol>
<li><p>Species ID questions will typically ask what kind of XXX something is and tag the question with XXX. However, the species is actually a YYY -- which gets pointed out in the answer -- so should it be tagged with YYY?</p></li>
<li><p><a href="https://biology.stackexchange.com/q/56476/16866">This question</a>. It asks about pigments in food, but the answers are rather plant/animal focused. Should a botany or zoology tag be added to the question although it doesn't ask directly about either one?</p></li>
</ol>

<p>My opinion is YES because people searching for similar questions later would benefit from coming across questions with tag-relevant answers. </p>

<p>Thoughts?</p>
 3624: <p>To me, the first should definitely be retagged (since the current tag is incorrect), but I'm hestant to the second one. The question doesn't have anything directly to do with either botany or zoology, even though those are used as examples in the answers.</p>

<p>Generally, obviously missing or incorrect tags should be edited, but I think tags should have a clear connection to the question (which is why I find you second example problematic). However, if answers show that the question is framed in the wrong way (indicating missing tags) or based on factually incorrect information the tags should be changed.</p>
 25568: <p>I absolutely agree with user @theforestecologis 's proposal. </p>

<p>Whatever our view to study is interdisciplinary or not; the nature is itself interdisciplinary... so there are connections or correlations among different areas and ways  of studies. </p>

<p>So theoretically it is possible an asker asked a question focused within subject or topic X; but its answer lies within subject or topic Y. So it is obviously okay to add or replace tags based on answer. </p>
 27148: <p>I mostly agree with @fileunderwater's answer but came across another example where I thought retagging was clearly appropriate so decided to make a separate answer:</p>

<p><a href="https://biology.stackexchange.com/questions/58976/what-is-this-small-tissue-structure-found-in-kidney">What is this small tissue structure found in kidney?</a></p>

<p>I added the "endocrinology" tag to that question, because the answer is that the structure referenced was in fact an endocrine gland. Based on the question, it wasn't really possible for the asker to know that tag was relevant, or for someone else who doesn't know the answer to add the tag, but I still think it's appropriate to add a tag in this circumstance. It isn't that the tag <em>isn't relevant</em> to the question: it is.</p>

<p>For your example (2), the reason I would say not to tag those things is in part because those particular tags are incredibly broad, and not particularly relevant to the content. Good answers to that question need not address either or both of zoology and botany, even if the current answers do.</p>
