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Would Great Danes and Chihuahuas be considered separate species if no other breeds existed?


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Can someone tell me whether my reasoning on this matter is sound. As I understand it, all the various breeds of dog, from Chihuahua to Great Danes, are currently by convention considered to be the same species. Even though it is virtually impossible for certain breeds such as Chihuahuas and Great Danes to interbreed with each other (if the Chihuahua was female it would die, and if it were male it couldn't impregnate the female Great Dane), because both breeds can interbreed with various medium-sized breeds, their genes can still flow back and forth (put very crudely, of course).

 

However, what would happen if all of a sudden every other breed of dog went extinct, and all we were left with were pure-bred Chihuahuas and pure-bred Great Danes? Would the fact that these two populations, at least if left to their own devices with no human intervention, would be reproductively isolated from one another then mean that they would have to be considered to be separate species? Personally I've always felt that by all rights they should be considered different species now simply because they look so different from each other, not only in their fur but also their anatomy and skeletal features. But I know appearance isn't everything in biology.

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Isn't this a repeat of a recent question?

 

Dogs would be considered ring species in one interpretation and sub species in another. If indeed the only dogs left were Great Danes and Chihuahuas and an outside source unaware of their origins were to inspect them they would qualify as separate species....

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But I know appearance isn't everything in biology.

 

Indeed - reproductive isolation is not the be all and end all of species classification.

 

Species are more rightly defined as "independently evolving metapopulations." http://sysbio.oxfordjournals.org/content/56/6/879.short

 

That is to say a species is a group of organisms, which through the process of sharing alleles with one another, share a common evolutionary history, which is distinct from other groups of organisms.

 

Despite rather seemingly convincing evidence for pre-mating isolation between a great dane and a chihuahua, they are only genetically distinct from one another at relatively very few genetic loci - that is to say most of their evolutionary history is very closely shared. Given most purebred dogs represent extreme selection on rare mutants in a population I'd personally suggest that they represent extreme outliers in a normal distribution of phenotypic variation - like a dwarf and a pro basketball player would in the human population. If the isolation was maintained for long enough, they may well end up being different species, but they're so recent in that they lie somewhere on the continuum from homogeneous population and distinct species, which is difficult to define. :)

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Isn't this a repeat of a recent question?

 

 

Not quite, although I did ask a related question about dog breeding and speciation a few weeks ago, I decided to ask this nonsequiter question as a new post rather than just adding onto the previous one.

 

At any rate, I also posted this question to another website and got some interesting responses. What does the term 'species' actually mean to biologists nowadays? Because the inability to produce fertile offspring can't be the only criteria, because dogs and coyotes are different species and yet are capable to interbreeding, as are wolves and coyotes. Is that simply because they have the same number of chromosomes, or is there more to it than that?

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Is that simply because they have the same number of chromosomes, or is there more to it than that?

 

Nope, some species display polyploidy: http://jhered.oxford.../9/379.full.pdf

 

As I tried to explain above - the defining feature of a species, at least under the general lineage concept is that they are a species is a group of organisms, which through the process of sharing alleles with one another, share a common evolutionary history, which is distinct from other groups of organisms.

 

The characteristics by which you decide that they have a distinct evolutionary history are secondary, and may be different depending on the particular species division you are considering, particularly when speciation is very recent. E.g. it could be fixed genetic, ecological, life history or morphological differences, evidence of either reproductive isolation or persistence of both species in the presence of hybridization through hybrids being at a selective disadvantage, etc.

 

As speciation is a multifaceted process, there's no one, silver bullet way of delimiting species. Hope that helps :)

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