I was thinking about chimps and bonobos the other day after reading “Our Inner Ape” by Frans de Waal. Chimps and bonobos live in non-overlapping territories in the wild, being seperated by the Congo River. Neither species swims, and the Congo River is quite wide, so a barrier exists which prevents their interactions in the wild. But de Waal also states that the two species interbreed in captivity, so what would happen if the Congo river sudenly changed in such a way as to allow easy travel for the apes into each others’ territories? In all likelihood, one species would wipe out the other, or they would interbreed to the point where there was only one species (after a sufficiently long time, and assuming no human intervention).
So, has this type of thing every been observed in the wild? Two species which can interbreed, but don’t do so due to a natural barrier that at some time becomes breeched? Have we ever seen two closely related species merge into one species?
Note: I only offer the first paragraph as background as to what got me thinking, and don’t want to open a debate on whether the scenario I outlined would actually happen or not.
Not sure if this is what you’re looking for, but the mitochondria in our cells was probably a separate organism that we develoved a symbiotic relationship with in our evolutionary past.
Doesn’t the definition of a species preclude it from breeding with something else? If bonobos and chimps breed together, they’re ipso facto the same species. So the thing you’re talking about doesn’t happen by definition, by my understanding.
But wait for **Colibri **to give you the straight dope.
There are many thousands of examples, and this is one of those things that can make the Biological Species Concept sometimes difficult to apply. There are many “secondary contact zones” where formerly separate forms have come together and now hybridize. If the hybridization is extensive enough, the forms are considered to be the same species. If hybridization is sporadic or limited to a narrow zone, they are nowadays usually conisidered to be separate species.
One well-known secondary contact zone is on the Great Plains. where at least 14 species or subspecies pairs of closely related birds come together and hybridize at least occasionally.
Such secondary contacts between previously isolated forms evidently have been caused by climate changes in the past. Today, however, many have been caused by habitat changes caused by humans, or by introduction of exotic species.
The definition usually includes the req’t that they interbreed in the wild. But even that is not a hard and fast rule-- wolves and coyotes often interbreed in the wild, but they are considered seperate species. That definitino lends itself to a lot of subjectivity, which is why a cladistic approach is often viewed as superior (ie, more objective). That, of course, would probably put us in the same genus with Chimps and Bonobos, or at least it would put us in some grouping with them that did not also include gorillas.
I’m no paleoanthropologist, but I recall that some believe that Homo neanderthalensis interbred with early Homo sapiens. Of course, you’d also have to believe that Neanderthals were a separate specie from early man instead of a subspecies (Homo sapiens neanderthalensis).
In contrast to discrete hybrid zones as found in the Great Plains, another possible scenario is one species expanding into the range of another over a wide area. In this case, the more common species may end up “swamping” the other through hybridizing it out of existence. This seems to be happening with Mallards and American Black Ducks in the eastern US:
Witness the difficulty in pinning down the taxonomy of the Red Wolf ( “hybrid swarm” dating to the 18th century or half million year old relictual species ), source of a great deal of controversy in the 1990’s at least.
The biological species concept just doesn’t work in a lot of situations. Species are gradiants, not distinct entities–they only appear to be distinct when the middle potions of the gradiant have died out or relocated. Look at dogs; are chihuahuas and St. Bernards the same species? They won’t (and I assume can’t) interbreed, but I bet you could find a medium sized dog that could breed with a chihuahua, then find a bigger one to breed with that, and so on until one can breed with a St. Bernard.
I guess it would make sense that birds would be prone to this sort of phenomenon-- what with their ability to fly.
Has this been known to have happened with any of the larger mammals, or even with the more fast breeding rodents?
Tamerlane: I was thinking of the red wolf when I posted that, although that wouldn’t fall into the category of this thread topic since both the grey wolf and the coyote remain as distinct populations.
Cliffy: I don’t think we can say the Neanderthal/Sapiens case is closed, but there is very little evidence that hybridization actually did take place-- or if it did that it happened to any significant extent.
There’s nothing at all special about birds. Hybrid zones and hybrid “swamping” occur in just about every kind of animal and plant that breeds sexually. This is very common. I just happen to be more familiar with avian examples.
With large mammals, sure. Perhaps the most notable example is the Banteng, the original wild cow of SE Asia. The evidence at the moment suggests that the wild populations are almost all to some degree intermixed with European and Asian cattle species, and that the hybrids, being better able to cope with humans, are outbreeding the pure pure-strain individuals. Pretty much the same story as the Mallard/Pacific black duck, although slowed somewhat because the first generation male hybrids are sterile.
I can’t think of any examples of rodents off the top of my head, but it would be astounding if they didn’t exist.
Do the original, unhybridized populations of European and Asian species no longer exist? That was what I was getting at. Two species turn into one, with the original species no longer existing as separate populations.
For awhile, it looked like the Baltimore Oriole was becoming so interbred with Western Bullock’s Oriole so much so that they were being combined into the Northern Oriole. It seems that the two species are now drifting apart again, to the relief of baseball fans.
I don’t quite understand what youi mean. Just as with the Mallard/Pacific Black Duck the two populations now show genetic continuity. There is simply no point at which we can say This is a mallard/banteng" and “This is a cow/black duck.” It’s just an arbitrary line in the sand. In that respect the original species no longer exists as a separate population.
Note that this does not mean that there are no longer any genetically pure mallards or banteng anywhere on the planet. It simply means that what were distinct species now form a genetic continuum.
I guess the point to relaise is that species is a purely human construct, not a natural delineation. The original species ceased to exist as soon as humans could no longer differentiate the two groups. In reality of course the original species will never actually cease to exist untiil the new taxon becomes extinct altogether. Some of the genetic material form both groups now exists in the new group. Even if there were no longer any genetically pure banteng left we still wouldn’t be able to say whether it was the banteng or the cow that was extinct. All we could say is that there were two distinct groups, and now there is one new group comprised of bits of both.
To use a human example, when East and West Germany reuinified, which country disappeared? In reality both did. There were two nations, then there was one comprised of parts of both. The new entity isn’t solely either of the original. In the same way the newspecies formed isn’t solely either of the original two species, it’s something distinct from both.
If you want an example of where there were two species and then just one of the original species no longer existed you are actually asking for an example of where one species replaced a closely related species leading to its extinction.
That’s not it exactly. The lumping of Baltimore and Bullock’s Orioles into Northern Oriole and their subsequent re-splitting had more to do with shifts in taxonomic philosophy rather than anything about the birds themselves. IIRC, the two forms were merged into Northern Oriole by the American Ornithologists’ Union sometime in the 1970s, and they were re-split in the 1990s. They were lumped because they hybridize extensively in a small area; it was later decided that because this area was so narrow they should be considered separate species. It was also determined that the two forms were not each other’s closest relatives; there is a Mexican species that is closer to one of them. (This is one of those things that makes some cladists insist that the Biological Species Concept is inadequate.)
The present dynamics of the hybridization is described in my link to the Great Plains hybrid zone above:
At what point do you decide that they are one species instead of two? There are all kinds of situations that occur, from narrow hybrid zones such as those on the Great Plains, to situations in which two populations intergrade over hundreds of kilometers, to those in which there appears to be a smooth cline in characteristics across a species’ range with no distinct zone of hybridization. In the latter case, it is often impossible to determine whether the cline developed in situ across one originally homogeneous population, or whether it is the result of a very old secondary contact between two formerly separate forms in which “hybrids” now occur across most of the species range.
Good points. I suppose one might say that if the two species will interbreed in the wild, then as soon as the barrier separating them disapears it no longer makes sense to call them different species. After all, it was the barrier that created the “artificial” distinction in the first place.
And yet, that isn’t what would happen in the real world, I don’t think. Go back to my chimp/bonobo scenario. If the Congo River suddenly disappeared (without destroying the environment) and chimps and bonobos started interacting with each other, would biologists suddenly declare that there was now only one *Pan *species? I don’t think so.
Perhaps my question doesn’t really have an answer, then. Or maybe you just make some quasi-arbitrary decision that once a critical mass of hybrids exists (pick a percentage), then it makes no sense to say there are two distinct species.
Oceanographers are currently surprised to discover that certain odd looking Cetaceans are simply hybrids of other better known ones. The genetic differences among porpoises, dolphins and whales seem to be minimum.
