[From a colleague:] The species problem is real, but I think that (a) it is way overblown in importance in the phil. biol. literature as a result of our fixation on metazoans; and (b) it may already have a pretty good answer (Paterson’s “recognition” concept).
Briefly, on (b): the idea is just that the “glue” holding species together is the fact that members recognize each other as members, which is a fact about their cognitive systems analyzable in terms of pheromones or whatever. Of course, there is also the fact that recognition has to be correlated with reproductive viability, which raises all the usual design issues. But I don’t
see that there are any deep problems here that are not deep problems of biology, generally.
As for (a): The reason this problem is overblown is that (i) it is not universal (it really only arises in sexually reproducing species), and (ii) it is a subset of the deeper problems of biological unity or holism. The deeper problems are: How do the individual cells in a
multi-cellular organism achieve unity? and: How do the macromolecules within a single cell achieve unity?
If we knew the answers to these deeper questions, the species problem would answer itself. At any rate, by merely the posing the questions in this way, one can see that the species problem is a relatively superficial one. The fact that it has been so central to the analytical phil. biol. literature just goes to show how superficial that entire literature is.