On the variety amongst dogs, or What is a species?

Factual Questions
1

What if the animal species taxonomy were suddenly to disappear. No knowledge whatsoever of the former taxonomy exists - heck, let’s say even the idea of DNA has been forgotten*. The scientific community must recreate the species taxonomy cold. How would the variety amongst dogs be handled? On what basis would someone categorize a St. Bernard in the same species as a chihuahua?

I presume a key criteria for speciation is members of one species can only produce viable offspring from others of the same species. I assume the St. Bernard and chihuahua can do so if artificial insemination were used. Is it physically possible for a male St. Bernard successfully mate with a female chihuahua? Would they voluntarily try? If we didn’t already know they were of the same species, would we even try artificial insemination?

There are species of penguin that differ far less than these two breeds of dog. Did taxonomists try interbreeding them before declaring them separate species?

What other species exihibit as wide a variety as dogs? Are these species’ variations always due to human intervention?

*maybe scraping DNA is too extreme, but I hope you see what I’m trying to get at

2

Actually, a biologist will inform you that the default requirement for speciation (in general) is production of fertile offspring.

As an example, horses and donkeys can breed with each other, but the resultant offspring is incapable of breeding further generations - hence horses and donkeys are not the same species.

A St. Bernard and a chihuahua would produce offspring capable of breeding further generations (with dealer’s choice of St. Bernard, chihuahua, others of the St. Bernard/chihuahua crossing, or any other dog of your choice), hence the original parents would necessarily be of the same species.

There are exceptions, but that is the general rule of speciation as far as taxonomy is concerned. If I recall correctly (and it has been a number of years), taxonomy is merely a system for consistent naming of varietals that meet the default requirement for speciation.

3

Ahh - the joy of posting at work. I meant to edit and add the following and timed out:

Keep in mind that, in nature there are a number of factors that dictate when or if breeding between creatures will occur, including hormonal cycling (heat, estrus, etc), geography, physical connectivity concerns, phereomones, etc.)

In the case of a St. Bernard and a chihuahua, there may be physical limiting characteristics, but smaller St. Bernard mixes and larger chihuahua mixes could certainly breed (and would given the chance) - which would tell a biologist that they’re the same species. Also, a bitch of either breed in heat presented with a male of the other species would give it a whirl - I’ve seen that personally. Being unable to complete the physical act of mating does not necessarily equate to speciation - the fact that they’d try argues toward being the same species. There will always be members of species that are physically incapable of mating with each other. (Example: There are male and female humans that would experience similar difficulty to St. Bernards and chihuahuas, and for the same reasons - reproduction for two such individuals is still possible, but unlikely without internvention. Same would go for your the dog breeds mentioned.)

4

[QUOTE=Aangelica]
Actually, a biologist will inform you that the default requirement for speciation (in general) is production of fertile offspring.
[/QUOTE]
Is that still the scientific definition of “species”, then? I’ve been told on these boards that that definition has been discarded, but in favour of what I’ve never been able to ascertain.

5

[QUOTE=Priceguy]
Is that still the scientific definition of “species”, then? I’ve been told on these boards that that definition has been discarded, but in favour of what I’ve never been able to ascertain.
[/QUOTE]

The Biological Species Concept defines a species as a population that interbreeds regularly in the wild. “Regularly” can be somewhat subjective, and it might be simply that a physical barrier prevents the breeding (eg, chimps and bonobos).

But that definition really has no meaning for domestic animals like dogs, since they don’t naturally occur in the wild. The default assumption for domestic species is to put them in the same species with their closest wild relative-- for dogs that is the wolf.

6

[QUOTE=Aangelica]
Actually, a biologist will inform you that the default requirement for speciation (in general) is production of fertile offspring.

[/QUOTE]

The problem with this definition is that crosses between clearly defined species occasionally do produce fertile offspring. Ligers are a partial example, with only the females being fertile.

Wikipedia has a pretty good article on why species is difficult to define precisely.

7

BTW, there have been a number of threads on the subject of what is a species, and I believe there even was one on the specific question about chihuahuas and St Bernards. Do a little searching, and you’ll find some good stuff. Keep an eye out for posts by Colibri– he’s the resident expert.

8

John Mace is correct. You can’t say that species produce fertile offspring whithout saying it must be regularly and in the wild. Otherwise, the whole idea gets shot to hell. There are countless possible hybrids and many of these are fertile as well but they still aren’t considered the same species. Some people consider wolves and dogs to be the same species because they can interbreed just fine. That may song ok but coyotes can breed with dogs and wolves as well and almost anyone would consider them a seperate species.

The concept of a species isn’t quite as scientific as some try to make it. There is a lot of ad-hoc analysis and negotiation that goes on when it comes to grouping certain species.

9

I understand, but given the proclivities of certain humans (not to mention my friends’ bulldog) there are probably better arguments.

I’ve heard the question bandied about as to what would happen if a large-breed male canine got a small-breed female canine pregnant- would the size of the babies cause childbirth problems? It seems to me that the babies would simply be malnourished and therefore undersized.

10

[QUOTE=stolichnaya]

I’ve heard the question bandied about as to what would happen if a large-breed male canine got a small-breed female canine pregnant- would the size of the babies cause childbirth problems? It seems to me that the babies would simply be malnourished and therefore undersized.
[/QUOTE]

Generally, the babies come out larger than they would if it was a small dogXsmall dog cross, but smaller than big dogXbig dog cross.

Your idea that the mother’s uterus is the problem is in the right vein, but it’s usually space and not nutrition at jeopardy. By and large, dogs are nutritioned well enough that the babies aren’t malnurished. However, once the uterus is small enough to be squishing the babies, the increased fetal stress triggers a hormonal cascade that leads to labor.

At any rate, there is significant risk of the baby being too big for the birth canal, leading to dystocia and the need for a cesarian section delivery. That is why it is usually recommended, if crossing vastly disparate sized animals, that the female be the larger of the two.

/hyjack

11

[QUOTE=Aangelica]
Actually, a biologist will inform you that the default requirement for speciation (in general) is production of fertile offspring.
[/QUOTE]

Actually, the requirement for speciation (which is defined as the formation of new species) is the production of infertile offspring, as a first step, and then no offspring at all between the two populations.

I know what you meant to say, but that isn’t what you did say. :slight_smile:

12

A St. Bernard can’t successfully mate with a Chihuahau, but a St. Bernard can mate with a German Shepherd. A German Shepherd can in turn mate with a Beagle, and a Beagle can mate with a Chihuahua. So if the notion of “species” is transitive, one must conclude that St. Bernards and Chihuahuas are the same species.

13

[QUOTE=Chronos]
A St. Bernard can’t successfully mate with a Chihuahau, but a St. Bernard can mate with a German Shepherd. A German Shepherd can in turn mate with a Beagle, and a Beagle can mate with a Chihuahua. So if the notion of “species” is transitive, one must conclude that St. Bernards and Chihuahuas are the same species.
[/QUOTE]

Are you speaking of the physical considerations of the dogs involved? If the gametes of a Saint Bernard and a Chihuahua could be combined in vitro, would they be viable?

I’m glad I saw this thread; the differences between some breeds of dog, in their physiognomy, might be greater than those between humans and chimpanzees, and yet we are not considered to be the same species. Although, there are some who would go so far as to consider us cogeneric.

14

[QUOTE=Chronos]
A St. Bernard can’t successfully mate with a Chihuahau, but a St. Bernard can mate with a German Shepherd. A German Shepherd can in turn mate with a Beagle, and a Beagle can mate with a Chihuahua. So if the notion of “species” is transitive, one must conclude that St. Bernards and Chihuahuas are the same species.
[/QUOTE]
Not necessarily; the concept of ring species allows for related geographically adjacent species to interbreed even though species toward the outer ends of the spectrum can’t interbreed (either due to genetic incompatibility or because their mating behaviors have diverged to the point that they don’t seek to breed with one another). There’s also the concept of symatric speciation and heteropatry, in which geographically colocated species which developed from a common species diverge from a common ancestor but do not breed for one of the above stated reasons. The late and esteemed Ernst Mayr, however, claimed[sup]1[/sup] that symatric speciation does not occur in recent classes (specifically, mammals and birds), although the enforced artificial selection of domesticated animals is a different case.

It’s also not clear that a St. Bernard couldn’t make with a Chihuahau, though there are likely to be natal development problems with a Chihuahau bitch and a St. Bernard sire, and the converse has obvious logistical difficulties. I think it’s probably that such a match could be done using artificial insemination and a suitable host. Whether that qualifies as strictly being defined as “natural” breeding is somewhat irrelevent given the very unnatural nature of the exaggerated characteristics of many AKC breeds, which were selected for show appearance rather than reproductive viability. Indeed, there are some domestic species that have difficulty or are essentially incapable of breeding without technological intervention (which is actually ideal for the farmer who wants to control the breeding practices of his livestock).

In any case, we define the domestic dog–all breeds–as being of the same species, and indeed a subspecies of the Grey Wolf (Canis lupus), and it is alleged that domestic dogs can at least sometimes produce fertile offspring with other canine species including coyotes (Canis latrans) and jackals (Canis aureus). It’s likely that all domestic dogs are genetically convergent enough to produce viable (if nonoptimal) offspring.

I feel compelled to note that there is a substantial variation size, appearance, and other phenotypical characterist of Homo sapiens, despite the fact that we have a relatively narrow range of genetic variation, with about 0.1% difference in nucleotide pairs. (Chimpanzees, which have a much smaller population size and thus we’d expect them to have a much smaller range of variation, have about 0.3% variation in nucleotide pairs.) So just because dogs (or humans) exhibit a wide range of variation in appearance doesn’t mean that they are highly differentiated in ways that would indicate impending speciation.

Stranger

[sup]1[/sup]“But this theory [Darwin’s theory of sympatric speciation] was not confirmed in any of the carefully studied cases of speciation in mammals, birds, butterflies, and beetles. In my 1942 Systematics and the Origin of Species, I showed that in these groups geographic isolation had been the exclusive mechanism of speciation and not a single case of sympatric speciation had been demonstrated.” – What Evolution Is, Ernst Mayr, pg 180.

15

[QUOTE=Stranger On A Train]
In any case, we define the domestic dog–all breeds–as being of the same species, and indeed a subspecies of the Grey Wolf (Canis lupus), and it is alleged that domestic dogs can at least sometimes produce fertile offspring with other canine species including coyotes (Canis latrans) and jackals (Canis aureus). It’s likely that all domestic dogs are genetically convergent enough to produce viable (if nonoptimal) offspring.
[/QUOTE]

All species of the genus *Canis *can interbreed and produce fertile offspring, even if certain breeds of domestic dogs might have some logistical problems doing so.

16

[QUOTE=Stranger On A Train]
I feel compelled to note that there is a substantial variation size, appearance, and other phenotypical characterist of Homo sapiens, despite the fact that we have a relatively narrow range of genetic variation, with about 0.1% difference in nucleotide pairs. (Chimpanzees, which have a much smaller population size and thus we’d expect them to have a much smaller range of variation, have about 0.3% variation in nucleotide pairs.) So just because dogs (or humans) exhibit a wide range of variation in appearance doesn’t mean that they are highly differentiated in ways that would indicate impending speciation.
[/QUOTE]

Although it’s true now that humans outnumber chimps and bonobos by a staggering amount, that’s a very recent phenomenon and probably wasn’t true as recently as 50k years ago. Hence our rather close genetic relationship with each other. We appear to have gone thru a series of genetic bottlenecks, with one of the severest occurring about 70k years ago, with possibly as few as 5,000 breeding females alive at the time.

17

[QUOTE=John Mace]
Although it’s true now that humans outnumber chimps and bonobos by a staggering amount, that’s a very recent phenomenon and probably wasn’t true as recently as 50k years ago. Hence our rather close genetic relationship with each other. We appear to have gone thru a series of genetic bottlenecks, with one of the severest occurring about 70k years ago, with possibly as few as 5,000 breeding females alive at the time.
[/QUOTE]
Sure. But the point is that the outstanding variation in appearance of H. sapies is no indicator of its genetic diversity, which is quite slender. That dogs display such a wide range of appearance does not indicate that they cannot or should not be classified as one species, and is also the result of enforced artificial selection which can rapidly produce wide variations in phenotypes from a relatively small, recent population.

Stranger

18

[QUOTE=Stranger On A Train]
Sure. But the point is that the outstanding variation in appearance of H. sapies is no indicator of its genetic diversity, which is quite slender. That dogs display such a wide range of appearance does not indicate that they cannot or should not be classified as one species, and is also the result of enforced artificial selection which can rapidly produce wide variations in phenotypes from a relatively small, recent population.
[/QUOTE]

Absolutely. I know you didn’t mean it this way, but your post seemed to indicate that the genetic variability in chimps was surprising since their population is so low. Rather, it’s a function of how the species evolved. During the genetic bottleneck we went thru 70k years ago, there were probably other populations of humans around, but those lines all went extinct. We might even consider the Neanderthal line (and possibly others) to be one of those lineages if we look back just a bit further.

Additionally with chimps there are at least 3 recognized subspecies, and maybe a fourth.

19

I’m more than happy to be corrected on the matter - as I mentioned, it’s been a number of years since I looked into it (and that was a speeding-bullet pass through in my undergrad biology classes - for some reason, it wasn’t an area my professors spent much time on).
(And thanks John! One of the problems with posting from the office is I tend to get interrupted mid-thought, which doesn’t do anything to help my coherence.)

20

[QUOTE=stolichnaya]
what would happen if a large-breed male canine got a small-breed female canine pregnant- would the size of the babies cause childbirth problems?
[/QUOTE]

I’ve done many c-sections where fetal/maternal size disparity was the problem. A Giant Schnauzer x Beagle was a recent one. Offspring tend to be healthy. It is not a cost effective means of producing dogs, however.