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.
I’m sure you’re aware of the use of often subtle differences in cerci in odonates which is often the only way to ID a species if other characteristics are very similar among two or more species. I’m thinking of bluets (Enallagma) in particular. But other than the field guides which show drawings or close-up photos of these appendages I personally haven’t seen any primary literature that really discusses the differentiation or function of them. But it might be out there.
This seems to be more of a question about the definition of a species. In what I’ve seen with terrestrial gastropods, dissection is only one tool used in combination with biogeography, ecology, conchology (where applicable), and genetics. Using just one of those (including genetics) is largely unhelpful in distinguishing “species”.
You might also take a look at the literature on mammalian bacula, which differ by species in many closely-related rodents. It seems analogous to the lock-and-key model for insect genitalia. I suspect the literature is more extensive than it is for insects.
Look at Usami et al. 2006 Biological Journal of the Linnean Society ; it is one of the only known instances (apparently outside of moths) where a true lock-and-key system exists as a pre-zygotic barrier. For cicadas, terminalia differences produce tactile stimulation (or lack thereof) but not actual pre-zygotic barriers (at least in the group I study). The question of “how much is enough” is actually pretty problematic if there isn’t a good bio-acoustic barrier for them.
Yes, that is precisely the topic I’m addressing. I’ve spent the day doing a deep dive on literature related to this topic and will offer a “reading list” shortly. For starters, I found this most relevant review article which I’m reading this evening:
When there is general agreement about what a species actually is then these sorts of questions will be resolvable but that general agreement is a ways off yet. Undergraduate courses can give the impression that accepted species conform to some general principles in which reproductive isolation is a central tenet but neither the organisms nor taxonomists always fit the theoretical pattern.
Evolution is a fact, by natural selection, by genetic drift, by hybridization, by whatever and it can lead to what gets called speciation. Some organisms (especially large ones) have become reproductively isolated into the sort of species postulated by Ernst Mayer. Some organisms (especially small ones) swap genetic material in ways that reduce the species concept to a characterization of phenotype. Taxonomists applying cladistic analysis to large groups of organisms (like insects) whose ecology is poorly understood generally don’t have the information to determine what reproduces with what and describe species using the available information. Paleontological taxonomy is all pretty much a matter of hypotheses.
The puzzle of how life evolves is fascinating. Facile descriptions that minimize the complexity of the processes and the diversity of the outcomes are frustrating.
As promised, here is a very preliminary reading list on the topic of gentialic differentiation in insects. Two important references as starting points include the following book and a major review article:
Eberhard, William G. 1985. Sexual Selection and Animal Genitalia. Harvard Univ. Press, Cambridge. 244 pp. [I found a used copy on Amazon.]
Shapiro, Arthur M., and Adam H. Porter. 1989. The lock-and-key hypothesis: Evolutionary and biosystematic interpretation of insect genitalia. Ann. Rev. Entomol. 34:231-245. https://doi.org/10.1146/annurev.en.34.010189.001311
The following articles also seem useful and relevant:
Burns, John M. 1983. Superspecies Atrytonopsis ovinia ( A. ovinia plus A. edwardsi ) and the nonadaptive nature of interspecific genitalic differences (Lepidoptera: Hesperiidae). Proc. Enomol. Soc. Wash. 85(2):335-358. https://www.biodiversitylibrary.org/page/16179748#page/355/mode/1up
Porter, Adam H., and Arthur M. Shapiro. 1990. Lock-and-key hypothesis: Lack of mechanical isolation in a butterfly (Lepidoptera: Pieridae) hybrid Zone. Ann. Entomol. Soc. Amer. 83(2):107-114. https://doi.org/10.1093/1esa/83.2.107
Costa-Schmidt, Luiz E., and Aldo M. de Araújo. 2010. Genitalic variation and taxonomic discrimination in the semi-aquatic spider genus Paratrechalea (Araneae: Trechaleidae). J. Arachn. 38:242-249. https://doi.org/10.1636/JOA_A09-75.1