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At what point can hybrids be considered a new species?

Posted by Nick_B79 z4 MN (My Page) on
Sat, Dec 6, 03 at 23:02

I'm stratifying seeds from Aesculus x carnea (Red Horsechestnut) and have read that, due to a chromosome doubling in the original parent tree, this hybrid is tetraploid and will not backcross with other Aesculus species successfully. Since this hybrid is now considered sexually distinct and true-breeding from seed, why could it not be considered a new species instead of simply a hybrid?


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RE: At what point can hybrids be considered a new species?

Species is a very fuzzy concept. It is also a contentiouse one. It all depends on which definition you accept.

There is the biological species definition, Specific Mate Recognition System definition, the Phylogenetic Species definition, amoung others.

If you are using the naturally biological species definition then there is a requirement that the species be naturally occuring.

Since A. carnea does occur in the wild, has a native range, is reproductively isolated, and reproduces directly it qualifies as a species by all the definitions.

I personally don't think whether reproductive isolation is "natural" or man made should matter in determining what a species is. After all isn't man just one more environmental factor from the point of view of other species. Thus I'm of the opinion that various breeds of dogs should be considered species. They are after all reproductively isolated and have been so for long periods of time. Just because there ecological niche is either symbiotic or parasitic on man doesn't mean they shouldn't be considered species. We no longer consider dogs to be of the same species as wolves even though they probably are an offshoot of wolves. Certainly dogs are not reproductively isolated from wolves for natural reasons, nor do they occur in the wild. So why not go a step further and recognize the breeds themselves as species.

I always thought that the opinion that man is unnatural is an artificial convention in the first place. Man is a indigenous species to this planet just like any other. Certain plants become reproductively isolated and speciate because of the behavior of bees. This is considered a natural process. Why should our behavior causing the speciation of other animals not also be considered natural.


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RE: At what point can hybrids be considered a new species?

MacDaddy,

"Species is a very fuzzy concept. It is also a contentious one. It all depends on which definition you accept."

I am not sure I accept any of them. Plant taxonomy has a lot of heritage classification that is questionable. From the standpoint of hybridization, the way I see it, it should be easy to cross varieties within a species, it should be more difficult to cross varieties of different species within the same genus, and it should be much more difficult to cross varieties that are members of different genera.

Apparently orchid breeders have no difficulty making intergeneric hybrids. To me, that brings into the question the taxonomy of the orchids.

I don't agree with your suggestion that dog breeds be considered to be separate species, simply because crossing takes place so easily and spontaneously. Since dogs and wolves cross so readily it seems questionable to me to consider them as different species. But I really don't understand canine taxonomy. I tend to think of wolves and coyotes as different species. Yet we had a dog once that was part coyote and she dug a coyote hole in our back yard to give birth to her puppies. So, I dunno. It seems a bit of a stretch to classify all canines as the same species.

I think we probably should define species and varieties in terms of DNA analysis. That can be objective rather than subjective. But when we do that, I suspect that the boundaries between species and varieties and between species and genera will become completely blurred into a continuum of similarity and dissimilarity. Which seems to be the actual case now with orchids.

And if we use DNA analysis as the basis, the ease or difficulty of hybridizing becomes irrelevant. The genetic engineers can do pretty much any crosses they want to, within the limits of state-edicted legality.

-- Burton --


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RE: At what point can hybrids be considered a new species?

Mating between different Canid species is quite easy. Does that mean we should consider the Abyssinian Jackal to be the same species as the Dog?
I think the American Kennel Club is as good a reproductive isolation mechanism as any mountain.
So I think one should keep ones mind open on the idea that dog breeds might just be species in their own right.


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RE: At what point can hybrids be considered a new species?

Are you saying that a great dane and a chihuahua might be completely different species? A Husky and a poodle? A Cocker Spaniel and a Border Collie?
What about cross bred dogs? Cocker Spaniel X Border Collie, Great Dane X Grey Hound? and the mongrels - crosses of crosses? Where would you draw the line. They are all the same species and their genetics has not changed considerably from the wolf, with which they still breed.
The various dog breeds are selected and isolated 'varieties' or 'strains' and approach homozygosity, the more they are inbred. Unfortunately they also often approach infertility the more they are inbred, and the odd 'throw back' may be little more than the the dog down the road having his way with your 'pure bred' dog. There is too little control in dog breeding to be sure someone hasn't cheated.
It is common, and popular for new breeds of dog and cat to be created by intercrossing, selecting, and stabilising, just as is done with the selection of plant varieties.
'hybrid vigour' is a term often used when varieties are crossed. the term 'hybrid' is not exclusive to crosses between species or genera. When the parents are relatively homozygous, the cross will be an F1, but further crosses will have very mixed chromosomes, and very mixed appearance.

Genetic variation can occur within species to the point where varieties seperated by distance will not breed, although they will cross breed within the species with close neighbors. For instance Possum populations in Australia. Possums from Qld behave like a distinct species from those in Tasmania, but all along the chain they will interbreed. At what point could you say they are seperate species?
Just a few roughly cast ideas. The lines are often very blurred.
Cheers, Jan


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RE: At what point can hybrids be considered a new species?

I am aware of ring species. My whole point is that there is not any clear cut lines between species.

I am aware of new breeds being produced like the Labradoodle. I don't see why that would neccesarily count against the "breeds as species" idea. Nor would the fact that mutts are produced. There are naturally occuring interspecies mutts in the wild, like with sarracenia.


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RE: At what point can hybrids be considered a new species?

If you are going to start looking at individual breeds of dogs as distinct species because of their differences, then there is only a thin line between that and considering different races of humans as species. The genetic difference between a labrador and a poodle are not very much different from the genetic difference between a Japanese person and a Norwegian.

You say "Species is a very fuzzy concept. It is also a contentiouse one. It all depends on which definition you accept"

That is fine, so long as you don't intend to try and force your 'accepted' definition on others. You can define species in any way you like for yourself, but it doesn't make you right. I think your thin line stops way further back than you are prepared to push it :-)


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RE: At what point can hybrids be considered a new species?

I wasn't basing it on genetic difference. I was basing it on breeding isolation. No such isolation currently exists between human races.

You seem to be sure that there is a substantial genetic flow between dog breeds. If that were proven to be true (not because they started from the same base genes but that new mutations are flowing) then I would not consider them separate species (or more technically subspecies). I don't see how this flow can occur given what I know about dog breeding. A mutt just wouldn't fit the breed specifications well enough to be selected as a breeder.

In fact I'm wondering whether certain breeds of dogs have diverged to the point where they can no longer breed naturally ala ring species. I'm sure a Great Dane can breed with a Husky which can breed with a Shelty which can breed with a Chihuahua. However I am not sure a
Great Dane and Chihuahua could even mate and produce offspring successfully.

I'm not too concerned about any politically correct implications. That is I am not going to let my understanding of nature be preordained by politics. I could discuss this but I don't however think this is the appropriate forum for this.

You don't have to get into the issue of dog breeds to discover inconsistent application of the concepts of species, subspecies and the like with regards to human races or "species" protected under the endangered species act. Why exactly is the San Fransico Garter protected? It isn't even a species but a subspecies of the most broadly distributed snake on the planet. These political issues often have little to do with science.


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RE: At what point can hybrids be considered a new species?

Why do you say a GD cannot mate with a Chi? Because of the technical difficulties or that they are genetically incompatible?
Given human intervention, they are quite compatible. A Chihuahua male will willingly oblige a GD female, and a litter of normal X breds result. The reverse of this cross would mean at best a caesarian birth, at worst death of the female dog.
Clydesdales and Shetland ponies have been cross bred both ways, with live births resulting. The Smaller female size limiting the size of the foal to some extent.
I don't know what involvement you have had with dog breeding, but as a Vet I have had experience with a vast variety of dogs, and can say that pure breds are only the tip of the iceberg. Even first cross dogs like my Poodle X Bearded Collie are in the minority. Most pet dogs and feral dogs, and many working dogs are far from pure bred.
You say a Mutt wouldn't fit the breed specifications to be chosen as a breeder.
You are assuming that a breeder is doing the choosing. Dog breeders are in the minority. The vast majority of dogs do their own choosing, and Mutts make terrific breeders, lacking many of the specific problems brought about by inbreeding.


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RE: At what point can hybrids be considered a new species?

Yes, I did mean the physical difficulties of a GD X Chi breeding. From what you say there is already a partial incompatibility in that a female Chi would require a C-section to give birth. I am sure a male Chi is willing but I don't know about the mechanics of such a small dog mounting a GD. Is that a common phenomena round your parts? :) You seem to think that somehow the breeders don't catch such instances. In that case, do such crosses end up being registered with the AKC as Chi's or GDs?

I don't think your Mutt argument holds water. Most snakes are common garters but that doesn't mean other snake species don't exist that can cross breed with them. So it is not material whether most dogs are mutts or whether most mutts choose their own mates. It could still be that pruebreds could qualify as species even if there was enourmouse one-way genetic flow from purebreds to mutts.

If breeders are indeed keeping their breeds genetically isolated then that is a isolation mechanism same as any other. Whether that is true or not I have no idea. Seems to me that for the most part it is true. They even keep registries of who the parents are.

Whether Mutts are more healthy is also not material to my argument. Cheetahs are genetically inbred and not very vigourous but they still qualify as a species.

Also note that inbreeding in and of itself is not the cause of genetic disease. The only thing inbreeding does is expose harmful recessives to natural selection. There are species which inbreed early and often which do not have problems (in particular termites). It is the fact that most species do not inbreed that allows harmful recessive genes to be prevalent in the gene pool. Breeders could weed these recessives out but they don't for financial reasons. It's expensive.

Inbreeding can be a problem if one has a small pool of individuals and they become so genetically similar that there is no variation in their immune defenses. Thus a germ that is exposed to the selective pressure of getting around a single individuals immune system automatically has the correct attack strategy for the next individual it infects.

This is one reason why "clone armies" are unrealistic. Even if you had a very healthy person to clone a germ would eventually arise that would decimate your army.

Plant enthusiasts have already experienced exactly this type of situation. Monoclone fields can be more prone to communicable diseases. One named variety may have had it's defenses breeched and may be more prone to certain diseases than other named varieties. Especially when the named variety is vegitatively propagated.


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