The definition of taxonomic characteristics

This greatly confuses me, particularly with an interest in wasps and the amount of re-writing of the taxonomy that seems to occur; are there any specific consistent rules as to what is a defining morphological, biological, physiological or behavioural characteristic? Or the priority of them below family level?

For instance, humans are all homo sapien, even though there is massive variability in appearance and physiology, but with insects the slight variance in a spot’s shape or length of a ‘tooth’ derives a new species/genus.

Why also does it only take one specimen to allow a new species definition, particularly when it could so easily be found to be either just sexual dimorphism, an anomalous mutation, a juvenile or just inherent variability within a species?

I’d love to dive into this and understand as much as possible as I continue down the rabbit hole of parasitoid wasps, as I see the phrase ‘catch-all group’ a lot as potential sorting pile, while wondering if there is always actually a need to be more specific just because a group is broad. Or that it points to problems higher in the taxonomy creating conflicts.

I’m sure this is a much debated topic, I’m curious about how and why things are how they are :)


I guess some species are just much more variable than others - humans are very variable, but also keep in mind that we are keyed in to notice differences in other humans when we might not notice differences in other species (e.g. differences in pheromones, or subtle morphological features that we can’t easily see). Species with large ranges or large populations are more likely to be variable than rare species. E.g. take a look at this somewhat uncommon species - all the observations are very similar and there isn’t much variation. Then look at this very abundant species and see how much variation there is! These days most invertebrate taxonomists base a lot of the determination on the genitalia, simply because if there is a large difference there then then the two species will be reproductively isolated. And once two species do become reproductively isolated, the genitalia often evolve more rapidly than the external features. A fair amount of genetic work also goes into taxonomic revisions these days as well.


The goal of taxonomy is to reflect relatedness between species (and higher ranks) not just similarity, and note that genetics which allows to look into “insides” of the species and its evolution is really new tool. In the past there was no other way than to compare many data about organisms and try to decide which traits are common or similar because of common ancestry and which because of convergence. People were not always right, especially they changed mind when new data appeared, so there is so much changes in taxonomy, variation described as different species and several species described as one. I doubt there are characters which are “universally” connected to species differentiation, it rather requires much study and looking at many individuals to decide whether a given trait is species-specific or not. There are some which are good candidates, eg mentioned above genital structures, as they sometimes are like key and keyhole in males and females. But eg in Bombus hortorum and B.ruderatus they are similar and you have to use other traits. Sexual pheromones are also good candidates for species-specific traits as they are used to recognise partners (and it’s good to mate with one’s own species ;) ) but I can imagine that there is eg some nest parasite which mimics chemicals of their host species. Also body colour in some groups will be good character and in others there will be much mimicry. There are some means which help very much to decide whether individuals are one or more species, like examining mating pairs, individuals from one nest or breeding in laboratory. Also examining many traits (both morphological, chemical, behaviouraletc) on the same group of individuals surely can help somewhat to decide which ones are more similar to which.


In insects a species with additional spot will also have different genitals, not the same thing with Homo sapiens, you see it in every insect key, somewhere you can go id without genitals, but somewhere it’s too hard, but it’s present everywhere because if they can’t mate physically, that’s not one species.

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As @justyna_kierat says, taxonomy is about relatedness, and the characteristics specific to each group are not necessarily transmittable to others. Characteristics to define the great apes cannot be used to define ungulates or fish. Characteristics for moths cannot be used for flies. Since genetics and genital dissection are fairly recent, most of the past characteristics were visual in nature, and that is still the dominant method used. Since species were also defined as mating = fertile offspring, characteristics to separate species were different in each genus. The species of the genus Euxoa are defined by different characteristics than the related genus Feltia. Sometimes species can not be distinguished by visual means, as in the case of Xestia c-nigrum & dolosa in Eastern NA.
At least this is how I understand it. There seems to be a fine line between variation caused by geographical isolation and physical variation. If the different variations found in humans were to remain geographically isolated for a million years, they may eventually form different species, or they may not. Sort of a crap shoot really - some Genera split off into many species, some are conserved as one species for a long time.
I’m not a taxonomist, BTW, so could very well be wrong about all this!

How to define species is such a difficult question, and rightly so because we as humans are just trying to put natural processes into simpler, communicable terms. The real answer is, it depends - on taxa, on the people doing the science, on the time period.

Truly it is an evolving process, much like the evolution of the organisms themselves. Parasitoid wasp species diversity is sorely underestimated by current taxonomy, and molecular methods will continue to split one species into several. What’s particularly nice about phylogenetics is that it isn’t the final answer to the question of “is this one species”, but that it creates a framework to map traits that could be important to species differentiation, like gall morphology, host species, geographic range, etc.

Here’s a great paper if you’d like some further reading


A very good question. Which morphological characters are needed to define a new taxon is really based on the opinion of the specific taxonomist studying that taxon- there are no universal rules. Unfortunately, this means that a lot of taxonomic characters from a century or two past are actually completely arbitrary.

Just as an example, in the harvestman family Sclerosomatidae, taxonomists in the early 20th century completely ignored genital characters and emphasized the number of femoral nodules to differentiate genera. However, more thorough morphological studies by later authors focusing on male genitalia have sometimes grouped together species with differing nodule counts, in some species male and female conspecifics have been found to have differing nodule counts, in others the nodule count may vary from individual to individual, and in a few rare cases, single individuals have been documented with different nodule counts on the left and right sides of the body.

In other words, the use of femoral nodules as a genus-level taxonomic character is completely unjustified and is responsible for creating a large number of artificial taxa that lack meaningful definitions, but this taxonomic system still persists to this day due to a lack of modern revisionary studies. Re-writing of taxonomy often occurs as a result of modern studies “fixing” these problematic historical systems (often using genetic data), but this is a time-consuming process; I don’t know much about parasitoid wasps specifically, but many diverse arthropod groups are plagued by a huge backlog of artificial taxa like these in need of revision.


Keep in mind that not every newly described species will remain valid over time. This was probably more true a century or more ago when rather small morphological characteristics were often used to describe a new species or subspecies. Genetic data has become greatly important to support modern recognition of both new and old taxa although morphology such as genitalia differences still has a place.

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Thanks for all the replies! It seems we’ve become much better at determining species and phylogeny (DNA/Microscopy etc), but there’s a big existing taxonomic mess that’s very difficult to unwind because a) it took a lot of effort from essentially not many resources to define in the first place and b) it’s a long process to change it? And it’s a global system.

Do new species where only a single specimen is found get flagged in any way to note the fact only one has been found or are they included within the trees/keys as all others are? It would be really interesting to be able to see/search and highlight ‘confirmed’ vs ‘holotype only’ species.

Would it make any sense in actually having two completely separate systems: the phylogenetic tree based on genetic traceability to historic ancestors and evolutionary development, then a second, separate ‘morphological’ tree that provide for identification. The former built ‘fresh’ with the latter refined to consistent keys for instance.

Wasps, particularly Ichneumonidae, seem a pretty good candidate for a reset due to the mess of conflicting and mis-used identifiers. Any takers? :p

I’m going to read some of the shared material now, not sure I’ll be less or more confused by the end! :D

Well, if there is a group heading in a perilous taxonomic direction, Parasitoid Wasps is a sure candidate:

Good Luck! G.


I’m not aware of any “central” list of species found as only one individual ever but in keys there usually is a brief information about biology and range, so in case of such one-specimen species it would surely be mentioned. Also in papers describig new species there should be such information.

I doubt it will ever be done, as two separate systems would make additional mess (and as new species aer discovered and new traits examined, also this “unnatural” system would be prone to changes). However, identification keys are often arranged so that you follow morphology rather than phylogeny - I mean, when you go the following steps in key you not necessarily differentiate among more and more related groups of individuals. For example, in bumblebees you might be asked first about colouration of your specimen, although groups of similar-coloured bumblebees consist of sometimes distantly related species which are simply simiar to each other because of mimicry.

There is also the point that different taxonomists have different ideas about what defines a taxonomic character, which is why there will never be a resolution between the lumpers and the splitters.

Lumper: “Well, really, all currently recognized species of maples have enough in common that we really should consider the genus Acer to comprise one, highly variable species.”

Splitter: “There are enough clear differences between Acer saccharum and Acer nigrum that they really should be in separate genera.”


I think this is a large part of the (well at least my) confusion. There’s a system in place for the classification, but no consistent ruleset for how to define the parameters.

Which I guess raises the question for me: if ‘species’ is technically defined by what an organism can/does mate with, why do we have so many confirmed species where we don’t even have a male and female (aside asexuality, bacteria etc) on record? It feels like there needs to be a separate term ‘pseudospecies’ or something that denotes a placeholder or candidate status. For the parameters for ID it becomes ‘character features’ that can be grouped in any way to allow for keying/location/relationships.

It feels like we have a large mess because of a determination to classify things into the formal system rather than acknowledging the need for an informal but complementary one. Almost like sunk cost fallacy: we can’t start fresh or ‘fork’ the current design because of all the effort put in previously.

I think I’m being a bit (lot!) ignorant here, but as I’m learning this field it does seem like a lot of the past is burdening the future and definitely complicating the learning process :p I’ll probably look back on this post in a couple of years and chuckle at myself :D

I don’t think species are defined by what they can breed with, because that would make hybridisation by definition impossible.

Also, finding out what it will breed with is not a very practical way of identifying a specimen, especially if it is dead.

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Yes true, I blame my faulty memory for not remembering that better. I guess it’s a matter of priorities in which parameters are used to define species and taxonomic fit. I have much more reading to do…

The ink used in the debate on species concept(s) could easily fill a swimming pool…

I recently stepped into this freely accessible book chapter: , the title of which is very relevant for what we all are here about:

Are Species Good Units for Biodiversity Studies and Conservation Efforts?

Biological species concept purports that organisms that can or potentially could interbreed are one species. However, that is only one species concept out of many, including morphological, phylogenetic, ecological, evolutionary, and several others. They all have their problems (i.e. BSC doesn’t work for asexual lineages) so considering more than one to define a species is important. In my opinion morphological + phylogenetic should be the bare minimum. It’s not very helpful when someone describes a new species in this day and age with no sequence data, and vice versa (at least for fungi).

You are right that the rules on how to define species is not consistent, but how strict can we realistically be when defining natural processes that are not strict at all? Evolution is an extremely complex and chaotic process we still don’t fully understand and maybe we never will. It’s important to consider what we use taxonomy for - why do we care about defining and distinguishing taxa? What purpose does it serve? The answer will likely vary depending on who you ask.

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Where were you during the Great Dandelion Debate?

Another recently published free ebook is “What, if Anything, are Species?” by Dr. Brent Mishler:

This is a collection of papers published in scientific journals over the past 30-40 years so fairly technical, but I as a non-technical naturalist did not find it too difficult to follow. Lots of good information about the various species concepts, and why there is no single universally applicable way to define species.

He also makes an argument for the PhyloCode, a phylogenetic naming alternative to the standard Linnean system. I don’t know whether that has any hope of being widely adopted or how well it would work in practice, but at least after reading through the book I had a much better understanding of what it is trying to accomplish.


This is fantastic. I’m still reading through it, but one thing strikes me: using species in conservation/ecology etc seems so full of argument because those areas fundamentally centre on the context of humans and more importantly, our interference in the environment. We don’t seem to want to understand biodiversity just to observe it as-is, we want to do something with it, and that’s where the arguing starts, because we don’t agree on what our role is or should be within the system.

Thanks for the link, thanks @twr61 too, that sounds quite interesting too.