Subspecies and species are incompletely and completely separated evolutionary lineages, respectively. At the macro evolution scale Darwinian evolution is not strongly supported by current facts, but at the species and subspecies level it is relatively simple in theory. In actual practice it can be difficult to determine evolutionary lineages, but that is a problem of application of our current limited knowledge, which more work helps to improve.
While this is the dogma we are all taught, I wonder how many people have really thought through what this definition would mean for taxonomy? Because at least as I interpret this statement, the problem is not that the concept is imperfectly implied. Not only is there no real attempt to apply this standard, we are not even on the same taxonomic planet as this definition.
Newly described splits are almost never justified on the basis of completely separate lineages, which is very difficult to verify. They are merely shown to be different in some way or that existing taxonomy is not monophyletic. But this is just the beginning of the problems with this definition. It is not violated only by recent splitting of minutely different taxa. It is also violated by some of the earliest described and most taxonomically stable species, which, if we were to follow it fully and honestly, would require these species to be merged. Despite being clearly distinct physiologically, ecologically, and genetically, English and white oaks, wolves and coyotes, horses and donkeys, tigers and lions, virtually all citrus species, and many, many others would all need to be lumped into singular species because they are all capable of at least occasionally producing fertile offspring which can then backcross and deliver exotic genes to their parent species. In fact, this species definition is probably closer to how genus is used in practice, although there are even some genuses which would need to be merged into a single species were we to pursue this idea.
For this reason, this definition of species is rejected by many, perhaps most taxonomists. Unfortunately, as far as I know there is no singular definition that satisfies everyone, because species is a social construct created by humans to make sense of the world, and not a real physical thing. The real questions are not what is the objectively real or true definition, but rather: which definition is most useful? Unfortunately, the answer differs between people and the various contexts they work in, which has led to the heated discussion in this thread and elsewhere.
2 Likes
For those interested in this topic, I suggest:
- starting a new Nature Talk topic for it, to keep from taking the current topic off-topic [EDIT: now done, moved here], but first,
- reading and understanding de Queiroz (1999) on the General Lineage Concept of species, and especially the section titled Realism and Antirealism.
5 Likes
I understand that in herpetology (my area of interest, cant speak for other taxa) at least this definition is generally rejected, but from the literature I have read this conclusion is unwarranted and not unanimous. Just because lineages can hybridize does not mean they arenât isolated from each other. In most of the examples you describe (I canât speak for plants) the lineages are actually completely separated, because even though freak events can happen and the lineages can hybridize, physiological, ecological, and genetic differences will keep them from forming a continuum together as with subspecies. They are on separate evolutionary trajectories that will not merge. And it is very rare for animals at least to hybridize and produce viable offspring. Horses and donkeys donât reproduce in the wild as far as I know, and I have never heard of captive mules being fertile. Some species can hybridize and produce viable fertile offspring, such as lions and tigers, but they experience something known as hybrid breakdown where descendants become weaker and infertile, eventually dying out and not affecting the population as a whole.
For those willing to delve into the subject, here is a book with plenty of different points of view in a single place:
Wheeler, Q.D., Meier, R. (Eds.), 2000. Species Concepts and Phylogenetic Theory: a Debate. Columbia University Press, 230 pp.
https://www.researchgate.net/publication/247609844
3 Likes
Well, no. There is no definition of species that applies universally to all taxa. The so-called biological species concept is a theoretical construct that works, mostly, for a lot of large vertebrate groups. For the rest (i.e. most) of the stuff taxonomists work with, itâs an approximation at best.
The idea of âspeciesâ predates theories of biological evolution. Much angst has followed from efforts to find a workable biological definition for it.
3 Likes
Yeah, that is true and I probably should have mentioned it, but I was mostly pointing out (in the original thread) that species arenât arbitrarily assigned for humans to catalogue (although that is certainly part of it) but are an attempt to accurately name physical realities. I will say I like the evolutionary species concept and in general in seems to me to be the best definition and I think it is reconcilable to an extent with the biological species concept (as I pointed out above), but my area of knowledge is vertebrates, so I wouldnât really know how applicable both are outside of that field. However, that statement isnât really based on the biological species concept, as I went on to explain. The evolutionary species concept makes more sense imo, but I still think that the definition of a completely separated lineage is most accurate. As I pointed out species can interbreed while still maintaining their isolation when regarded as an entire lineage.
To be fair, Iâm pretty sure this is the definition of species that Mallards are using. 
2 Likes
I think of species in two ways: there are the real entities in nature which conform more or less with our various definitions of what characterizes them as discrete biological forms, and then there are the taxonomic species which is how we try to categorize those real entities into a Linnaean hierarchical system. The former do exist in all their messiness and complicated histories. The latter are how we try to make sense of those real entities through nomenclature.
Of course, any species, no matter how defined, is an ephemeral thing that is real only within a certain period of evolutionary time. And good luck setting a clear boundary of when it came into existence.
7 Likes
Thatâs really a great way of describing science in general
I wonder if botanists have already witnessed instant speciation in real timeâŚ
Any time one is involved in crop breeding or horticultural selection, one is at least potentially witnessing the formation of new species. Whether they subsequently go on to reproduce themselves without further human aid may take more than one human lifetime to determine, but could be followed in serial scientific studies.
Tragopogon mirus and Tragopogon miscellus are well-documented to have formed in North America from parent species introduced from Eurasia, so formation of these tetraploid lineages likely happened within the past 200-400 years.
These examples may be about as âinstantâ as speciation can get in plants, but maybe others know of quicker examples.
Speciation is much easier to see within a human lifetime for organisms with faster generation times and stronger selective pressures, such as in microorganisms causing human disease. (Think development of antibiotic resistance in human bacterial pathogens, emergence of new pathogens, etc.)
5 Likes
There are a couple of tetraploid parthenogenetic species of Aspidoscelis (whiptail lizards) that were created by hybridizing a pair of naturally-occurring species in the lab. They have been given binomial names but neither has been confirmed in the wild although quite likely could exist there. INat doesnât recognize them. The hybrid origin of species in this genus has been studied for many years and there is much discussion in the literature about how we should categorize and name these unisexual lineages ⌠something botanists have already grappled with but herpetologists have less familiarity with.
1 Like
I was not thinking specifically of the eventual survival of horticultural plants, more about the results of polyploidy (as found in various ferns), or lineages of natural hybrids having popped up here and there (IIRC the case for some Sorbus around the Adriatic): were there witnesses to register such an âinstantaneousâ event? (is there an official minimum requirement of size/longevity for such populations to attain the status of potential species?)
No, there is not. Whether the hybrids are artificial or spontaneous, one can only witness the instant that a potential species is formed (e.g., the first fertile F1 hybrid). To verify that it then goes on to become an actual species requires longer-term observation through multiple generations.
If the F1 hybrid(s) immediately start self-replicating unaided, establish a stable natural population, and show no signs of being âre-absorbedâ by one or both parental lineages (or any other lineage), then one might be able to say, retrospectively, within their lifetime, that one witnessed the instant a new species formed.
But as yet, at that instant, it wasnât really a new species yet, only potentially. Other events had to occur first. Speciation is a process that takes at least a few generations, not really an instantaneous event.
1 Like
A few generations seems pretty much instant (in my geology book at least 
). Thanks for the clear reminder, I take it neontology has restrictive criteria for actual species and speciation events sometimes.
1 Like