The Dog’s Origin and Current Relation to Wolves
By Jody Haynes
Scientists have been arguing over the origin of the domestic dog for centuries. Although most now agree that dogs are descended from the gray wolf, Canis lupus, Dr. Robert K. Wayne, canid biologist and molecular geneticist at UCLA, has shown, through his DNA research, that dogs are much more closely related to the gray wolf than anyone had previously suspected. In fact, due in large part to Wayne’s genetic research, the authors of the Mammal Species of the World—which is the primary, internationally accepted reference on mammal species—reclassified the dog in 1993 from its separate species designation of Canis familiaris to Canis lupus.
This article is a review of three papers—published in 1993, 1997, and 1999—representing the work of Wayne and his colleagues as they have strived to determine the origin of the domestic dog, both with respect to the species that is/are the dog’s ancestor(s) as well the point in time that the divergence event(s) took place.
Wayne’s 1993 paper provides a prelude to the story by citing genetic evidence of four major divisions in the Family Canidae, the first of which contains all of the wolf-like canids—gray wolves, coyotes, jackals, domestic dogs, etc.
Wayne (1993) elucidated the genetic affinities of three of the members of this canid division, as follows: “The domestic dog is an extremely close relative of the gray wolf, differing from it by at most 0.2% of mtDNA sequence.... In comparison, the gray wolf differs from its closest wild relative, the coyote, by about 4% of mitochondrial DNA sequence.” To summarize, these data suggest the following: (1) gray wolves and coyotes are closely related; and (2) gray wolves are 20 times more closely related to dogs than they are to coyotes.
Wayne stated later in this paper that “[t]he earliest remains of the domestic dog date from 10 to 15 thousand years ago...; the diversity of these remains suggests multiple domestication events at different times and places. Dogs may be derived from several different ancestral gray wolf populations, and many dog breeds and wild wolf populations must be analysed in order to tease apart the genetic sources of the domestic dog gene pool” (Wayne, 1993).
In their follow-up research, Wayne and his colleagues set out to test the hypothesis that domestic dogs arose from the gray wolf at numerous times and places, as opposed to two competing hypotheses, which suggest (1) a single origin of the domestic dog from a gray wolf ancestor, or (2) a single main divergence event, followed by numerous subsequent intermixing events. To do this, Wayne again chose to analyze rapidly evolving mtDNA1 sequences.
Wayne et al.'s 1997 paper summarized these ‘follow-up’ mtDNA analyses. In it, the authors stated that several different methods of phylogenetic2 analysis supported the grouping of various dog mtDNA haplotypes3 into four distinct clades4. Figure 2 in this 1997 paper shows a ‘gene tree’ of possible phylogenetic relationships among the wolves and dogs studied. It suggests that the mtDNA of the dogs falling into clades 2 and 4 are more closely related to the mtDNA of wolves than they are to the mtDNA of the dogs in clades 1 and 3.
If these analyses represented the entire genomes5 of the organisms studied, this figure would suggest that dogs are polyphyletic, which means that the term “dog” would no longer refer to a genetically coherent group of organisms, but rather a group of similar-looking organisms that did not have a shared genetic ancestry. However, a single gene tree may not be representative of the actual organisms involved (i.e., dog breeds, wolf subspecies, etc.), and thus cannot be used to infer phylogenetic relationships with any certainty.
Knowing this, Wayne et al. (1997) then cited a separate, more robust analysis of mtDNA control region sequences (which represents a different mtDNA gene than previously analyzed) that strongly rejected the monophyly of the dog haplotypes studies. This analysis suggests that all dog mtDNA haplotypes did not arise from a single divergence event (i.e., are not monophyletic), thereby rejecting the “single origin” hypothesis.
Wayne et al. (1999) nicely summarized these earlier two papers by stating that the clustering of dog haplotypes into four distinct clades “suggests that either wolves were domesticated in several places and at different times or that there was one domestication event followed by several episodes of admixture between dogs and wolves.”
Wayne et al. (1999) continued by stating “[w]hichever the case, the results imply that dogs have a diverse origin involving more than one wolf population.” The authors then concluded by stating, “the domestic dog is a genetically diverse species that likely originated from wolf populations existing in different places and at different times.”
To summarize all three papers, then, we can say the following:
v All wolf-like canids are genetically related and are grouped into a single division of the Family Canidae
v The gray wolf’s closest wild relative is the coyote, with which it shares 96% of mtDNA sequences
v Domestic dogs are 20 times more closely related to gray wolves than wolves are to coyotes
v “Dogs are gray wolves, despite their diversity in size and proportion” (Wayne, 1993)
v The “single origin” hypothesis—which suggests that dogs arose from gray wolves a single time—has been rejected
v Wayne's original “different times and places” hypothesis has not been rejected
v More work needs to be done to differentiate this “different times and places” hypothesis from the “single main divergence, secondary intermixing” hypothesis, which has also not been rejected
Vila, C., J. Maldonado, and R. K. Wayne. 1999. Phylogenetic relationships, evolution, and genetic diversity of the domestic dog. The Journal of Heredity 90(1):71-77.
Vila, C., P. Savolainen, J. Maldonado, I. Amorim, J. E. Rice, R. L. Honeycutt, K. A. Crandall, J. Lundberg, and R. K. Wayne. 1997. Multiple and ancient origins of the domestic dog. Science 276:1687-1689.
Wayne, R. K. 1993. Molecular evolution of the dog family. Theoretical & Applied Genetics 9(6).
1 mtDNA – refers to DNA in mitochondria, which are organelles inside the cells of all plants and animals that contain DNA that is separate and distinct from the DNA in the nucleus of the cells, which is referred to as nDNA
2 Phylogenetic – genetic inter-relatedness between two or more species; such relationships are often displayed graphically in one or more hypothetical ‘trees’
3 Haplotype – a set of analyzed genetic markers from a given source of haploid DNA; haploid refers to a single copy of DNA making up the genetic code, as in mitochondria
4 Clade – a purported monophyletic group; monophyletic refers to a group of organisms in which all members share a single common ancestor at some unidentified point in the past
5 Genome – refers to the entire set of genetic material (DNA) that an organism possesses, which includes both nDNA and mtDNA in animals