The edible frog exists only as an F1 hybrid between the pool and marsh frog. It’s a good example of a hybrid species.
One of the real problems we have in discussing whether a population of animals is a species or not is that we have several concepts that make the whole issue quite murky.
One of these concetps is the concept known a “hybrid species.”
It is certainly true that hybridization can create a new species, and it is certainly true that hybridization between related species is not as uncommon in the wild as was commonly thought.
These two issues make concept of “hybrid species” fairly murky.
For example, in the last few years, we have discovered that modern human populations, though derived from a species that originated in East Africa, occasionally hybridized with other human species, including Neanderthals. A tiny part of the genome of any humans whose ancestry is not within sub-Saharan Africa is Neanderthal.
Does that tiny bit of Neanderthal make us a hybrid species?
Many coyotes, especially those living in the East, have a little bit of dog and wolf DNA. They are still overwhelmingly coyote in their genetic make-up, but they do have some genetic material from another species.
Does that make them a hybrid species?
I don’t think we can say that Eastern coyotes and modern humans are a hybrid species. These organisms just happen to contain genetic material from another species, and in that same vein, I don’t think the red wolf counts as a hybrid species either. It is a coyote with some wolf genes, in the same way I’m a modern human with some Neanderthal genes.
After all, the red wolf wouldn’t exist at all, if humans weren’t actively keeping them from breeding with coyotes, which should be a major red flag about the unique species status of this animal.
I think a true hybrid species can exist with larger mammals, with the possible exceptions of the mule deer and Pere David’s deer. European bison may also be hybrids between ancient Eurasian bison and the aurochs.
But there actually are species that exist only in a hybrid form.
Perhaps the best example of this hybrid form is the European edible frog (Pelophylax kl. esculentus).
It is a hybrid between the pool frog (Pelophylax lessonae) and the marsh frog (Pelophylax ridibundus).
And the hybrid form cannot breed true. If you breed two edible frogs together, the tadpoles are too deformed to survive.
However, the edible frog can interbreed with either parent species, but the resultant offspring are very similar to the parent species on which there has been a doubling up.
The “kl.” in the edible frog’s scientific name stands for “klepton.”
A biological klepton is “a community of populations with a hybrid genome derived from the same parental species, reproductively dependent upon sympatric species that play the role of sexual host.”
These animals are not uncommon in fish and amphibians.
I’ve never heard of one in mammals or birds, though I’m sure someone more knowledgeable than I am might have some examples I don’t know about.
We need to have a better way of defining a hybrid species.
An organism that has some genetic material from a related species that got there as a result of crossbreeding is not a good example of a hybrid species. My guess is that when we get really sophisticated genomic analyses, we’re going to find that virtually every mammal species has some genes from related species that got there through some very limited and very distant crossbreeding.
But does that mean they are hybrid species?
I really don’t think so.
Real hybrid species do exist, and hybrid speciation is one of the most under-valued ways in which species can come into being.
It’s just that we need to be careful about making these declarations about a new species being formed through hybridization. Yes, it does happen, but if we’re not careful, we can wind up creating a definition of a hybrid species that essentially becomes meaningless.
The species concept itself is murky. If we didn’t know them so well, and their geography wasn’t so widespread and distorted by humanity, we might well combine dog, wolf, and coyote into one species. I remember an SJ Gould article about some limpet genus in the Caribbean where 300 described ‘species’ were reclassified as one species (sounds like dog breeds, no?). I doubt anyone studied fertility in the second generation of hybrids of these former species. And then, as you pointed out in an earlier post, there are cryptic species. How do you use fertility-related definitions of species on those aphids where the female reproduces without mating? What species is a plant chimera created when cell division goes wrong and one part of the plant is diploid and the other part is polyploid? (For and amusing, from an animal perspective, discussion of ploidy and crossing, see http://forums.gardenweb.com/forums/load/daylily/msg021425157966.html . . .
discussing hybridization of diploid and polyploid species. One sentence to illustrate: “Some triploids can have acceptable fertility with tetraploids. In general, a triploid’s fertility will be better with a tetraploid than with a diploid.”)
Evolution produces lots of variation . . . including variation in cross fertility patterns. I doubt that any two barriers to fertility are exactly the same when you get down to the nitty gritty.
Plants are weird.
Of course, if they had brains, they’d probably think mammalian speciation is pretty bland.
I’ve heard tell that the original popular appeal of Linneaus’ work was the kinky sex in the plant world.
Plants, in turn, are bland compared to fungi, who go from haploid to diploid through direct exchange of cytoplasmic material (conjugation). They have ‘mating types’ instead of sexes. Some species are known to have as many as 64 mating types. At least in some species, homozygosity is a barrier to mating.
But even if you stick to the animal kingdom . . . each speciation event has its own history and the resulting barriers have their own particular characteristics. Peggy Richter put it better than I . . . the result is not “clear cut”, and nature is inclined to tromp all over our categories.
Lest we forget, Gregor Mendel also used plants (peas for the most part) to develop his theory of genetics.
Interesting to me you mention the European bison as we visited the small herd of these at nearby Whipsnade Wild Animal Park last week. They are in the moult and looking very scruffy right now. Unless I am imagining it, they look to me more designed for living among trees than the American bison. Also I have read that the European bison (or wisent) was crossed with American bison early in the twentieth century at a time when there were few examples of either species available to zoos in pre second world war Europe. So I’m wondering what percentage of American genes may be present in the animals eventually reintroduced to east european forests post war. I’m going to try to look up on that, but meanwhile can anyone enlighten me on the subject?
They are.
There are still wood bison in northern Canada that are adapted to living in more forested areas, but the Eastern American bison that used to live here were much more adapted to living forests.
Bison are most closely related to yaks, but both species of bison have incorporated genes from the aurochs lineage. Most American bison have some cattle ancestry. There are very few pure ones left. The Caucasian population of Eurasian bison has some cattle in it, but they all have what might be some older aurochs ancestry. It may have happened that as the ancient bison spread from Central Asia into Europe, they occasionally mated with aurochs.
Something similar has happened with kouprey, a possibly extinct species of cattle endemic to Cambodia. During the Pleistocene, they managed to incorporate a Cambodian banteng’s mtDNA into their population. That means that a banteng cow bred into the kouprey population at that time.
Also you mention the recent revelation that a tiny amount of neanderthal genes are present in homo sapiens, but am I right in thinking that this applies specifically to people of european descent and if so does this imply that the crossbreeding which brought it about probably occurred in Europe, as I believe there is no evidence of neanderthal genes in people of pure African descent?
From what I’ve read, the current thinking is the hybridization occurred in the Levant and then the descendants spread throughout the world, except for Africa south of the Sahara. However, there are people in Africa that have Caucasian ancestry, like Ethiopians and others. So the Neanderthal genes could be found down there too.
“pure African descent”. To qualify that slightly, before I am picked up on it, I am aware that people on the African continent contain various strains, such as the capoid people who originally came from northern regions of the African continent (you can see it slightly in the faces of some north african people of berber descent) and form part of the bushman and hottentot inheritance. Also there are the hamitic/nilotic strains (tutsis?) and those of the somalis and ethiopeans. But do any of those contain neanderthal genes or are those only to be found in northern Europeans? As you can tell, I am only superficially informed on the subject.
I think Nature is less “clear cut” than humans would like it to be regarding what is a species, a sub species or a simple ‘variation within a species”. So the boundary areas can overlap. Nature doesn’t much care (nor do the animals) if they don’t fit into nice cubbyholes humans would like to use for classification. Yet we do need to identify for purposes of conservation and simply because we like to classify things.
To me, a species can contain Introgressions from another species and still be counted “unique” and deserving of seperate identification. OTOH, one or two differing characteristics (color being one used to identify bird and insect differences) do not a species (or even a subspecies) make. A 50/50 is clearly a hybrid (such as a fin/blue whale cross) but I would also count the F2 backcrosses (ie fin x fin/blue whale or blue whale x fin/blue whale) as hybrids. The F3 and beyond would be “introgressions” assuming no further combinations occured.
I’m not keen on the “if it’s physically isolated” being one of the rules — many animals are only isolated within the last 20,000 years ago or so, and that isn’t, imo, sufficent, if the offspring are fertile. So for me, the following apply:
1. If they CAN interbreed and have both male and female fertile offspring more often than not, they are “one species”. If it’s only females that are fertile or fertility is the exception, then they aren’t. So lions and tigers are different species but Amur and Bengal tigers are one species.
2. if they CAN interbreed and have fertile offspring but it’s clear that they have differing behaviors, timing of when they breed, etc, that would preclude interbreeding on any but an exceptional basis or only with human interference, then they are probably sub species.
3. Human caused isolation or mere geographic isolation don’t count. Hence, the puma of Florida and the puma of California don’t count as subspecies. It’s clear that at one time a more or less continuous connection existed for puma.
4. rare interbreedings of species are true hybrids. I’m not so sure about if we should care about interbreedings between subspecies.
I believe that you can have a seperation (via evolution) into species that then combine back — hence Human/Neanderthal or fin/blue whale. This may be highly beneficial from an evolutionary standpoint as it allows for fast tracking of any beneficial characteristics developed by either of the parents in the general population. I think it happens far more often than we think it does.
Peggy Richter
By these criteria, are dogs and wolves and coyotes subspecies?
My view is that dogs and wolves are the same species. Mech writes about female wolves that will solicit male dogs. However, these are usually not the main breeding females of a pack, and those wolves that do get pregnant by male dogs usually don’t have litters that survive. But sometimes they do. The genomic research shows that dogs and Old World wolves split 27,900 years ago, which is actually not that long ago. Dog and wolf DNA is very similar– as similar as wild subspecies of wolf are to each other and in some cases, more so.
Coyotes and wolves/dogs can interbreed and produce fertile offspring, but there are greater behavioral barriers that prevent them from doing so. I don’t think dogs regard coyotes as being other dogs. Even the friendliest of dogs, like golden and Labrador retrievers, that love being around other dogs, seem to hate coyotes with venomous passion. There is something about coyotes that just sets dogs off. I know they do occasionally interbreed, especially if they don’t have any other mates to choose from.
Golden jackals very rarely interbreed with dogs, too, even though they produce fertile offspring. I’ve not come across any accounts or genetic literature that says that wolves and golden jackals have interbred in the same way wolves and coyotes have. I had a reader who had grown up in India leave a comment that a male pariah dog in her hometown mated with a female golden jackal. Puppies were born, but if they survived to adulthood, they would have bred back into the golden jackal population. I wouldn’t be surprised to find out that golden jackals have a lot of secrets in their DNA. They vary so much in appearance and are so wide ranging– from Austria to Cambodia in the Old World and all of the Middle East, North and East Africa.
Ethiopian wolves also can interbreed and produce fertile offspring with dogs and probably with golden jackals and the African wolf subspecies, which recently was identified. In fact, I think its possible that either African wolf or domestic dogs have crossbred with them in the past, for the initial mtDNA studies found them to be almost a subspecies of Canis lupus. The genome-wide analyses revealed them to be much more distantly related to the wolf and dog species than the mtDNA study suggested.
In my studies believe our Retrieverman hits on the essence of this murky quesstion. The focus likely might be if offspring can reproduce. I have noted that inbred bloodlines offspring frequently just fail to be able to reproduce? Nature just takes over and eliminates the ability to reproduce.
I agree with you that dog, wolf and coyote on deep examination appear to be varieties of the same species. Like Marcus photos of Great Danes from United States show lines. They look like …(now no one get in a panic with me) Saluki cross breedings to me.
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