Philosophers who study the problem of biological function often begin their deliberations by reflecting on the functions of parts of animals, or the behavior of animals. Applying theories of biological function to unconventional or borderline cases can help us to better evaluate and refine those theories. This is the case when we consider whether parts of transposable elements —bits of “selfish” DNA that move about within a host genome—have functions of their own, that is, whether the parts of TEs have (...) the function of helping the TE move about within the genome. Here I argue that whether or not the parts of TEs have functions depends crucially on whether collections of TEs form “populations,” by which I mean, here, a group of individuals of the same type that impact one another’s chances of persistence or multiplication, by impacting one another’s access to a shared resource. I think there is suggestive, but not conclusive, evidence that some TEs have functions of their own. Considering the problem of TE functionality, then, has value both for philosophy and for biology. (shrink)