[thaumasiotes]

> but for Homo sapiens that "other group" would be of a different species and we thus we shouldn't be able to produce fertile offspring with them.

That's one definition of "species" (specifically, it's "the biological species concept"), but like all the other definitions, it doesn't work and does not correspond to most usage of the term. You can read modern genetic work discussing how homo sapiens interbred with hominids of other species. You can also observe how easily different species interbreed today, like wolves (canis lupus, obligatory pack animals) and coyotes (canis latrans, solitary).

[Groxx]

It also wouldn't surprise me if "can reproduce with" isn't a transitive property (edit: actually, I'd be very surprised if it always is. genetic stuff is never that simple). So even if you try to collapse species into that strict definition, you'll still have edge cases that don't fit.

Categories are hard.

[bigbubba]

You're right, it isn't necessarily transitive. So called 'ring species' are cases where it isn't.

[scarmig]

You don't even have to go as exotic as ring species. Climb up the evolutionary tree through each of your ancestors to some slime creature a billion years ago, and then climb down to your pet dog. Each connected node in that step can interbreed, but you can't mate with Fido.

I guess it's kind of a temporal ring species.

[thaumasiotes]

Ring species are so called because their breeding relationships form a cycle. You're only describing a chain. You would have described a ring species if you could mate with your dog.

(Also note that while it's perfectly possible to walk up from humans to an ancestral slime creature, and then back down to dogs, most of the steps along that path will be shared between humans and dogs, like a Y with a very, very long leg. The recent common ancestor of humans and dogs would have already been a mammal.)

[scarmig]

The point about the most recent common ancestor is well taken, but, at least according to my layman's level understanding of ring species, the two ends of the "ring" aren't expected to interbreed. Wikipedia supports that definition (and if it's wrong, should be updated by someone more knowledgeable than myself).

[thaumasiotes]

You seem to be right about the idea of "ring species". Papers from the past several decades all introduce the concept the same way.

[thaumasiotes]

> It also wouldn't surprise me if "can reproduce with" isn't a transitive property

"Can reproduce with" is not a transitive (or even a permanent!) property among individual humans. (For a more general transitivity allowing for the fact that it doesn't make sense to ask whether males can reproduce with males.)

[chmod775]

>You can also observe how easily different species interbreed today, like wolves (canis lupus, obligatory pack animals) and coyotes (canis latrans, solitary).

Their taxonomy predates the more modern biological classification systems though. Things have been reclassified before and I expect they will be again.

[A bit of a rant:] Any classification system that can't tell you something about the objects it classifies is objectively useless for anything but naming things. You might as well use a random name generator, draw lines in the sand in geometrically pleasing ways, and pretend it's a "system".

[thaumasiotes]

> Any classification system that can't tell you something about the objects it classifies is objectively useless for anything but naming things

They are both canis, you know. The difference in species designation tells you very little because that's not a problem that can be solved. It's not an issue of misclassification. The specific designations latrans and lupus tell you a lot.

[ithkuil]

Canis is the genus. Even barring classification mistakes, cross-species breeding across genus is not impossible, it's just very less likely to produce viable and/or fertile offspring.

A classic example is the breeding between Equus Caballus (horse) and Equus Asinus (donkey): it works and you get a Mule.

The genetic difference between the horse and the donkey is non trivial, they even have a different number of chromosomes (64 vs 62). Since the offspring gets half of their chromosomes from the father and half from the mother, the mule gets an odd number of chromosomes (63) which clogs the gamete production machine and renders the animal sterile. (See https://genetics.thetech.org/ask/ask225).

Now, dogs and wolves don't have that problem. Does it mean they are same species and that the classification was applied to eagerly?

[throwanem]

I mean, Canis dates to Linnaeus, so that it's paraphyletic should be the presumption.

[ithkuil]

what about Equus? that's also introduced by Linnaeus AFAIK; could you please explain what you mean; I'm actually a noob so please state things explicitly so I can understand things (I'm ok with looking specific things up, but it's harder to look up "between the lines" things)

[throwanem]

Sure! I mean, I'm an amateur myself, mainly having become interested in the subject because of my ongoing fascination with wasps. But, being also an old birder, I wanted to develop a general sense of what "kind" of wasp I'm looking at when I see one, the same way I can tell an accipiter from a buteo from an osprey. That's turned out to be a surprisingly complex proposition! So here's what I've learned about taxonomy and cladistics as part of my research into wasps. Again, I'm an amateur, so if any actual molecular taxonomists should happen by, I hope they'll correct the goofs I am surely about to perpetrate.

Taxonomy, a specialization of biology, is the study of how organisms can be usefully classified. A major aspect of that study is the determination of how species are categorized, and how ancestral relationships are recognized. When this is done through molecular phylogeny (ie direct analysis of genetic relatedness and ancestry), the result is a directed graph called a "phylogenetic tree", whose nodes denote species and whose edges denote descent. Such a tree is called "monophyletic", meaning that it contains both, and only, a specific common ancestor and all descendants of that ancestor. Any subgraph from a single node of a monophyletic tree is also monophyletic. In this way, it's possible to establish a relatively unambiguous understanding of descent and relatedness among possibly widely varying organisms.

There are other kinds of classification which often produce groupings that make an intuitive kind of sense in terms of similar characteristics and the like, but which don't accurately represent relatedness. The Linnaean taxonomy is one of these, and the two terms used in cladistics to describe its categories of error are "paraphyly" and "polyphyly".

A paraphyletic grouping is one which contains a single common ancestor, but fails to contain all of its descendants. Many hymenopteran families are like this, such as Crabronidae, which is "all Apoidea that aren't bees" - bees and ants being actually descended from basal wasps, which were themselves descended from an even more basal shared ancestor with modern sawflies. Paraphyletic groupings can be useful, but aren't to be relied upon to accurately describe descent, and are replaced with monophyletic groupings as the necessary research and analysis is performed.

A polyphyletic grouping is one which fails to contain a common ancestor. For example, the European paper wasp (P. dominula) and the eastern yellowjacket (V. maculifrons) closely resemble one another in size, body shape, markings, and much behavior. Based on that, a Linnaean taxonomist might classify them as separate species within a single genus. This (hypothetical) genus would be polyphyletic; while these two wasps are indeed closely related, they're not that closely related, sharing a common ancestor with several other genera including all other social wasps and a sizable fraction of solitary species.

How exactly this all relates to the concept of "species" is hard to really define, at least for me as an amateur, but I think maybe also for professionals in the field. The classical definition, that a species consists of all individuals able to interbreed and produce fertile offspring, and none which cannot, fails to explain cases like wolves and coyotes, lions and tigers, or horses and donkeys - all pairs which are classified as separate species, but nonetheless are able to produce fertile offspring at least some of the time. Phylogenetics offers a distinct species concept [1], defining a species in terms of interrelatedness and innovation in traits as compared with ancestors - I suppose the classical definition could be called "horizontal", while the phylogenetic one could be called "vertical". Notably, the phylogenetic species concept appears to place no significance on interfertility or the lack thereof, which would mean that Equus, Canis, and Felis are not necessarily paraphyletic at all - they might be, but that wouldn't be defined on the basis of interfertility.

In the limiting case, I suppose that means each individual organism could be considered to constitute a unique species, although that is probably nonsense and certainly too granular to be useful. In any case, I hope all this answers your question, and provides some ground for further study. I've certainly learned something today, just looking up references! Thanks for the impetus to further systematize my understanding of the subject.

[1] https://www.oxfordreference.com/view/10.1093/oi/authority.20...