Why do some genera have so many species?

I personally like large genera: it’s more of a challenge to learn them all! I’m trying to learn the buprestid genera Acmaeodera, Anthaxia, Castiarina, Chrysobothris and most importantly, the large animal genus Agrilus. It’s difficult, but when you look at the species, it really doesn’t make sense to split them–they are so similar–, the diversity in these groups has just exploded!

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Leaving aside @nathantaylor’s argument in favor of accepting paraphyletic taxa, it does seem that monophyly is a basic requirement for any newly described or redescribed genus. That doesn’t mean that there are not plenty of paraphyletic genera that haven’t yet been resolved. And it certainly says nothing about how far up or down the tree one chooses to split out each monophyletic group that we choose to treat as a genus.

As @ddubois2 and @nathantaylor point out, it’s not just a matter of taking a genus with 2,000+ species and splitting it into, say, 10 genera each with 200 species. The problem is that the current genus will often have lots of early branches with relatively few taxa. Each of these would need to become its own genus before you could get to the point of making monophyletic groups for the larger clades.


I was surprised when reading a textbook on plant systematics and taxonomy when it noted that, aside from species (where there is still some subjectivity), there is great subjectivity in all higher-level taxa, like genus, family, etc.

I strongly wish that some of the subjectivity were removed, and genera were limited to groupings of species that have at least some potential to produce hybrid offspring (whether or not it is fertile.)

For example, it would make a lot more sense to me to group the white oaks together as one genus, the red oaks as another, and then perhaps have the “Cerris”, “Protobalanus”, and “Mesobalanus” oaks grouped together too, since each of these groupings readily hybridizes with each other, but there aren’t generally (ever?) hybrids across groups.

Instead they’re lumped together into this unwieldly huge genus, Quercus.

A lot of groupings have been reclassified in this way. For example, you have Eutrochium, Ageratina, and Conoclinium split off from Eupatorium. And it all makes sense to me. Not only is each of these genera visually similar to each other and dissimilar, but hybrids are relatively common within each genus, and uncommon across them.


So many planet genera could easily be split into several, already having well defined groups (I’m talking genera like Quercus and Pinus. Instead they are in stupidly huge genera that are not helpful in the slightest.


Quercus and Pinus do have subgenera, although perhaps not enough to help in the ID process. But they are what they are … speciose and often tough to ID.


I do use the subgenera for Quercus and Pinus! I use them for when you don’t have enough information to ID to species.

For Pinus, there are two subgenera: Pinus (hard pines) and Strobus (soft pines). The hard pines have what are called “sheaths” around the needle bundles that are persistent, while the soft pines have sheaths that fall off. This means that the branches are soft pines have a “smoother” appearance. There are also a number of other traits, such as number of needles per bundle (usually 2-3 for Pinus, 5 for Strobus), whether the cone has a prickle or not (usually for Pinus, often lacking for Strobus). But these ones have exceptions that you need to be aware of when identifying.

I’m less familiar with Quercus, but I often use Sect. Lobatae (red oaks) and Sect. Quercus (white oaks) when I’m too lazy to bother with IDing oaks, in eastern North America. While there are exceptions, broadly speaking white oaks have blunt lobes while red oaks have bristle-tipped lobes.

I’m curious if you all know of any other subgenera that can be relatively easily identified.


Fun read: https://en.wikipedia.org/wiki/List_of_the_largest_genera_of_flowering_plants

I’ve never heard of two of the top five… some botanist I am.


Good question.

Binomial nomenclature (formalized by Linnaeus in the mid 18th Century) began as an exercise in cataloguing and the designation of species was an extension of the Biblical notion in which the kinds of animal and plant were divinely created by a supreme entity. Species were species because that’s how they were created. Genera were a cataloguing device for grouping similar species. Taxonomy was the business of assigning names to newly discovered things and placing them in the appropriate location in the catalogue. As more people got involved in more parts of the world it became increasingly difficult to keep everything sorted into agreed categories or to keep categories from overflowing with things that didn’t belong together.

The introduction of evolutionary biology and Mendelian genetics in the 19th Century fomented a great deal of heated debate about how species arose and changed but the idea that we needed to question what a species actually is took longer to develop. It’s a question that still hasn’t been answered adequately, probably because the question is based on false premises. With what we know now, assuming that there are species as laid down in scripture and trying to force data about all living things to fit the assumption is not scientifically defensible. Modern thinking and techniques have clarified that the idea of species as universal static forms is wrong on important points. In the world of very small organisms, for example, there is a tremendous amount of gene flow among organisms of what we have thought of as distinct species, including some that are apparently quite different. It’s why antibiotic resistance is such a problem. Even in multi-cellular organisms there are mechanisms for randomizing gene position on chromosomes (for example) that can lead to quite radical, population-level changes in relatively short times.

Anyway, genera remain cataloguing devices but as others have noted they are now increasingly being reconsidered to reflect relatedness or some analogue of relatedness (i.e. clades). It is not impossible under the modern perspective that a rapid radiation could lead to a large number of discrete(ish) species in a large genus but it is relatively unlikely. As more work is done it is likely that the large genera will fade away.


For what it’s worth, the more recent estimates put Euphorbia over 2,000. Plus, four photosynthetic systems: C3, C4, CAM, and C2. It is probably the only genus with all those systems and the only genus that has a C4 photosynthetic tree.


The irony of mentioning Euphorbia is that Poinsettia (once a separate genus) is morphologically very different from, say, the crown-of-thorns spurge or, really, any of the spurges that look like cactuses. You have to get really technical to decide that they are the same genus – and wasn’t that done before cladistics?

I have yet to see a picture of dinos with bird beaks. Or with the tail reduced to a pygostyle. Or the forelimb bones fused into wings (although the unrelated pterosaurs have a similar trait).


A lot of the problem stems from authorities being too lazy to deal with it I think. For example, Phylloscopus includes both traditional leaf-warblers, but also south-east asian birds like Bianchi’s, Martens, Alström’s, etc. warblers that look nothing like the other members of the genus in terms of plumage, structure, or habits. A seeming leftover from the colonial ages that should have been fixed a century ago.

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Astragalus are widespread in Europe and iNat map shows that in fact in big part of the world.


It doesn’t make them any less dinosaurs that they are, you can find all those traits in other dino groups in more mild form.


You’re a little backwards on this, They were treated as a separate genus, Seicercus, until the late 90s- early 2000s. The unscientific leftover of a colonial age is the idea that there are two discrete genera that follow the plumage division you noted.

Take a look at this phylogeny tree, which conveniently is very thorough, and labels the birds formerly considered Seicercus as such. As you can see the grouping formerly known as Seicercus is not its own closest relative. Half of it is put in a different part of the genus Phylloscopus. To separate off the core group of Seicercus into its own genus would require the genus be treated as 5 distinct genera, namely from that diagram, 1) P. maforensis to P. nitidus, 2) the core of the Seicercus group, 3) P. budongoensis to P. ruficapilla, 4) P. cebuensis to P. coronatus, and 5) P. ibericus to P. sibilatrix.

I’m not advocating for one treatment over another, but it seems to me like creating 5 genera out of Phylloscopus, with 2 or more genera having basically typical looking members might be more disruptive, and give more pushback than keeping them all at one genus. It’s a reminder that visual appearance is not the best decider of phylogeny. Because the physical appearance is subject to intense selective pressures divergent and convergent evolution work readily on it and have muddled non-genetic classification of organisms.

Edit: Just got around to checking, 4 of the 5 genera required to split Phylloscopus as I described would have “typical looking” members.


I think this is quite pertinent. Those groups most closely related to humans are, and have been, the most ‘important’ groups taxonomically. We have historically had detailed taxonomy for vertebrates, and it falls off with the smaller invertebrates don’t. I’m not a professional taxonomist, but it’s always bugged me that mammals are a class, and so are the insects (which have a huge range of forms). It also amuses me how taxonomists of groups with fewer species get so detailed about small differences. Some snake that live under specific rocks in the Namibian desert belongs to a sub genus and several sub species, even though it’s basically the same snake as others in the same part of Africa. Originally ‘worms’ was a group of legless round things that no one really cared about. That has changed, but I do think science history has played a big role in what we see as important.
And folks, I am not a taxonomist, so please don’t take any of this personally :slightly_smiling_face:


There is a funny case going on in the fly genus Drosophila:
This genus contains more than 1,500 species, and the phylogenetic relationships within the genus are actually pretty well known.

Drosophila is definitely a paraphyletic taxon, as for example Zaprionus, Scaptomyza or Mycodrosophila are treated as separate genera but, although they look quite different, form clusters deeply within the genus. Right now, up to 15 genera might be included in the ‘supergenus’ Drosophila.

The branch (subgenus) containing such well known species as the laboratory pet and multiple noble prize winner D. melanogaster or the invasive Spotted Wing Drosophila (D. suzukii) is a species rich group with more than 300 species and it branches off basally from most other members of the group – so in principle it would not be difficult to split up the genus into more digestible chunks.

However, when in 1823 a Danish guy named Johan Christian Fabricius (a student of Carl Linnaeus) described the type species, it happened to be Musca (=Drosophila) funebris and not D. melanogaster. Apparently, he and/or his wife never had overripe fruit lying around in their home and always emptied their waste in a timely manner, because WHY ELSE WOULDN’T YOU START WITH THE MOST COMMON MEMBER OF THE GENUS??? D. melanogaster was then named seven years later…

Now we’re in a right mess: D. funebris is only distantly related to D. melanogaster, so the latter would have to be renamed to Sophophora melanogaster. The importance of the laboratory fruit fly is so great, however, that this would cause major turmoil in the scientific community. There have even been petitions signed by hundreds of scientists directed at the ICZN to make exceptions form the nomenclature rule in this case, such as to accept D. melanogaster as the type specimen or to allow paraphyletic groups. They were not successful. So, who dares to make the final step and kick D. melanogaster out of the genus? ;-)

The alternative would be to inflate the genus and include all those separate genera into one huge, amazingly diverse genus…


If I were into flies, I might say “f*** it” and do it myself…

There’s no thrill like the thrill of attracting the ire of the entire scientific community!


Most scientists only get to do this once. And so they wait for the end of their careers to do it. And by that time, they’ve spent a lifetime not doing it, and so they don’t.


The large and widespread frog genus Rana was split up some years ago and the common leopard frogs, including Rana pipiens, known to biology laboratory students all over, were kicked into the genus Lithobates. That revision is still not accepted by many and there are various interpretations of the phylogenetic data that either accept the new name or keep the frogs in Rana, with a few different subgenera being proposed to fix the situation. Some of the arguments feel personal, like the name change was an assault on a sacred tradition. In my mind, I still call the frogs in my area Rana but use Lithobates.

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But they look right like other Lithobates and nothing like true Rana, why they don’t accept it other than personal arguments?

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