I’m not technical enough to understand whether “basal slippage rate” is simply another term for “tandem repeat sequences,” or a second factor that amplifies the effect of “tandem repeat sequences,” and makes them even more susceptible to rapid genetic change, but the short version is, there is indeed something special about dogs/wolves, that predates domestication, that predisposes them to genetic variation.
Biologist Raymond Coppinger convincingly posited the hypothesis that wolves “domesticated” themselves long, long before humans did any conscious selection. Wolves became dogs originally by choosing to hang around human settlements and eat food waste and feces rather than doing the wolf thing of hunting in packs. All dogs have much less cranial capacity than wolves (brains are calorically expensive and garbage doesn’t need a lot of brains to hunt down). If you check out what the commensal dogs, aka pariah dogs look like worldwide, you’ll see there is physically much less of a jump to modern selected dog breeds than from wolves.
Another thing: breeds that are selected for a physical appearance that leads to winning in the show ring can get bizarrely exaggerated very quickly, since that venue rewards exaggeration. The appearance of many breeds is very modern. If you compare dogs of a breed now and fifty years ago, you’ll see what I mean. Dwarfy dogs had real legs, smashed-in face dogs could breath normally, dogs that now have endless hair had normal coats. A generation is as short as two years, so concentrated effort in a single direction by a group of breeders can make a remarkable change.
Only if you define the puppy from two cocker spaniels to be a cocker spaniel. There is natural variation in litters, often enough to produced offspring that aren’t close representatives of the breed.
Right, it takes endless culling of puppies that don’t conform to the breed standard to maintain the breed. Usually it’s not thought of as “This puppy is not a cocker spaniel” but rather “this puppy is a bad cocker spaniel”.
Interesting to speculate what the outcome might have been if this evolutionary tactic had occurred not in wolves but some other wild mammal species. Would we now have a millennia-old tradition of pet pigs? Working pigs, companion pigs, lap-pigs, show pigs? A boy’s best friend is his pig. A piglet is for life, not just for Christmas. Do you know that there are some cultures that actually eat pigs? :eek: GROSS!!
Now I’m wondering how biologists reconstruct the domestication of our other most common pet species, the cat. Did wild cats “self-domesticate” with prehistoric human societies in something of the same way?
That’s certainly true, but the example was in response to a post about dwarfism in humans. Generally speaking, two cocker spaniels will produce dogs that most would recognize as cocker spaniels. The traits of a cocker spaniel are genetically consistent. It’s rare that a pup would be so divergent that it would look like a different dog entirely. You certainly couldn’t breed two cocker spaniels and get a German shepherd (or a dog that looks like one.) Human dwarves, on the other hand, are no more likely to produce dwarf offspring than non-dwarf humans.
That’s not true. Achondroplasia is a dominant gene. If you have no copies, you develop normally. If you have one copy, you develop dwarfism. If you have two copies, you die before or just after birth.
So if Achondroplasiacs mate, 1/4 of their offspring are normal, 1/2 will have dwarfism, and 1/4 will be stillborn.
Achondroplasia when neither parent is affected is caused by mutation, not by carrying the gene.
Anyway, the point is, the reason that when two cocker spaniels mate you get offspring that look a lot like cocker spaniels is that most of the traits that cause cockerspaniality are homozygous. If you breed two mutts together, even ones that look a lot alike, you can easily get pups that look very different from each other, because mutts will have heterozygous traits.
Wolves, like most wild animals, have a lot of genetic diversity. They’ve got some genes that (all else being equal) would produce large size, and some genes (again, all else equal) that would produce small size. Most wolves will have some of each of these, and so they’ll mostly cancel out, and you’ll get a normal-sized wolf. But some wolves, by chance, will happen to have more big genes than small genes, and end up being larger than the average wolf. Take the largest wild wolves, and breed them together, and you’ll still end up up with a mix of genes again, but one that averages a bit larger than the original population. Keep repeating this process, and you end up with a St. Bernard, almost all of whose size genes are the big ones, and almost none of which are the small ones. So St. Bernards are a lot less diverse than the wild wolves they descended from.
Run the process the other way, always selecting the smallest wolves, and you end up with a chihuahua, whose size genes are almost all the small ones. Again, they’re much less diverse. But you can’t breed a chihuahua out of St. Bernards, since the St. Bernards just don’t have the small genes to begin with to select (and likewise, you can’t breed a St. Bernard from chihuahuas).
Now, if you (somehow) bred chihuahuas with St. Bernards, you’d end up with dogs with a (very specific) mix of big genes and small genes. Let those dogs breed with each other, and you’d end up with a random variation of sizes similar to what you find among wolves (from which you could, again, breed to very large or very small dogs).
You might end up with big wolves and little wolves. So far, no one has ever been able to domesticate a wolf, although I don’t know how intensive the effort has been.
True, when I said “you end up with a St. Bernard”, what I meant was “you end up with an animal the size of a St. Bernard”. There are some traits common to all dogs that you might or might not end up with, and of course there are traits associated with those breeds that have nothing to do with size, like coat pattern.
Note that if you just let dogs mate and breed at random, you won’t get a wolf back. Instead, you’d get the recognizable yellowish, medium-sized mutt, the pariah dog mentioned upstairs.
Per the documentary, they tried to see if they could domesticate wolves. It didn’t work, as the wolf puppies, although tamer than wilder ones, definitely did not behave like dog puppies.
The majority of large breed dogs have a mutation in one of their genes that is not present in the small breeds of dogs (a variation in an IGF-1 gene).
It has been found (more or less), that dogs probably came from a smallish subspecies of wolf.
Although they differences look impressive, selective breeding by humans can have a relatively fast and profound change. One example is bulldogs (as per my “school mascot”, the ugly Uga). If you look at the early Uga, from the beginning-mid century, they look somewhat recognizable as a “bulldog”, but definitely different from the barrel-shaped ugliness found today. If someone has the opportunity to pass by the UGA vet college, and is interested, there is a nice display on selective breeding and skull shape changes, complete with skulls, in one of the main (public) lobbies. The transformation is awesome (and somewhat sad).
Another point I’ve seen made about dogs, is that they change shape & proportions a lot from puppyhood to adulthood (compared to cats, say). This gives selective breeding more to work with, by selecting for various juvenile traits to be retained until adulthood (or to exaggerate adult ones). Takes ears for example; your “generic dog” starts with floppy puppy ears that turn into upright pointy adult ears; with selective breeding they stay floppy. A cat on the other hand starts out as a kitten with upright pointy ears, and ends up as an adult with upright pointy adult ears; there’s no “floppy ear stage” for breeders to aim for. It’s much faster & easier to breed for modifications of existing traits than it is to hope for a mutation that adds a trait you want to breed for.
Some species do seem to be much easier to selectively breed than others.
Obviously some factors are the genotype and phenotype variation and proportions in the population. As well as obviously things like litter size and reproductive age making some species more convenient for humans to breed.
But we don’t quite understand all the contributing factors, and exactly why dogs are one of the easiest species to breed novel characteristics.
I seem to remember that domestic cats are descended from a small African wild cat which is naturally easily tamed. Other similar looking small wild cat species, not (the European wild cat is not, for example). I could be remembering this wrong. Miners in the western US used to tame Ringtail Cats, which raccoon relatives, not felids; apparently they are also naturally easy to tame. There are probably a fair number of tamable pet prospects out there in the wild, just some caught on and some didn’t.
The documentary** KarlGrenze **mentions above (if it’s the one I think it is) along with other references in this thread explain some of the incredible malleability of dog genes. It’s an animal breeders dream to have that much inherent diversity to work with. If you wanted to breed a new form of a cheetah you’d be stuck looking for a significant mutation in animals that are all as closely related as siblings.
As **Karl **also mentions, it is suspected that domesticated dogs descended from some sub-species of wolf. It does make sense that humans would start domestication with something a bit different from a gray wolf whose cute offspring are likely to eat your children. But we haven’t been able to narrow down when dogs were first domesticated yet. We may never uncover enough evidence to determine how it all got started.
Some anti-evolutionists have tried to justify their steady state idea by claiming that dogs if left to intermix without human intervention would return to the wolf form. But wild dog populations don’t do that, and though I don’t know the details, they’ve probably lost some of the genes that wolves have, or their genes have diverged sufficiently that there’s no path back to the wolf characteristics that would be advantageous to the dogs in their environment.
I’ve seen this referenced before on the boards, but as it’s germane to the discussion, here is a cite to the domesticated silver fox experiments out of Siberia. Here’s the story I first read on it from National Geographic and one I haven’t read it from Slate. Bottom line - the study has managed to create two cohorts of foxes - one thoroughly domesticated and one completely vicious toward humans. And they did so in less than a half-century. From the National Geographic story:
Some of the morphology of the domesticated dog may be explained by “a domestication phenotype”. Again, from the National Geographic article: