Over a considerable period of time could I breed a strain of (say) dogs up to the size of elephants by careful selection of mates for maximum size, or is there some genetic cut off point point where they will “top out” size wise?
Are you sure you know what question you want to ask?
All mammals, including elephants and whales, have evolved “over a considerable period of time” from a species about the size of a rat. Since at some point the ancestors of these animals had to pass through an intermediate of dog size then it should be clear that it’s possible to breed dogs up to the size of an elephant.
There is no genetic cut off point at which any species tops out in terms of size. That is because animals mutate and with a sufficent period of time and the right selections those mutations can be exploited for almost any trait of any extreme.
Instead there is a genetic cut off point at which the species stops being that species. In order to breed whale sized animals from shrews it has been necessary to incorporate a line period of line breeding and genetic isolation from the shrew population. As a result it is no longer genetically possible for whales and shrews to interbreed and even less physically possible.
Your dogs are gioing to run into the same problem. To breed an elephant sized animals from a dog you will need to isolate them genetically form the rest of the species. By the time you’ve reached the size you want the species will no longer be interfertile.
The other possible question is that you are suggetsing somehting akin to getting the “biggest” genes form all available living dogs and combining them into one uber-pooch. That will ensure they remain the same species because no new genes are being introduced. But such an animal has amaximum size limit of the largest mastiffs. Thats; the genetic potential for dogs as they exist today. Without new mutations being introduced you can’t get bigger then that.
So even if I bred largest mastiff male to largest mastiff female over say 100 - 200 years or so, at some point the pups would “top out” re size potential. That answered the question. Thanks!
Chances are that they are already “topped out” right now. It’s unlikley that even in 200 years of breeding you would produce more than a few extra ounces in weight from the largest mastiffs without introducing new mutations into the line.
Out of curiosity, have dogs already gotten to the point where the largest and smallest examples of the species are physically unable to breed? I can’t imagine a mastiff and a chiahua managing to breed with much success, even though genetically everything is still compatible.
'Nother dog/gene question:
My daughter asked the other day whether you could do a blood test to identify the breed of a dog, or the composition of a mutt.
I responded that I wasn’t sure. While I assumed there was a genetic basis for the various breeds, I didn’t know that those differences had been identified. (Apologies for my mangling of the appropriate terminology.)
Blake: Why would an elephant sized dog necessarily be a different species*? A chihuahua is about as small relative to a mastiff as a mastiff is to an elephant. And there are dogs even tinier than chihuahuas out there. Of course, we got the smaller dogs by breeding down in size, not up, but why is there an “up” limmit, and how do we know we’re already there?
*ignoring for the sake of discussion here the problem of assigning species designations to domestic animals.
Guess I should have done my own search before posing my question.
Appears the breeds are readily distinguishable upon DNA testing.
My bad.
The larger the dog, the shorter the lifespan:
From http://www.worsleyschool.net/science/files/stbernard/dog.html
The heart of a dog can’t scale up to that size without some significant changes in “design”. I’m looking for some more detailed cite for the heart being the culprit in short lifespans.
Yeah, that’s the one reason I could think of. Certainly the physical properties of blood and the capabilities of the circulatory system combined with the strength of gravity make scaling up more complicated than scaling down. But, still, how do we know what the specifics are? Do cows, for example, have some adaptation that dogs don’t have to overcome the problems imposed by gravity? Or bears (to pick something more closely related to dogs). How do we know when we’re at the limit?
Although insemination usually must be done artificially, a mastiff and chihuahua can produce viable offspring.
If the mastiff is the male, that must be a bitch of an experience for the mother…
Sorry.
Race horses have been carefully bred for speed for about 500 years now.
As I recall, there has been only a rather small increase in speed (+2%?) since records were started. And that increase seems to be dwindling off in recent decades.
So some people think that breeding faster race horses has basically ‘topped out’.
Of course, this is controversial – it’s disputed by a whole lot of people who make their living in the race horse industry.
I guess it wouldn’t be strictly necessary, but the changes we are talking about would require an extended period of line breeding to produce and the introduction of some truly novel mutations. The chances of managing to see those mutations arrive while at the same time time maintaining genetic compatibility is extremely remote. SO remote I think it;s safe to say it;s impossible.
The general rule of thumb for mammals is that it’s easier to scale down than up through neotenic selection, and that the lower limit is about 50% less than the size at which the juveniles become ambulatory.
That rule of thumb isn’t perfect, but it’s pretty close and it’s genetically sound. That’s because mammals start out small and grow big and thus have the genes and physiology already in place to code for a perfectly functional miniature. In theory it would be possible to develop a miniature wolf in 1one generation by simply eliminating all genes that code for growth after about 8 weeks of age. That would be the extreme of neotenic dwarfing and would result in a wolf of tiny size that is capable of surviving without its parents, and it could be accomplished without adding any new functional mutations, simply selecting for mutations. Miniature dogs aren’t produced just that way, but they do display a lot of neotenic traits which suggests that this failure to grow is largely responsible for their size.
Breeding animals up is much harder because animals don’t have any inbuilt template for gigantic forms. As a result you have absolute limits on size imposed by physiology. Aside from the problems of respiration and cardiac function a dog the size of an elephant would break every bone in its legs simply by trying to run. Even if we could somehow scale up the bone strength and density, the way dogs run is totally unlike the way elephants or even bears run and totally unsuited to large animals. Scaling up dogs to elephant size would require major behavioural changes aside from the purely physical.
How do we know we are at that limit? We don’t, but it is very likely we are because people have been trying to breed dogs for extreme sizes for millennia now. If there were genes or combinations of genes that allowed for extra size out there then we would almost certainly have seen them expressed. Most of the increase in dog size seen in the last 100 years or so can be attributed to nutrition and medical care rather than genetic size.
Yeah, that does make sense. I suppose if we could breed a cow-sized dog, someone would’ve figured out how to do that by now.
Let me toss out one exception to the above: Greyhounds. While they’re relatively large dogs (I’ve seen one adult about 40 pounds and several at 90+) they tend to live to the 12-14 range. I once knew one retired racer who I’m told made it to 16 but that’s the oldest I know.
I’m also unaware of much that they’re genetically prone to other than some lines being prone to osteosarcoma (we lost one this way).
In short, in the racing community, any congential defect means the dog doesn’t breed ever. Only healthy dogs get ANY chance of being bred and even then a fair number won’t.
In fact, domestic species, and particularly dogs, are largely neotenic modifications of their wild relatives. Canis lupus familiaris retains the many of the juvinile characteristics of immature wolves.
Note, however, that forced downscaling or removal of growth characteristics can have other implications, specifically hormone production and metabolism regulation. In fact, this is part of the larger problem; the fact that body size isn’t a single, uncoupled phenotype but is the result of a confluence of hundreds or thousands of disctinct but interrelated characteristics that are each controlled by scores of not-necessarily-unrelated alleles that composed the pertinent genotypes, plus environmental factors that affect development (i.e. nutrition, habitat size, climate, et cetera).
So you can’t just tune a specific genotype to control size; if you make the legs longer, for instance, you also need to increase blood flow capability, joint load capacity, et cetera. As can be seen from dog breeds that have been bred for gigantism, gross directed selection for a small set of characteristics results in functional incompatibilities, chronic health problems, and other physical and developmental problems that natural selection, by virtue of its holistic application of environmental pressures, would streamline or eliminate.
Extremes of physical characteristics–such as the extended neck of the giraffe–are hardly rare in nature but are developed as a result to extreme pressures and availability of an otherwise untapped resource or outcompete a predator/prey/competitor. As with the giraffe, these features will develop only to the point that it is physiologically possible to sustain by making only modifications to existing genotypes. Developing genuinely unique characteristics is rare and gradual, and most characteristsics–say, the strength of limbs or lift provided by wings–suffer from geometric/physiological scalability limits. A dog the size of an elephant, provided that you could birth the thing in vitro, would almost certainly collapse upon itself, even if it had a metabolism that could sustain such growth. (It would, of course, be almost certainly unsustainable in nature, as it would require a massive protein load; note that the only survival large mammalian predators are in the ocean, whereas large land mammal predators like the dire wolf and the short-faced bear died off with the extinction of even more massive herd prey animals.)
So yes, there are limits to how large you can or would want to breed an animal, at least without dramatically modifiying the anatomy and physiology of the animal in question. Scalablity of various phenotypes is not linear nor proportional, so overally scaling is a complex, multi-variant process.
Stranger
Slight hijack, would it be possible to make a tiny elephant, or polar bear?
There are pygmy elephants in existance now, although I don’t know if they qualify as “tiny”. Since we’ve bread tiny dogs, tiny horses and tiny goats, I don’t see any reason why not, though. Probably depends on your definition of “tiny”, though.
Not that it is reality or anything, but wasn’t the “Old Rich White Guy” at the beginning of the Jurassic Park novel showing off an elephant that they had downsized to the size of a rat? Your post brickbacon reminded me of that.
You can tell that it has been forever and a day since I read through the book.