Posts Tagged ‘sheep domestication’

I’ve written several posts about dog domestication and how the foundation of modern Western “breed” dogs has distorted our ability to figure out where dogs originated.  For several  years, it was strongly contended that dogs were derived from East Asian wolves and that there was only a single domestication event. Most of this studies that point to East Asia as the place of origin have used either mitochondrial DNA and–most recently– y-chromosome analyses. However, a study that examined the largest part of the genome of dogs and wolves found that most domestic dogs share genetic markers with Middle Eastern wolves.  And of course, the oldest of all dog remains are found in a region that runs from Central Asia to Belgium– the earliest, in the Altai Mountains of Siberia, dates to 33,000 years before present.

How can these different findings be reconciled?  I think the best way is simply that dogs were domesticated over a long process that involved several different populations of wolves.  Mark Derr in How the Dog Became the Dog lays out a very complex scenario in which humans and wolves associated with each other over tens of thousands of years.  For most of this time wolves living near humans had little morphological differences from wolves not associated with humans, but then, during the Last Glacial Maximum, the wolves associating with humans began to develop the morphological traits associated with domestication. These same forces created a general gracilization in humans, and they likely had the same effects on the wolves that were living with and relying upon humans for sustenance.  The shortened muzzle trait, which has been found in several specimens that are over 30,000 years old, is perhaps the earliest trait to have developed in the wolves that became dogs. Derr posits that the region running from the Carpathians to Central Asia would have been where the first anatomically distinct dogs would have been found– and that is precisely where the oldest dog remains to date were found. (The manuscript for How the Dog Became the Dog was submitted months before this discovery was published.)

For whatever reason, the dogs that descend from Middle Eastern wolves wound up contributing the most to modern dog lineages, though there is some influence of East Asian wolves in East Asian dog breeds, just as many Scandinavian dog breeds also descend from the a mitochondrial DNA matriline that originated in a European wolf bitch. When wolves were more common, they exchanged genes with dogs and the wolves that became dogs– and vice versa.

All of these issues are very complex and are often contradictory. And they are further exacerbated with a real methodological problem that exist in most genetic assays of domestic dogs:  they tend to include too many Western breed dogs. Western breed dogs are genetically depauperate compared to the village dogs of Asia and Africa, which have both proven to be quite genetically diverse. Because the African dogs are more genetically diverse than those of Asia and also possess Middle Eastern wolf genetic markers,  it is very unlikely that dogs were domesticated in East Asia in a single domestication event.

Now, what if we had a parallel domestication from which we could compare to the domestication process in dogs?

Well, it turns out we do.

Dogs are the oldest domestic animal. There is no debate about it.

However, the question of what the second domestic animal was has always been debated.  The debate is between two species:  the goat and the sheep. Evidence of domestication for these two species occurred from 9,000 to 11,000 years ago. I think the bulk of the evidence suggests that sheep were the earlier of the two species. The first evidence of domestic sheep appears  around 11,000 years ago in the Middle East, which means that sheep domestication predates the large-scale horticultural societies that would develop in the Fertile Crescent by about a 1,000 years.

We’ve known for a while that sheep are pretty genetically diverse even as domesticates, but the extent of that diversity was only recently revealed in a study that was recently released in PLoS Biology. This study examined over49,000 SNP’s in over 2,800 sheep from  74 breeds.

The study found that these sheep breeds had more diversity than most dog breeds. Many sheep breeds have effective populations in excess of 300 individuals.  The various breeds of sheep we have are the result of breeding for just a few traits, and the two most widespread traits, wool and hornlessness in both sexes, are the result of dominant alleles that are easily transmitted across populations. Sheep breeds were largely not created through inbreeding. They were mostly created through selectively breeding from a genetically diverse population. Sheep were likely domesticated over a large range over a relatively long period of time. There was no greater diversity in the Middle Eastern sheep breeds than those from other places, and what’s more, there was a lot of crossbreeding between breeds over the years.

The study is entirely about sheep, but I think it shows a real weakness in the genetic studies I’ve seen in domestic dogs. Most studies on domestic dogs do not fully account for the amount of loss in genetic diversity that exists in breed dogs. However, almost all dogs in the West are either breed dogs or derived from crosses between breeds. It has been calculated that modern breed formation may have taken away over a third of all the genetic diversity in domestic dogs, which are still a pretty genetically diverse population.

They become even more so when non-breed village dogs from Africa and Asia are included in the analysis.  Dogs from these populations actually suggest that dogs were domesticated in the way sheep were. However, because most dogs in the West derive from dogs that were intensely inbred and/or kept in closed registry populations, a lot of dog genetic diversity was squandered.

Sheep also show that breeds can be founded from genetically diverse populations. Inbreeding is not a requirement for breed formation or maintenance. In the oldest dog “breeds,” like the greater tazi landrace, there is still a lot of genetic diversity when the landrace as a whole is considered one breed. However, when one considers these dog to be separate entities, as they typically are in the West,  they don’t have much. These dogs existed for thousands of years with selective breeding from a genetically diverse population, just as domestic sheep breeds have been. But within the Western closed registry framework, they have lost a lot of that diversity.

Dogs were not created by inbreeding wolves. Sheep were not created by inbreeding wild mouflon.  Instead, through domestication, certain traits were selectively bred from animals presenting novel mutations. Inbreeding would fix these novel traits more quickly, but the genetic evidence shows that most of the oldest forms in dogs predate modern breed formation. That is, dogs like sighthounds, mastiffs, and mountain dogs that existed thousands of years ago didn’t appear by inbreeding but through selective breeding from diverse populations.

The intense inbreeding and linebreeding that exists within the closed registry system for most domestic dogs in the West has created a major distortion in our understanding of how dogs were created.  Dogs that exist within those populations are not very representative of dogs throughout their history.  But the dogs of Africa and Asia point to the possibility that was once there.

If sheep breeders can produce wool, milk, and meat animals that have produced so effectively over thousands of years without resorting to using genetically depauperate populations, why can’t dog breeders?

I think the answer lies in two places:  the Victorian blood purity cult that is the modern dog fancy and the traditions of how dogs are bred. Both of these things have almost metaphysical reasons for existing as they do, for both fly in the face of what modern science says about the importance of genetic diversity in maintaining sustainable, healthy populations.

The other problem that exists with dog genetic diversity is that the economics simply don’t work anymore.  When breeds were being founded, they weren’t as different from landraces. Wealthy individuals kept scores of dogs in their breeding programs, so they had large founding populations. Most people can’t do that today, which is one reason why virtually every breed that has been founded after the Second World War and then placed within a closed registry system has had severe health problems. Further, I don’t think most people want to do the intense culling that one must use to use very tight breeding effectively.  In those days, there was a lot of culling of the weak.

Today, we have situations where it is hard to maintain genetically diverse populations in dog breeds. Crossbreeding is not allowed in any of the established registries, and the blood purity cult won’t allow such heresy to go without fatwas. Dogs that are born through crossbreeding may only participate in competitions if they are spayed or neutered, and their genes never contribute to the next populations. There are some exceptions to the blood purity cult, like the Dalmatian backcross project and the use of Central Asian tazis in the UKC saluki. But in general, blood purity is seen as an ideal. Further, many dogs that are purchased from breeders are given spay-neuter contracts, and more and more municipalities are passing mandatory spay and neuter.

All of these factors are pushing dogs into more and more genetically depauperate populations that, over time, will become less healthy– regardless of what sort of culling and “selecting against disease” continues to happen within the system.

Sheep were lucky.  Because almost every sheep that exists today has some practical purpose, there was never a big push to turn them into fancy animals. By the time of the Industrial Revolution, the bulk of dogs in the West were no longer needed for anything, except as sporting animals and objects of conspicuous consumption. And those factors led to the big push to “improve” them through “scientific” breeding, which was often little more than inbreeding or very tight linebreeding.

Perhaps the best juxtaposition of two dogs that can explain what happens when dogs from genetically diverse populations become “improved breeds” can be found in AKC salukis and tazis from Central Asia. Tazis from Central Asia can live into their twenties, but the average lifespan of a Western breed saluki is something in the 11 to 12 year range.

The tazi was developed from a genetically diverse population, while the saluki is a Western cutting off of dogs of that general type into something called a breed.

If dog breeds had been able to continue on as selectively bred from diverse populations in the West, the results would have been different.

For the very simple reason that large predators tend to exist at lower densities than their prey, fewer wolves would have been involved in dog domestication than would wild sheep have been involved in their domestication– at least in the initial stages. However, over time, we could be talking about a large number of wolves working their way into the dog population, so it may have been a wash.

But sheep very clearly show us that we don’t have to inbreed to create and maintain breeds.

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