For some years now, I have been delving into the topic of concepts of species limits and definitions. It has always been a topic of interest for me, but it has come into more pertinent focus as I have increased my interest in moth identification. Examination of genitalic differences in moths seems to have begun in earnest in about the 1920s. Under the general banner of the “lock and key” model, it has been assumed that the physical structure of male and female genitalia (among other factors) work together like a key and lock to inhibit or limit mating opportunities between divergent “species”. The corollary to this hypothesis, then, is that moths which differ recognizably in genitalic structure represent different species. Decades of work using genitalic structure to define species differences have ensued. As investigators looked closer and closer, smaller and smaller differences in genitalia have become the basis for species separation. Among some groups of moths such as the Macariini (Geometridae) and Phycitinae (Pyralidae), the species-specific differences published in formal monographs (e.g. the Moths of North America fascicles) have at times become exceedingly arcane and small. Fundamentally, I can’t find any overarching discussion in the literature about the limits to such species discrimination. I don’t doubt that seasoned investigators can find small differences in genitalia, but I can find no discussion to answer the fundamental question: What difference do the differences make (e.g. to the moths themselves)?
I think the concepts of species limits based on genitalic differences is a topic ripe for investigation and critical discussion.
At present, I’m still in an information gathering mode. IF anyone is aware of critical discussion, comparisons, or even experimental evidence which might address the question of “how much differentiation is different”, I would like to learn about relevant literature.