Species complex and "species group" are often used interchangeably, as for example in Jinzhong FuCameron J. WeadickXiaomao ZengYuezhao WangZhijun LiuYuchi ZhengCheng LiYing Hu (2005). "Phylogeographic analysis of the Bufo gargarizans species complex: A revisit". Molecular Phylogenetics and Evolution37 (1): 202–213., the full text of which shows both words used throughout with no distinction between the meanings. I believe this is generally the case, although some authors prefer one term to the other, and there seems to be a tendency to use "species complex" for situations that are better understood (and have been shown to be complex). Sminthopsis84 (talk) 16:59, 5 December 2013 (UTC)
The species group mentioned in the ICZN rules just means a group of (related) species. This meaning is already covered in both clade and taxon, there should be no reason to retain the Species group article other than as a re-direct or disambiguation. Petter Bøckman (talk) 12:33, 29 December 2014 (UTC)
The matter is actually more complicated: there is a whole complex of closely related article stubs here on Wikipedia that needs to be disentangled. After doing some reading and research, it seems to me that: (i) species group and species complex are synonyms, both refer to a group of very close, but distinct species; related to this are more narrow concepts, that is (ii) cryptic species (complex) for morphologically nearly identical species (with the definitition of "nearly identical" being totally subjective), (iii) sibling species for two very close sister species, (iv) species flock for a species group/complex living in sympatry, and (v) superspecies as a way to treat a species group taxonomically as long as the individual species cannot be properly circumscribed. See Bickford et al. 2007, TREE for a good overview.
I would suggest to have one general article that includes the more narrow concepts. "Species group" gives slightly more hits on Google than "species complex", and the difference is more clear-cut on Google Scholar, thus I would merge "species complex" into "species group". I don't see enough material to retain the other stubs as separate articles, and would thus also merge "cryptic species complex", "sibling species", "species flock" and "superspecies" as sections into the "species group" article. Tylototriton (talk) 08:44, 30 December 2014 (UTC)
@Petter Bøckman:@Tylototriton: useful discussion. I've added some material here, and redirected microspecies which was inappropriately discussed only at apomixis. I'm a little bit reluctant to make "Species group" the page that all others direct to, because it is also used in more general ways, e.g., "Copepods may form the largest biomass of any animal species group", to mean species combinations (e.g. as environmental indicators), and in "crown group" etc. in phylogenetics. I don't think I'm understanding the nomenclature meaning, since the ICZN glossary says "The species group includes all taxa at the ranks of species and subspecies [Art. 45.1]", which might mean that it is a convenient way of discussing the nomenclature of those ranks, without saying anything about groups of species -- could that be true? Sminthopsis84 (talk) 16:22, 31 December 2014 (UTC)
You have a point there about more general uses of "species group". Also, I noticed "species complex" is the term included in the "speciation" template. Hence, I support a merger of "species group" into "species complex". Regarding the nomenclatural meaning, I think you are right, "species-group names" seems to refer to all species and subspecies names. Should be discussed elsewhere, maybe under Species#Placement within genera? In any case, I would contribute to expanding this article, I had actually started to work on Cryptic species complex but I think it should be included here. I'm most interested in the evolutionary and ecological aspects though, less so in taxonomy and nomenclature... I'll try and merge the Species group content for a start. Tylototriton (talk) 10:50, 1 January 2015 (UTC)
Performed the merger. Obviously, a lot remains to be done; I'll try and continue to work on this article. Tylototriton (talk) 11:31, 1 January 2015 (UTC)
I am currently working on updating this article, aiming at an eventual GA nomination. Still shuffling around pieces of information, hence the "patchiness". I will next merge in contents from superspecies, sibling species, and species flock, and then gradually expand and properly reference the individual sections. Tylototriton (talk) 11:03, 7 February 2015 (UTC)