So, I’ve been going back and forth about creating species complex taxa for Euphorbia. The utility of creating these complexes is extremely clear to me and there have been many times where I wished I could use them. Problematically, though, the complexes I have in mind don’t meet all the criteria (my notes followed by “-”; the last two points are excluded since they are simply procedural and not criteria for recognition):
- “Species complex is monophyletic (i.e. sibling groups of species)” - The phylogenies are not well resolved enough to know this with high confidence.
- “Complex is recognized in the literature” - This is incredibly hard to apply. Sometimes they are noted but rarely as species complexes per se. More as groups which have sometimes been given section or subsection recognition.
- “A named subgenus, section, or series does not already exist for the group” - Sections exist in Euphorbia. They are necessary to make sense of a genus with over 2000 species. Sections in Euphorbia are equivalent to genera in most other groups.
I will also some implicit criteria that can be extracted from the following statement: “Species sometimes intergrade and there are places on the tree of life where adding hard range map boundaries is arbitrary and/or identification to species level is often not possible.”
- Species intergrade.
- Creation of species boundaries on continuous variation is arbitrary.
- Identification to species level is not possible.
I wish to create the following complexes and note which criteria they meet (there are a lot more, but these three are the ones I’ve thought about most recently):
Euphorbia dentata complex:
Well known to cause problems with identification. Can be structured to include E. dentata, E. davidii, E. cuphosperma, and E. tubadenia, though E. shiediana would probably need to be added to make the group roughly monophyletic (E. shiediana’s placement is unknown but clearly closely related to the other 4). Several of these members are extremely difficult to ID without seeds. Some species express population variability that makes them difficult to separate even with seeds.
Criteria: 1. Maybe met? 2. Depends on whether you include previously recognized taxa as “recognition”. 3. Failed, obviously. 4. Probably true. The group needs more work to see if this is absolutely true. 5. Probably true. See 4. 6. Without seeds, this is often true. sometimes true with seeds.
Edit: The way I created it, it is synonymous with the E. dentata alliance in Mark Mayfield’s 1997 dissertation ( A systematic treatment of Euphorbia subgenus Poinsettia (Euphorbiaceae)).
Euphorbia cyathophora complex
Causes problems when we don’t know what the roots are like. Includes E. cyathophora, E. hormorhiza, and E. elliptica. E. pinetorum could be included, but I think it’s distinct enough to not worry about.
Criteria: 1. Phylogeny not complete enough to know. 2. Hard to say. 3. Failed, obviously. 4. Essentially true in most characteristics, especially when looking at E. elliptica and E. cyathophora. 5. Probably true. 6. True without roots unless future taxonomic work reveals more useful characteristics.
Euphorbia hyssopifolia complex
An extremely well known problem. Includes at least E. hyssopifolia, E. nutans, E. vermiculata, and probably a couple of others that I can’t remember off the top of my head (I need to review McVaugh’s work) in North America (though the two do not appear sister phylogenetically). In South America, probably includes E. lasiocarpa as E. hyssopifolia can have hairy fruits and E. lasiocarpa can have glabrous fruits.
Criteria: 1. Probably not, but the taxonomy is so fuzzy that we may not know until a complete taxonomic revision of both the South American and North American taxa is complete. 2. Sort of. Wheeler described this as the “Pandora’s box” of Euphorbia in the 40s. It’s gotten better since then, but there’s still a lot of difficulties. 3. Failed, obviously. 4. True in all ways except seed characteristics. 5. Probably not true. 6. Definitely true without seeds.