The biological species concept remains the standard. If two populations live in the same area, reproduced sexually, and don’t interbreed (in the wild, producing fertile offspring), they are separate species, definitely. This is the ideal.
We’d like to use this standard always. But other organisms don’t care about our need to have neat, mutually exclusive species names to use. They don’t always cooperate.
Hybridization is fairly common between closely related species. Whether it causes us to treat the two taxa as one single species or not depends on its consequences. Are the hybrids fertile? Are they fertile but unacceptable as mates to one or both of the parental taxa? Are they rare? Are they common? Are the parental species becoming less distinct or more distinct with time?
Sometimes hybridization between species introduced in North America is so extensive that they aren’t separate species any more, at least on this continent (Lolium perenne/multiflorum, Raphanus raphinastrum/sativus, some Tamarix species that get ID’d as T. ramosissimum on iNaturalist but are actually hybrids, others). Dr. Mary Barkworth calls this process “despeciation.”
We can’t use only that criterium because it fails in many cases with fertile hybrids, so that standart is artificial, more than some other human concepts.
Yes, species are (in part) artificial. We need words to talk about different kinds of plants, animals, etc. We need neat, mutually exclusive words; species names. Other organisms don’t give a damn about our needs for neat words.
I personally view species concepts as a list. You keep going down the list until you find one that works for the organisms you’re working on. Biological species concept is at the top of the list. If the organisms occur in the same area and they don’t interbreed (or only rarely hybridize to produce fertile offspring) it’s the one to use. If not, you continue down the list. Phylogenetic species concept (any two populations you can consistently distinguish are different species) is often what we really do use, though it can lead to stupid results. (I’m taking currently taking a pleasant romp through Taraxacum microspecies, all with the same ecological niche*, all similar, some of them interbreeding in one direction but not the other.) Sometimes it comes down to the taxonomic judgement of the people who study the taxa. It’s a mess, but that’s just real.
Edit: All the Taraxacum microspecies I’m dealing with have the same ecological niche; they are more diverse elsewhere.
Or for that matter, Olympic gull, Larus glaucescens x Larus occidentalis. In its range, it far outnumbers either parent species. Yet the parent species are still considered separate species, and the hybrid is still considered a hybrid.
I do not believe a break in the ring violates the common definition of a ring species. For decades, the most common example of a ring species given was Ensatina, and these descriptions standardly mentioned a break at the southern edge of the ring.
Also, the Biological Species Concept is not an accepted Gold Standard. Some decades ago there were those pushing for it to be accepted as such, but now very few biologists think that is likely or even desirable. Biological processes are so diverse that any one species definition must either be too vague to be useful, too focused to be generally applicable, or too arbitrary to make sense. The BSC, in different situations, suffers from all three of these faults.
The Ensatina salamanders constitute another controversial “ring species” example. There are are breaks in the ring, with genes not flowing across those breaks, and therefore many people think the “ring” consists of several species. This is controversial, of course; people disagree.
Considering the Biological Species Concept the gold standard does not, to me, indicate that it’s the only useful species concept. It means that we use it when we can. When it fails, we look down the list for some other species concept to apply. Even when we name species for which the BSC doesn’t work, we take into consideration how much variation occurs in good biological species as a kind of check on how to apply whatever species concept we do use.
But my point is that not every biologist, not even most, use the BSC when they can. Phylogenetic species concepts are much preferred by many biologists working with genetic data, who not only have no direct evidence of who might mate with whom under which circumstances, but whose fields fell on the opposite side of the species wars from Ernst Mayr. Ecological species concepts are often what works for community ecologists. Paleontologists have their own traditions that don’t generally reference interbreeding at all. Etc.
I am a practicing plant taxonomist. I use the BSC when I can. Others I know use it too as the standard. In practice we often use the PSC because we don’t directly know about interbreeding, but we use it to find the edges to gene flow that characterize the species as defined by the BSC. It seems to me and to many others that the PSC used alone (without reference to the gold standard BSC) results in a proliferation of splitting that is not useful, as each tip on a cladogram can get named.
If any of mine are in the side convo, go for it. I would like to be able to get back onto the original question.
I think that a related issue to the original question is: why do some highly speciose genera have one very widespread species and numerous highly localized species? I believe that the answer to that question will directly shed light on the original question.
Might be a silly question, but is it possible that these species are just arbitrarily delimited? For example, maybe the widespread species could easily be split up into numerous localized species, or vice versa where the localized species could be lumped together.
Right, but what I am trying to get at is, what defines the difference between a hybrid swarm, like the Olympic Gull, and a good species considered to have originated through hybridization, like the African Wolf, considered to be a descendant of a 72% Gray Wolf/28% Ethiopian Wolf ancestor. Why is the Olympic Gull defined as a hybrid swarm, and the African Wolf defined as a good species?
I think as any new species it should drop the ability of hybridization significantly, would it be a geographic reason or something new in gene flow of this hybrid, better two things together.