user24354: At the moment, Biology SE has tags cat (species), dogs (species), feline (family), mammals (class). A bit messy.
My proposal:

Species — Either singular or plural. Hence, either tags cat and dog or tags cats and dogs. Let us aim for consistency.

Families — Instead of tag feline, how about creating tag felidae (with synonym feline or felines)? Since many people like dogs, wolves and foxes, how about creating tag canidae?


I am not proposing that tags be created for all species and all families of mammals, but rather to species and families of mammals familiar to humans — such as Bovidae, Canidae, Equidae, Felidae, etc. After all, laymen seldom ask questions about animals they have never heard of.

Related:

Should we have tags for nomenclature + taxonomy or only taxonomy?

Tagging hierarchy - best practices for adding more "general" tags

Creating tags for taxonomy ranks?



Update
Not everybody uses Stack Exchange the same way. For example, to read biology-related questions on Stack Exchange, I created a filter and am notified whenever new questions with the tags that I follow are posted on Biology SE, Medical Sciences SE, Bioinformatics SE, etc.

Does this remind you of RSS? Google may have sunset Reader, but the spirit of RSS is still very much alive in certain corners of Stack Exchange. Interestingly, not many seem to be aware of it!
Suppose that I am particularly interested in canids. If I follow zoology I will be flooded with new questions, most of which are of little interest to me. Obviously, I would prefer to follow tag canidae.
Suppose that the mean time between new questions with the tag canidae is 2 months. It is tempting to conclude that this tag is too specific and, thus, unneeded. I believe the exact opposite is the case. The rarer, the better. I don't want to visit Biology SE for sightseeing, to read its front page hoping I will encounter something that interests me. Instead, I want to be notified whenever a new question with a tag that I follow is posted and I want to visit Biology SE mainly to read questions that I am already interested in. I don't think I am the only one with such proclivities.

user22057: It should first be emphasized that retagging and adding tags to old posts has negative, as well as positive, consequences for the list.
One negative consequence is that old retagged questions come to the top of the list again and dilute and displace new ones that then do not receive the attention they may deserve. This is unfair on posters who have put effort into an original question*.
A second negative consequence is that many old retagged questions are of very low quality and should have been left to die. The main reason, I believe, that they have not been closed is that this list does not have sufficient members with the privilege to close questions. One of the reasons for this is the low quality of questions a new academic encounters on first viewing this site. A vicious circle.
This is why, when I realized the consequences, I abandoned a project of my own to recatagorize the tags for metabolism. If I ever get back to it I will only do a few questions at a time, spread them over a reasonable time period, and ignore poor-quality questions.
I therefore believe that tags should only be invented or rationalized if there is a clear need. In my opinion, “A bit messy” is not a sufficient justification.

*Of course other actions on a question, such as edits bring a question to the fore. But here we are dealing with individual questions — not batches — and editing is an integral part of the SE model for producing good questions and answers. In addition, there are limits on the number of edits an author can make to prevent abuse of the system.

24354: <p>At the moment, <strong>Biology SE</strong> has tags <a href="https://biology.stackexchange.com/questions/tagged/cat" class="post-tag" title="show questions tagged &#39;cat&#39;" rel="tag">cat</a> (species), <a href="https://biology.stackexchange.com/questions/tagged/dogs" class="post-tag" title="show questions tagged &#39;dogs&#39;" rel="tag">dogs</a> (species), <a href="https://biology.stackexchange.com/questions/tagged/feline" class="post-tag" title="show questions tagged &#39;feline&#39;" rel="tag">feline</a> (family), <a href="https://biology.stackexchange.com/questions/tagged/mammals" class="post-tag" title="show questions tagged &#39;mammals&#39;" rel="tag">mammals</a> (class). A bit messy.</p>
<p>My proposal:</p>
<ul>
<li><p><strong>Species</strong> — Either singular or plural. Hence, either tags <a href="https://biology.stackexchange.com/questions/tagged/cat" class="post-tag" title="show questions tagged &#39;cat&#39;" rel="tag">cat</a> and <a href="https://biology.stackexchange.com/questions/tagged/dog" class="post-tag" title="show questions tagged &#39;dog&#39;" rel="tag">dog</a> <strong>or</strong> tags <a href="https://biology.stackexchange.com/questions/tagged/cats" class="post-tag" title="show questions tagged &#39;cats&#39;" rel="tag">cats</a> and <a href="https://biology.stackexchange.com/questions/tagged/dogs" class="post-tag" title="show questions tagged &#39;dogs&#39;" rel="tag">dogs</a>. Let us aim for consistency.</p>
</li>
<li><p><strong>Families</strong> — Instead of tag <a href="https://biology.stackexchange.com/questions/tagged/feline" class="post-tag" title="show questions tagged &#39;feline&#39;" rel="tag">feline</a>, how about creating tag <a href="https://biology.stackexchange.com/questions/tagged/felidae" class="post-tag" title="show questions tagged &#39;felidae&#39;" rel="tag">felidae</a> (with synonym <a href="https://biology.stackexchange.com/questions/tagged/feline" class="post-tag" title="show questions tagged &#39;feline&#39;" rel="tag">feline</a> or <a href="https://biology.stackexchange.com/questions/tagged/felines" class="post-tag" title="show questions tagged &#39;felines&#39;" rel="tag">felines</a>)? Since many people like dogs, wolves and foxes, how about creating tag <a href="https://biology.stackexchange.com/questions/tagged/canidae" class="post-tag" title="show questions tagged &#39;canidae&#39;" rel="tag">canidae</a>?</p>
</li>
</ul>
<p>I am not proposing that tags be created for <strong>all</strong> species and <strong>all</strong> families of mammals, but rather to species and families of mammals <strong>familiar to humans</strong> — such as Bovidae, Canidae, Equidae, Felidae, etc. After all, laymen seldom ask questions about animals they have never heard of.</p>
<hr />
<p>Related:</p>
<ul>
<li><p><a href="https://biology.meta.stackexchange.com/q/575/24354">Should we have tags for nomenclature + taxonomy or only taxonomy?</a></p>
</li>
<li><p><a href="https://biology.meta.stackexchange.com/q/3551/24354">Tagging hierarchy - best practices for adding more &quot;general&quot; tags</a></p>
</li>
<li><p><a href="https://biology.meta.stackexchange.com/q/3568/24354">Creating tags for taxonomy ranks?</a></p>
</li>
</ul>
<hr />
<h3>Update</h3>
<p>Not everybody uses Stack Exchange the same way. For example, to read biology-related questions on Stack Exchange, I created a <a href="https://stackexchange.com/filters">filter</a> and am notified whenever new questions with the tags that I follow are posted on <strong>Biology SE</strong>, Medical Sciences SE, Bioinformatics SE, etc.</p>
<p><a href="https://i.stack.imgur.com/DsDfO.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/DsDfO.png" alt="Top 3 questions on my bio filter" /></a></p>
<p>Does this remind you of <a href="https://en.wikipedia.org/wiki/RSS" rel="nofollow noreferrer">RSS</a>? Google may have sunset <a href="https://en.wikipedia.org/wiki/Google_Reader" rel="nofollow noreferrer">Reader</a>, but the spirit of <a href="https://en.wikipedia.org/wiki/RSS" rel="nofollow noreferrer">RSS</a> is still very much alive in certain corners of Stack Exchange. Interestingly, not many seem to be aware of it!</p>
<p>Suppose that I am particularly interested in canids. If I follow <a href="https://biology.stackexchange.com/questions/tagged/zoology" class="post-tag" title="show questions tagged &#39;zoology&#39;" rel="tag">zoology</a> I will be flooded with new questions, most of which are of little interest to me. Obviously, I would prefer to follow tag <a href="https://biology.stackexchange.com/questions/tagged/canidae" class="post-tag" title="show questions tagged &#39;canidae&#39;" rel="tag">canidae</a>.</p>
<p>Suppose that the mean time between new questions with the tag <a href="https://biology.stackexchange.com/questions/tagged/canidae" class="post-tag" title="show questions tagged &#39;canidae&#39;" rel="tag">canidae</a> is 2 months. It is tempting to conclude that this tag is too specific and, thus, unneeded. I believe the exact opposite is the case. The rarer, the better. I don't want to visit Biology SE for sightseeing, to read its front page hoping I will encounter something that interests me. Instead, I want to be notified whenever a new question with a tag that I follow is posted and I want to visit Biology SE mainly to read questions that I am already interested in. I don't think I am the only one with such proclivities.</p>
 22057: <p>It should first be emphasized that retagging and adding tags to old posts has negative, as well as positive, consequences for the list.</p>
<p>One negative consequence is that old retagged questions come to the top of the list again and dilute and displace new ones that then do not receive the attention they may deserve. This is unfair on posters who have put effort into an original question*.</p>
<p>A second negative consequence is that many old retagged questions are of very low quality and should have been left to die. The main reason, I believe, that they have not been closed is that this list does not have sufficient members with the privilege to close questions. One of the reasons for this is the low quality of questions a new academic encounters on first viewing this site. A vicious circle.</p>
<p>This is why, when I realized the consequences, I abandoned a project of my own to recatagorize the tags for metabolism. If I ever get back to it I will only do a few questions at a time, spread them over a reasonable time period, and ignore poor-quality questions.</p>
<p>I therefore believe that tags should only be invented or rationalized if there is a clear need. In my opinion, “A bit messy” is not a sufficient justification.</p>
<hr />
<p>*Of course other actions on a question, such as edits bring a question to the fore. But here we are dealing with individual questions — not batches — and editing is an integral part of the SE model for producing good questions and answers. In addition, there are limits on the number of edits an author can make to prevent abuse of the system.</p>
