A smart philosopher of biology I know claims to be researching "whether there are (really) species, as opposed to just individuals". So far as I can tell, he is investigating whether biological explanations that appeal to species are not really always better put in terms of individuals. That is, he's studying whether talking about species serves a certain kind of (ineliminable?) role in biological explanation.
That definitely seems worth worth investigating - especially since there are so many cases where the distinction between different species looks very unprincipled. (Because of ring species and ligers it won't do to just say that two things are the same species if they can produce fertile offspring.)
But it seems strange to me that he puts this in terms of `investigating whether species really exist'. This is because, presumably, he thinks couches really exist, and yuppies too, even though we could surely phrase an adequate biological and scientific theory in such a way as not to entail any sentences of the form Ex couch (x) or Ex yuppie(x).
What I THINK might be going on is that he thinks objects need to earn their keep, in a way that concepts don't. That is: it's fine to apply scientifically useless predicates like "...is a yuppie", but not to introduce scientifically useless *objects* like species. On this reading he would be fine with saying that dogs exist, or that two newts are consepecifics, but not with saying that there are (abstract) objects called species.
But I don't see quite how one would motivate this differential treatment. (Admittedly this may have something to do with my current adherence to the merely logical notion of objecthood). Also, the problems for the notion of species looking unprincipled seem to apply just as much to claims about being a dog or being two animals being conspecific.
*Obviously some scientifically useless objects are bad to introduce, like the flying spagetti monster but that's because their existence would entail false claims about the distribution of matter in space-time. In contrast, just proposing new ways to think of the same old distribution of matter etc. in space-time as constiuting objects e.g. (tables, vs. half tables, vs. complete livingroom sets vs. dearths of tables) in different ways, seems harmless.
[edit: there is something SLIPPERY about the way I am using the concept/object distinction here. must think more about this, and ask the philosopher of bio]