Effectiveness of using genitalia for arthropod identification?

Nature Talk
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I’ve found that in Lycosidae exclusive use of genitalia for species identification has had some negative results and wonder if this is true in any other taxa. The advent of DNA barcoding provides an additional, less subjective means of testing identifications. For example:

  1. In 2021 a new Hogna species was discovered based on general appearance and behavioral differences. The species is present in at least a dozen states and common in two, and it is pretty obvious which named species it has been conflated with for over a century. As research progressed we found male genitalia were virtually identical, and female genitalia, though slightly different, were not reliably separable due to individual variability. More recent DNA barcoding confirmed they were indeed different species and not even closely related. Formal publication should take place next year.
  2. Three species in the Pardosa genus were synonymized in the late 1950s. In 2010, DNA showed this was wrong. The researchers noted there were differences in general appearance but didn’t elaborate.
  3. Vogel (1970) noted distinct regional variations in the genitalia of one Pardosa species.
  4. We found some specimens in collections identified with existing, genitalia reliant keys that were way off.

It seems that although genitalia is a significant trait in Lycosidae, it is subjective, and there are now too many similar Lycosid species for it to be used in isolation. An added shortcoming is that it can only be used for adult specimens while pattern on juveniles are often quite distinctive themselves.

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My guess - and it’s only a guess - is that taxonomy for many species used to proceed via close examination of museum specimens. Museum specimens have often lost all color and sometimes even patterning, so structural differences are what remains to be seen. Plus, since the concept of species used to be, essentially, if they can interbreed, they are the same species, looking at structural differences in genitalia seemed to be the way to go. Nowadays, I think taxonomists (those precious few!) have a broader view of how to distinguish species.

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This is analogous to Linneaus’s attempts to categorize plants based entirely on their anthers and pistils. It kinda worked in some cases, but it eventually became clear that looking at more characters gives a more complete picture than just the reproductive parts.

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Well, hopefully the upshot will be that we will stop being told “these can only be distinguished by dissection.”

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I doubt it. If anything, we’ll be told, “You need to dissect out the genitals, but also I need a better view of the third segment of the antennae, plus the hairs on the pseudoplibillium can’t be seen well enough except under far UV.”

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Those are similar to the roadblocks I encountered. I didn’t have access to those options so I started trying to improve my live photos and figure out why there seemed to be so much individual variability in general appearance. The latter came down to recognizing developmental and sexual differences. Once those were accounted for, there was a lot less apparent variability, and then I could develop long lists of field identification traits. Reconciling my field identification with the scientific literature has remained a big problem for some genera.

I had originally thought the roadblocks were intentional, but the more I learn the more I think it’s the only answer they had to offer, and that perhaps they realized the traditional methods didn’t work very well.

P.S. it’s been a while since anyone’s thrown up those roadblocks in front of me regarding Lycosidae.

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Eric, you and I are facing the same situations in both spiders and moth taxonomy. Different looking moths named as separate species even though the genitalia don’t differ consistently, OR species distinguished by genitalic differences even though the moths are essentially identical. The examples are innumerable. And then along comes barcoding. Sheez…

The major problem with any of these taxonomic frameworks (phenotypic, genitalic, genetic differentiation) is the lack of any objective definition of a species concept.

Going back several decades, the prevailing idea was the “biological species concept” of Ernst Mayr and others that there had to be some barrier to (successful) interbreeding for two populations to be considered separate species. The array of alternative species concepts has multiplied in the intervening years. There are volumes written about this and I won’t go into them here.

Fundamentally, there is nothing objective about the concept that “if the genitalia differ recognizably and consistently, then the two populations are separate species”. The implication is that if the genitalia differ, that somehow must impose a barrier to successful interbreeding. But how do the spiders or moths respond to those differences? What level of genitalic differences will impede their breeding efforts? To my knowledge, no one has ever defined this, much less attempted to investigate it for any group of organisms or set of species. As a result, at times the descriptions of genitalic differences between two populations of moths (or substitute any other type of organism) read like a parody of the whole topic when an author describes minor differences in the shape of the valvae or uncus of two different moths and declares them separate species.

As for genotypic differentiation, the 2% barcode rule drives me crazy. It is arbitrary and the foundation of it is extremely thin. As I’ve read back in the literature, one broad-based study (on butterflies? if I am remembering correctly) found that the 2% barcode difference correctly predicted the post hoc species ID on the order of 95-98%. Fine. But there was variability in that statistic and what about the 2 to 5% which failed to be classified properly? There is no objective “species concept” in this. Virtually all subsequent barcode-based taxonomic work is founded upon just the one or two original studies which settled on the 2% level of differentiation. It’s a house of cards.

Even with large volume genomic sequencing now being employed, while we may have more confidence in genetic differentiation, I’m still finding a lack of objective definitions of a species concept. Recent papers often offer a very circular logic: “These clades [in a phylogenetic tree] separate out at the level commonly regarded as species-level differentiation, therefore we’ll name these as separate species.”

An example of a study that I recite frequently which attempts to do taxonomy in a comprehensive manner is that of Cong et al. (2020) regarding the North American Junonia (buckeye) butterflies.* They really try to take in the whole biology of the set of populations under study. The outcome is a quagmire but I applaud their efforts!

* Cong, Q., et al. 2020. Speciation in North American Junonia from a genomic perspective. Systematic Entomology (2020), DOI: 10.1111/syen.12428.

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Not sure I’m qualified to offer a definition of species, but first I try to group a mass of living things into phenotypically separable groups. In carving potentially unnamed species out of the mass, I consider appearance, habitats and behaviors. The past practice of describing species based on dead lab specimens, sometimes with seeing only one sex, seems awfully archaic. I look for durable breeding populations and become familiar with them where they live.

The first test is whether I can reliably distinguish two similar species in a habitat where they co-mingle. This creates functional field identification guidance and opens up the iNat observation database to begin creating range maps. The final test is DNA barcoding which either confirms or sinks the process. I can’t say I’ve had a lot of experience, but in the first case I was sure sure long before the DNA results and would have bet $1,000 on it. Two more cases in the same genus appear to have already passed the DNA barcoding, but we still have a limited familiarity with their daily lives and only fragmentary county level range maps.

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The way I see it, genitalia differences are just like any other physical difference between species. Some groups of similar species happen to have wildly different genitalia, and others have no consistent differences. Same goes with the popular “DNA Barcoding” technique for arthropods. It’s one little bit of a single mitochondrial gene, which in a lot of cases can be very helpful for separating species, but in other cases provides no help.

I think this is the take-home message. No single trait should ever be used in isolation to define species. It seems some people get the idea into their heads that “species” need to have different DNA barcodes, or need to have different genitalia, or need to have different outward appearances to make them separable in photos. None of these are good species concepts. Take it all into account- phylogenetic distance of the populations, internal and external anatomy, life history, biogeography, isolation mechanisms, etc. If the result is that two populations behave differently and isolate from each other in nature via anatomy, physiology, and/or phenology, then go ahead and name them two species. If the only consistent way to distinguish between them in-hand is to look at the genitalia, then so be it. If their DNA barcodes are identical, then so be it. Both the positions of “genitalia should never be relied on as a means of separating species” and “if the genitalia are different they must be named different species” are equally ludicrous, in my opinion.

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I can live with that. Before we looked at examining the genitalia of item 1 above in detail, I was pretty sure the genitalia would be inconclusive or else someone would have noticed the new species already. At that point we already had dozens of other reasons why it was an unnamed species so it didn’t matter one way or the other.

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I don’t think I would put that third one on a par with the other two because different outward appearances encompasses a lot more than a single trait.

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You can look at different parts of the genetic codes and have two different sexes to consider so the first two aren’t exactly single traits either. Separable in photos also has different levels. I’m normally satisfied if I can separate 90% of specimens, however in situations where one of the species has a lot of individual variability, I have to be happy with a lot less. Each situation is unique.

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