Why do some genera have so many species?

Well, Euphorbia was brought up so I guess now I have to comment! :-)

I’ve been in favor of breaking up Euphorbia for a long time and Chamaesyce, Pedilanthus, Monadenium, Synadenium, Tithymalus, and others have been recognized. This taxonomic battle has been going on since at least the early 1800s and became quite a back and forth during the 1900s with various experts either deciding to lump all but the more divergent forms together or split out dubious groups. Few considered all genera that are now in Euphorbia to be in one genus but preferred subfamilial taxa like tribe or subtribe (see Euphorbiinae). There are several things we now know that get in the way, though.

The most basic way to explain the situation is that you have a body plan shared throughout much of the genus with many alterations from and reversions back to that form (viz. subg. Esula and sect. Nummulariopsis) in all four major clades such that there is little cohesion except at monophyletic tips where noval characteristics have developed (like C4 photosynthesis Chamaesyce/sect. Anisophyllum or spurred cyathia in Pedilanthus/sect. Crepidaria). In other words, you can recognize the noval groups at genus level, but only if you recognize two groups that have very similar body plans as different genera too. In otherwords, if the more conserved members across three of the four clades went extinct, you might be able to split them easy enough. But with them extant, you can’t really do that cleanly.

A more familiar example is the problem of recognizing birds at the same taxonomic rank as reptiles when birds “are” reptiles in that they descended from and are nested within reptiles. The phylogenetic term is paraphyletic. Personally, I’m one of a those weird taxonomists that think it might be worth accepting paraphyletic taxa, but the vast majority frown on that position.


I was thinking of Euphorbia when I asked the question so I’m kind of pleased others brought it up… hehe…

and while I love birds, as a plant person, it feels (relatively) as though there’s like three of them


I am still mad this went back to Euphorbia

sounds good to me


Some species have minor differences and evolved pretty recently yet there can be hundreds of those in the genus, if they’re closely related there’s no need to split the genus. With time of some million years it would make more sense to do so.)
Regarding birds they’re reptiles, there’s nothing that separates them, they’re the same as other dinos and have no truly unique traits, it’s just how human sees them through science history.


I laughed so hard at this!


The most accurate reply is, “well, it’s complicated.”

There are a bunch of potential ways to define a species, but each fails in some manner or another. As a result there are a lot of potential ways to define species.

An example of the way a definition of species may fails is to look at the most commonly taught one, the Biological Species Concept, which basically states that if individuals can produce fertile offspring together then both belong to the same species. Well, this definition fails when it comes to asexually reproducing organisms, and there are too many exceptions to it. For example llamas and camels are clearly different (although related) species, no-one would claim them to be the same species, but they can produce fertile offspring (assisted fertilization, male camel, female llama). Macaques frequently hybridize producing fertile offspring, both anthropogenically and naturally at the margins of their species ranges, with rhesus/pig-tailed macaque and rhesus/long-tailed macaque hybryds being especially common. This sort of thing has increasingly been found to be the case in animals and is far more common in plants. As a result of these exceptions and problems, despite the prevalence of this definition being taught in school and in undergrad it’s not really used all that much in professional circles any more.

Here are a couple of incomplete lists of some of the ways species are presently defined:


As an evolutionary biologist, I have never encountered any definition of a genus other than “a group of species closely related enough to be considered by taxonomists to be of the same genus.” In other words, there is no modern scientific concept behind ‘genus.’ There are thousands of definitions of species, and even when these don’t contradict each other (they usually do), they are often difficult to apply and arbitrary. But as far as I know a genus (as well as a tribe, a family, etc.) is entirely up to the preferences of the community of specialists in that area.

The variety of biological processes, biological outcomes, is far too great for any of our definitions to consistently fit soundly or make sense.


@chrisangell and @earthknight I appreciate your responses.


Oho, a thread about massive genera that need to be split?

Bulbophyllum, Epidendrum, Maxillaria, Pleurothallis, and Oncidium have entered the chat


Oh man, the talk about Euphorbia reminded me of the controversy around the genus Acacia, specifically the classification of Acacia species in Africa versus the Australian wattles. I think, personally, that this was one of the worst cases of vote rigging and biased academic red tape I have seen, but then I am a layman and there isn’t much I can do about it.


there are some phenomena acting on the number of species/taxa for a certain genus:

  1. Evolutive history (what now has few taxa/species could have had more in the past and vice versa)
  2. Environmental/geological history (isolation and environmental conditions are powerful speciation drivers)
  3. Reproduction strategies (think about genera in which agamospecies are frequent)
  4. Attitude of scientists towards splitting or lumping (some genera that have undergone a strong splitting would need a taxonomic reconsideration)

In any speciose genus, you’re likely to find some subsets of the group that clump together more closely than others in terms of characteristics or genetics. So the subgenus is often used to bring some degree of order to the oversized genus. But taxonomists seem not to like the subgenus category all that much. Most subgenera are either ignored or eventually elevated to full genus. This has been true in chipmunks in which there was recognized anywhere from one genus (Tamias) with three subgenera to three genera (Tamias, Eutamias, Neotamias) with no subgenera. The sciurid taxonomists seem to have settled on the latter arrangement. But there really isn’t much functional difference between a subgenus and a genus except that you can ignore the subgenus if you wish when using the scientific name.


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!

1 Like

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.