In nature there’s no way of knowing that, since the river does prevent interbreeding. And the critical thing is what happens in nature, not what happens in captivity. As a side note, would you object less strenously if they were seperated by an ocean instead of a river?
No, because we can produce (often fertile) hybrids of all types of animals in captivity which do not interbreed in the wild. All the species in the genus Canis, for example, are interfertile.
Well, lucky for us the barrier was overcome before the modern definition of a species was developed! But you could probably only make a case for subspecies back then because there were no real isolates (except maybe Tasmanians) as there was plenty of travel back and forth between Alaska and Siberia (by boat) before Europeans arrived. In fact, the entire Eskimo popultaion is a relatively recently arrived ethnic group (compared to Amerinds).
But there’s still a certain amount of subjectivity that goes into defining what is a species and what is not. If a freeway gets built that seperates two populations of the same species, do they become two species? Probably not. It’s biology, after all, not phyusics or mathematics.
How could we know for sure? Maybe if the Congo River suddenly became easily crossable, the more violence-prone chimps would kill all the bonobos.
But how long were they separated? 20,000 years? And were the environmental pressures different enough, given T time, for them to evolve to non-interbreeding animals? My guess is there wasn’t enough time for them to diverge sufficiently.
Besides, the latest theories I have read about populating the Americas postulates repeated invasions/migrations from Asia and maybe even trade. Each wave probably interbred with the previous wave of (now) natives, decreasing the difference between the continents’ homos.
We know they didn’t, but do we know they didn’t because of time? Although geographical separation is often a factor, surely two completely separated populations do not always evolve into separate species, right?
It would be astounding if a large, slowly breeding mammal like H. sapiens could evolve into a new species in a time frame on the order of 10K years. I believe the common thinking is that given enough time, isolated populations will always evolve into non-interbreeding populations. IIRC, the rule of thumb for mammals is that it takes on the order of 1M years for populations to become distinct species. That’s a pretty sloppy way of looking at it, though, since it’s “number of generations” that is important, not the mere passage of “x” amount of time. If nothing else, genetic drift pretty much assures speciation at some point.
The other key factor to keep in mind is how large the breeding population is. Smaller, isolated populations can diverge more quickly than large populations which span multiple geographic zones.
The critical question, though, isn’t whether they do interbreed, but whether they would, given the opportunity. Suppose, for instance, as a hypothetical: A chimp is climbing on a tree branch which overhangs the river, when suddenly, the branch breaks. The chimp can’t swim, but by instinct or luck, hangs on to the floating limb, and manages to keep its head above water. Eventually, it washes up on the bonobo side. Would the chimp just stay aloof from those alien wierdos from another bank, and remain in a state of forced celibacy, or would it join in on the hot monkey orgies and produce a bunch of hybrids? Of course, this would also depend on whether the bonobos let the chimp mate with them. And of course, we could equally well postulate a bonobo, by a similar fluke, ending up on the chimp side of the river. Or a severe drought, which let many members of both populations cross freely.
My point is, geographical isolation does not, in itself, constitute speciation. It may well facilitate speciation, if it persists long enough, but it’s not inevitable.
I think the point is that we have any way of knowing that.
Chimps don’t take kindly to outsiders, not even other chimps. If it were a female, it’s possible that she would be accepted into a troop. But there are very real and very different gender behavior patterns between chimps and bonobos, and a female might not be able to adjust to the differences in the wild. If it were a male, it would almost certainly be killed. But it still wouldn’t matter. One or two random matings do not a single species make. The barrier is real and cannot possibly be breached on any regular basis.
At any rate, chimps and bonobos are classified as distinct species. That is a fact. BTW, I used to be a lumper in my youth, but as I get older I find myself more accepting of the splitter mindset.
AFAWK, the geographic barrier has persisted for at least 1M years. Is that long enough? But the thing is, bonobos and chimps are not only morphologically different, they are behaviorally quite distinct as well. So, we have two physically and behaviorally distinct populations that have been isolated for a very long time and who have virtually no possibility of mating in the wild. What do we do?
To a large extent, this whole discussion turns on what one uses for a definition of a species. For a good description of the different application of the BSC and the PSC, see this thread that I started 1 year ago (to the day!). In particular, note Colibri’s response in post #25.
Yeah, a female chimp might get all prudey when she sees how bonobos use casual sex, both straight and gay, as rewards, to maintain group cohesion, and because everybody likes a quickie now and then. But give her a little time and some way-overripe fruit and she’ll come around.
Possibly ala Deliverance. Like I said, those bonobos are horny little bastards.
Unless the matings result in viable offspring, in which case they are subspecies of a common ancestor.
Oh, and my annoyance with splitters comes from the Leakeys, who seem to claim every bone they find is a new species. Rather than digging up Louis and wringing his neck I became a strict BSCer.
Yes, they did seem to want to create a new species for every bone fragment they found. But others are just as bad. Not only are they sure their new find is a new species, but they usually want to claim that theirs is the one and true human ancestor. It gets pretty silly.
But you could probably only make a case for subspecies back then because there were no real isolates (except maybe Tasmanians) as there was plenty of travel back and forth between Alaska and Siberia (by boat) before Europeans arrived. In fact, the entire Eskimo popultaion is a relatively recently arrived ethnic group (compared to Amerinds).
