Man's best friend is much more than a household companion - for centuries, artificial selection in dogs has made them prime examples of the possibilities of evolution. A century and a half ago, Charles Darwin recognized how the incredibly diverse dogs supported his revolutionary theory in his famous book On The Origin Of Species. At the time, he believed that dogs varied so much that they must have been domesticated from multiple canine species. Even still, he speculated that:
if... it could be shown that the greyhound, bloodhound, terrier, spaniel and bull-dog, which we all know propagate their kind truly, were the offspring of any single species, then such facts would have great weight in making us doubt about the immutability of the many closely allied natural species
If only Darwin knew what we know now, that indeed, all dogs did descend from one species! While humans have been breeding dogs for over ten thousand years, it was until recently that strict standards and the emphasis on "purebreds" has led to over 400 different breeds that are some of the best examples of the power of selection. Those that doubt whether small variations in traits can lead to large levels of diversity clearly haven't compared a Pug to a Great Dane - I mean, just look at them compared to their ancestor:
We've turned a fine-tuned hunting animal, the wolf, into a wide variety of creatures, from the wolf-looking shepherds to the bizarre toy breeds. Before domestication, dog's life was tough. But when people pulled specific wolves out of their packs and began breeding them, we changed everything. There were some traits that made this easy - the social structure of wolves, for example, made them predisposed to belonging to a community. Still, we opened up a number of genetic traits and allowed them to express variety that would have been fatal in the wild. We not only allowed these traits to persist, we encouraged them. We picked dogs that were less aggressive or looked unique. And in doing so, we spurred on rapid diversification and evolution in an unbelievable way.
Take their skulls, for example. Like other members of the order Carnivora, dog's skulls have a few distinctive characteristics: relatively large brains and a larger-than-normal structure called a zygomatic arch which allows for bite power and chewing. But years of hand-picked puppies has led to an amazing amount of skull diversity in dogs. A study recently compared the positions of 50 recognizable points on the skulls of dogs and compared them to each other and other members of the order Carnivora. There was as much variety in the shape of the skulls of dogs as in the entire rest of the order, and the extremes were further apart. 
What does that mean, exactly? It means that the differences between the skulls of that Pug and Great Dane I mentioned before (on R) are greater than the differences between the skulls of a weasel and a walrus. Much of this variation is outside the range of the rest of the order, meaning dogs' skull shapes are entirely unique. In just a few centuries, our choices have created unbelievable variety in the heads of dogs - more than 60 million years has created in the rest of the carnivores.
The amazing diversity of dogs is a testimonial to the possibilities of selection. And it's not just their skulls that vary. A joint venture between the University of Washington and the Veterinary School at UC Davis mapped the variation in the genomes of a mere 10 different breeds of dogs. They found that at least 155 different regions of the dog's genome show evidence of strong artificial selection. 
Each region contained, on average, 11 genes, so it's harder to identify exactly what about each area was under the most selection, though there were clues. About 2/3 of these areas contain genes that were uniquely modified in only one or two breeds, suggesting they contain genes that are highly breed-restricted like the skin wrinkling in the Shar-Pei. Another 16 had variations in 5 or more breeds, suggesting they encode for traits that are altered in every breed, like coat and size.
While we usually think of evolution as a slow and gradual process, dogs reveal that incredible amounts of diversity can arise very quickly, especially when selective pressures are very, very strong. It's not hard to see how selection could lead to the differentiation of species - just look at the breeds of dogs that exist today. There's a reason that you don't see many Chihuahua/Saint Bernard mixes: while it's entirely possible for their genetics to mix, it's just physically difficult for these two breeds to actually do it. Just imagine what a poor Chihuahua female would have to endure to give birth to such a mix, or how hard it would be for male Chihuahua to mount a female Saint Bernard. Indeed, dogs are well on their way to speciation.
Of course, it's at this point that I have to mention that while I have talked about "dogs" this entire time, they're not actually a different species. Wolves are Canis lupus, while dogs are merely a subspecies of wolves, Canis lupus familiaris. Despite centuries of selective breeding and the vast array of physical differences, dogs are still able to breed with their ancestors.
When you take away the selective breeding done by humans, a number of these unique traits disappear. But feral dogs don't just become wolves again - their behaviors and even looks depend greatly on the ecological pressures that surround them. Our centuries of selective breeding have opened a wide variety of traits, both physical and behavioral, that may help a stray dog survive and breed.
A good example of what happens to dogs when people are taken out of the picture can be found in Russia's capital city. Feral dogs have been running around Moscow for at least 150 years. These aren't just lost pets that band together - these dogs been on their own for awhile, and indeed, any poor, abandoned domesticated canine will meet an unfortunate fate at the hands of these territorial streetwalkers. Moscow's dogs have lost traits like spotted coloration, wagging tails and friendliness that distinguish domesticated dogs from wolves - but they haven't become them. The struggle to survive is tough for a stray, and only an estimated 3% ever breed. This strong selective pressure has led them to evolve into four distinct behavioral types, according to biologist Andrei Poyarkov who has studied the dogs for the past 30 years. There are guard dogs, who follow around security personnel, treating them as the alpha leaders of their packs. Others, called scavengers, have evolved completely different behaviors, preferring to roam the city for garbage instead of interacting with people. The most wolf-like dogs are referred to as wild dogs, and they hunt whatever they can find including cats and mice.
But the last group of Moscow's dogs is by far the most amazing. They are the beggars, for obvious reasons. In these packs, the alpha isn't the best hunter or strongest, it's the smartest. The most impressive beggars, however, get their own title: 'metro dogs'. They rely on scraps of food from the daily commuters who travel the public transportation system. To do so, the dogs have learned to navigate the subway. They know stops by name, and integrate a number of specific stations into their territories.
This dramatic shift from the survival of the fittest to the survival of the smartest has changed how Moscow's dogs interact with humans and with each other. Beggars are rarely hit by cars, as they have learned to cross the streets when people do. They've even been seen waiting for a green light when no pedestrians are crossing, suggesting that they have actually learned to recognize the green walking man image of the crosswalk signal. Also, there are fewer "pack wars" that once were commonplace between Moscow's stray canines, some of which used to last for months. However, they remain vigilant against the wild dogs and wolves that live on the outskirts of the city - rarely, if ever, are they permitted into Moscow. When politicians thought to remove the dogs, their use as a buffer against these animals was cited as a strong reason not to disturb them.
Moscow's exemplary dogs show how different traits help dogs adapt to different ecological niches - whether it be brute strength for hunting in the truly feral wild dogs or intelligence in the almost-domesticated beggars. Some wonder if the strong selection for intellect will make Moscow's metro dogs into another species all together, if left to their own devices.
Dogs make it easy to understand and demonstrate the core principles of evolution - variation and selection - and how they can make such a dramatic impact on an animal. It's no wonder that Darwin took cues from domesticated animals when formulating his theory of evolution. However, there's still a lot to learn about the processes that have shaped our best friends, and what future lies for them. How much time will it take to completely separate dogs from wolves, into their own species? What areas of the genome are key to doing so? In studying dogs and wolves, we may gain insight into how speciation occurs and when a threshold of change is met for it to do so. Seeing how much change has occurred already makes you wonder what surprises our canine companions still have in store for us as they, and we, continue to evolve together over the next ten thousand years.
Like this post? Check out the next installment: Evolution: Watching Speciation Occur
Citations:
Drake, A., & Klingenberg, C. (2010). Large Scale Diversification of Skull Shape in Domestic Dogs: Disparity and Modularity The American Naturalist DOI: 10.1086/650372
Akey, J., Ruhe, A., Akey, D., Wong, A., Connelly, C., Madeoy, J., Nicholas, T., & Neff, M. (2010). Tracking footprints of artificial selection in the dog genome Proceedings of the National Academy of Sciences, 107 (3), 1160-1165 DOI: 10.1073/pnas.0909918107
Poyarkov, A.D., Vereshchagin, A.O., Goryachev, G.S., et al., Census and Population Parameters of Stray Dogs in Moscow, Zhivotnye v gorode: Mat-ly nauchno-prakt. konf. (Proc. Scientific and Practical Conf. Animals in the City ), Moscow, 2000, pp. 84 87.
Vereshchagin, A.O., Poyarkov, A.D., Rusov, P.V., et al., Census of Free-Ranging and Stray Animals (Dogs) in the Coty of Moscow in 2006, Problemy issledovanii domashnei sobaki: Mat-ly soveshch (Proc. Conf. on Problems in Studies on the Domestic Dog), Moscow, 2006, pp. 95 114.
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This is one of your best posts to date, I think. Very interesting about the Moscow dogs.
These Moscow dogs are fascinating!!
There's a reason that you don't see many Chihuahua/Saint Bernard mixes: while it's entirely possible for their genetics to mix, it's just physically difficult for these two breeds to actually do it. Just imagine what a poor Chihuahua female would have to endure to give birth to such a mix, or how hard it would be for male Chihuahua to mount a female Saint Bernard. Indeed, dogs are well on their way to speciation.Is it really known for certain that all dog breeds have compatible genomes?Of course, itâs at this point that I have to mention that while I have talked about âdogsâ this entire time, theyâre not actually a different species. Wolves are Canis lupus, while dogs are merely a subspecies of wolves, Canis lupus familiaris. Despite centuries of selective breeding and the vast array of physical differences, dogs are still able to breed with their ancestors.Given the morphological diversity among dogs, perhaps a different definition of species would be appropriate in this case. Lots of other 'species' can interbreed and have fertile offspring.Oh, and yes, those Moscow dogs rock.
That is a good point, Bjørn. I guess it really depends on how we define a species. I don't think it's been tested, but I'm confident that if you took sperm from a Chihuahua and put it in a female Saint Bernard that the puppies would be just fine. But does that mean they're the same species? As we define them now, species aren't solely based on breeding, though if it can't occur, it is a clear indicator of distinct species. Maybe we should look more for natural tendencies - as in, if you put a horny male of X in a room with Y in heat, do they mate? Do they recognize each other as the same species? But even that has it's issues and problems as a working definition. Further aside, what's the point of defining species in a domesticated construct? In the wild we might talk about populations, conservation, etc, but when we're talking about dogs that are reared and bred by people, what is the meaning or importance of defining species?
I know that if you inseminate a Shetland pony mare with a Clydesdale stallion's sperm, she drops a foal that's Shetland sized when it's born. But it grows much larger than any pure Shetland pony, if not as large as a Shetland. (This has been done.) Probably the same thing would happen with your Chihuahua/St Bernard mix. Anecdotally, my sister has a dog that looks like a giant Aussie shepherd - its father was actually a Great Pyrenees, but nothing of his came over into this F1 litter than his size. Of course, Pyrenees and Aussies are much closer in size to each other than Chihuahuas and St Bernards!
This is a remarkable piece of writing ... so incredibly awesome.
I've been told that when a tiny female dog mates with a large male, the delivery isn't inordinately problematic, not nearly as you might think. "Nature just takes care of itself." Call the dog expert warehouse and ask them to send one over!
Excellent piece on dogs [I'm a fan of, and we breed, Portugese Water Dogs] Recently an article in SCIENCE examined the genetics of certain coat variations in dogs, concluding that quite a few variations were due to differences in only 3 genes. But the study omitted the most significant difference, between non-shedding [hair] coats and shedding [fur] coats. Neither authors nor Journal responded to the question of cause of this difference. Do you know, or know of, the genetics of this difference? non-shedders include our PWDs, Poodles, some others.Richard Frankel [PhD, Physical Biochem.]
Came back from your blog about the Research Blog Awards to read this, and I agree its a worthy candidate for Research Blog of the Year. I just wondered about the last sentence of your "Take their skulls" paragraph. You contrast "a couple of centuries" of our choices with 60 million years [evolution] for the rest of the carnivores. If human beings have been around for a million or two of those sixty million years, what evidence is there for when men began domesticating dogs? Surely it would be more than a couple of centuries ago, though if you count from the origins of the Westminster Dog Show you might be closer.
"They've even been seen waiting for a green light when no pedestrians are crossing, suggesting that they have actually learned to recognize the green walking man image of the crosswalk signal." It seems to me waiting for a green light would indicate they have not learned to distinguish the walking man figure.
You do realize that dogs are colorblind, don't you?
"How much time will it take to completely separate dogs from wolves, into their own species?" Well that is a leap!!!
Manipulation of the variety within a species is observable throughout the plant and animal kingdom, however the transformation of one species into a completely new species has never been observed in any way and in dogs the boimolecular changes that would have to occur have absolutely no relation to the diversity you see in "manmade" descendants of wolves.
For the sake of science and truth you should be ashamed of suggesting to your readers otherwise, at least until you can site a proven mechanism (in science that means an observable, reproducible example) for "evolution" of one species into another, let alone a sequence of such evolutions producing a line of different species, each the progenitor of the next .......pure fantasy.
Caleb - colorblindness is irrelevant. The positions of green and red lights are fixed, so, like colorblind humans, these dogs are likely using the POSITION as the cue - when the top light is lit, go, when the bottom light is lit, stop.
tsad,
It seemed to me that she was asking a question, not declaring something. Asking questions and studying to try to find the answers does seem more in line with the scientific process than going to someone's website who holds an opposing theory to one you hold and trying to break it down.
Thank you for reminding us that the theory of evolution is a series of concepts building upon each other and that some of the corners need to be shored up as they leave the structure unstable. Assuming that many of us do not hold "the word of god" as written, edited and re-edited by man to be a firm foundation upon which to build concepts, can you present a more solid model that we might want to review?
A friend of mine had a pug that got impregnated by a big german shepherd. We were all afraid she would explode, but she did fine and had an appropriately sized puppy. The birth was even easier because the puppy had a slightly pointier head than a purebred pug would. So many breeds these days routinely have cesareans because they've been bred into a small body/giant round head phenotype. The puppy grew up looking like a pug on stilts.
My own dog is half Bassett and half Great Dane. She is also one funny looking dog.
Great post. What an impressive clan of dogs! The only quibble I'd have is with the phrase 'This dramatic shift from the survival of the fittest to the survival of the smartest ...'. I don't really like the word fitness; I use fittedness, as in 'those best fitted to their surroundings'. Of the early immigrants, those descendants who were better at reading and responding to the signals in the human-designed environment survived better and better than their wilder cousins could. 'Fitness' isn't just about muscles and big sharp fangs. For the metro dogs 'smartness' is the core of their 'fitness'.
It didn't take the first evolution denier--sorry; I mean skeptic--long to land. [Whispers:] If we ignore it, it might go away.
Re the definitions of species: A biological species is defined as all those organisms that can interbreed with each other and produce fertile offspring. This can be precluded by various reproductive barriers, both pre- (e.g. spatial, temporal, mechanical or behavioural isolation) and postzygotic (e,g, hybrid inviability, infertility or breakdown). Mechanical isolation almost applies to the oft-cited Dane/Chihuahua pairing! The 'horny' question relates to behavioural isolation, just one way 'compatible' organisms can be prevented from mating.
"I don't really like the word fitness; I use fittedness, as in 'those best fitted to their surroundings'" - ChicagoMolly
They're the same thing. Well, fitness is technically an organism's ability to produce offspring that themselves reproduce, but it depends on the suitability of each to their surroundings, whether in terms of morphology, physiology, behaviour, brain power, etc. I must point out that such a key term in biology will not go away simply because you "don't really like" itâ¦
Very interesting post. Easy to read and to understand.
Street dogs are smart everywhere. In my city in Latinamerica, I have seen street dogs using the pedestrian bridge while the moronic humans dodge cars in the street bellow. An when there isn't a bridge, you see some dogs waiting in the corners for the light, or maybe, for the smart humans to cross and then they follow.
Very cool about the Moscow dogs.
Here's a link for the first in a series of articles on the selective crossbreeding of boxers and Pembroke Welsh corgis to introduce a natural bob tail gene into the boxer. It turned out to be a fast process to get the cross back to dogs that met the boxer standard.
Sorry, I missed that the link paste failed. It took a bit before I was allowed to post another entry too.
http://www.steynmere.com/ARTICLES1.html
We had a neighbor years ago who had a dog who was half Chihuahua, half German Shepherd. (We never did find out which parent was which.) Princess was slightly larger than a standard-issue Chihuahua, with larger and more upwards-pointing ears, Shepherd-marked fur, and a Shepherd-fluffy tail. Nice dog, without the nervous mien of most Chihuahuas.
Fascinating piece! I would love to see a similar article on the pressures of artificial breeding selection on cats.
See Emma Townshend's "Darwin's Dogs." (By the way, she's the daughter of Pete Townshend of The Who.)
cool, cool.I love dogs.
It'd be totally fascinating to see what urban animals (dogs, cats, birds, mice etc.) would evolve into if the current selection pressures remained the same for a considerable time, say, 10000 years or so.
@Caleb J -- dogs aren't entirely colorblind -- that's a myth. They don't have the full range of color vision that primates do, but they do apparently have dichromatic vision, blue-yellow. They are only red-green colorblind. Nevertheless, the fixed position of lights is something that they can understand.
@Bjørn Ãstman --
or how hard it would be for male Chihuahua to mount a female Saint Bernard
Er, not that hard. The female will simply lie down. OR the other way around -- the male's generally pretty good at crouching. One of my friends as a teen had a shih tzu which she refused to spay -- fathers of the inevitable puppies included at least one German Shepherd and a black lab. The pups all came out small and then ended up looking more like...well, a version of their dad which had been put through a boil wash and then had their legs chopped off at the knee.
Oh, I have also encountered a Clydesdale mare which had been impregnated by a shetland pony. Figure *she* must have lain down. (Offspring was "normal horse" size, about halfway between its parents.)
Critters get damned ingenious when they're horny. It's not just humans that do that, you know.
This reminds me of the Border Collie x Newfoundland experiment that Jasper Rine and Elaine Osterman at UC Berkeley did in the early '90s to study inherited behaviors in dogs. The study didn't reveal the sort of genetic data that they wanted, but they learned quite a bit about inherited behaviors. Don't think it ever resulted in a paper. The F2s looked like Bernese Mountain Dogs (black and white without the brown parts).
The most fascinating thing about dog genetics is that so LITTLE of the genome is altered between a Great Dane and a pug. And most of the changes aren't in coding regions, but in all the noncoding control regions surrounding the ORFs, which are too often dismissed in popular writing as "junk DNA".
Great piece on dogs. The Moscow dogs is a really interesting tale. As for the question about speciation, I'd say it is difficult to address without a very stringent definition of species. For instance, assuming that a chihuahua (at one end of the spectrum) cannot breed with a St. Bernard (at another end) due to post-zygotic incompatibility, if they are able to each breed with other common dog breeds thereby connecting them, are they the same or different species at that point?
"the transformation of one species into a completely new species has never been observed in any way "
what about the transformation of Homo sapiens into Helacyton gartleri?
http://en.wikipedia.org/wiki/HeLa
"the transformation of one species into a completely new species has never been observed in any way"
This statement is just plain wrong. The number of new plant species arising from polyploidy and hybridization, whose first appearance has been directly observed (and directly proven by recreating the new species from the ancestral one(s) in the lab!) numbers well into the hundreds.
Christie, i pointed out your misuse of "infamous" to help out. i liked your article, but to fix your mistake and delete my comment without any sort of acknowledgement seems awfully unethical. I think the right approach would have been an editor's note at the end, or maybe even just not deleting my original comment. The fact that you're a grad student and a finalist in a 3 Quarks Daily contest is only more of a reason to follow the rules...
Qatch out commenters, Ms. Wilcox will delete your comments if she deems them unflattering...
Adam -
I didn't mean to offend you. I saw your comment, and I decided that while my intention was to say "infamous" in the sense that Darwin aroused such feelings in many due to his manuscript, I understood that you took it as me somehow negatively reflecting on his work and in turn decided to rephrase my intro so that others would not confuse my meaning and think that I somehow disapproved of Darwin's insights. I did not see it as unflattering so much as confusing. Once I clarified my viewpoint, I didn't think your comment contributed to the discussion, so I deleted it. I did not see it as 'unflattering' so much as unnecessary once I'd changed that phrase. If it will make you feel better, I won't delete comments like yours in the future.
Christe,
Genetics is one of my favorite subjects. In college, I needed a few extra credits and I took a graduate course on âPopulation Geneticsâ. The prerequisites were courses in calculus and statistics. One interesting fact that came out is how random crossing of domesticated plants (wheat for example) will revert to the wild form. Wheat will regress back to look exactly like the original wild form after only a few generations of random crossing. It is interesting that feral dogs seem to revert to the typical âmuttâ that looks quite like the metro dog in the picture. That is, medium size, short hair, tan color with a black mussel. One can observe this all over the world. This tells me that dogs are really a separate species that evolved from the Wolf like the coyote. The ancient wild dog (probably extinct except as feral dogs) adapted itself to humans much the same way as Metro dog. The dogs traded protection (alerting against danger) and companionship for food and shelter. Darwin may have been more right about dogs than some people give him credit. There are four distinct dog groupings. The original ancestors may actually have been four subspecies of the original wild dog. I enjoyed reading your article.
Thanks.
Art
I am dying laughing at the commenters' descriptions of breed mixes.
Thanks Christie!