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A history on the early forms of dog domestication

Received 2010 Mar 1; Accepted 2010 Mar 9.

  1. Darwin's [ 5 ] entire theory of evolution by means of natural selection provides a material explanation for diversity of form in nature.
  2. They roll to their feet, eyes now trained on the rabbit.
  3. Rather, he suggested that domestic dogs 'descended from several wild species'. He collides with her haunches, sending her back into the gate.

Abstract The increased battery of molecular markers, derived from comparative genomics, is aiding our understanding of the genetics of domestication. The recent BMC Biology article pertaining to the evolution of small size in dogs is an example of how such methods can be used to study the origin and diversification of the domestic dog. We are still challenged, however, to appreciate the genetic mechanisms responsible for the phenotypic diversity seen in 'our best friend'.

The Origins of Dogs

Background Size and shape are the hallmarks of the mammalian radiations and these two features are emblematic of the remarkable diversity of families, orders and genera of mammals that vary widely in form, yet share a common ancestry. The ordinal-level diversity of mammals is especially noteworthy, reflecting conformational changes in the skull, dentition and postcranial skeleton that result in forms as divergent as bats and whales.

From a paleontological standpoint, diversification of many orders occurred over a relatively short period of time [ 1 ], which makes changes in form even more curious.

One potential model for understanding the genetic basis of evolutionary change in mammalian form is the domestic dog, Canis familiaris. Domestication and strong directional selection for example, artificial selection for phenotypic and behavioural traits have resulted in morphological diversity within the domestic dog unparalleled in any wild mammalian species.

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In approximately 15,000 years the level of morphological divergence among dog breeds has exceeded that seen between many genera of wild canids [ 2 - 4 ]. Today, the 400 plus breeds of dogs vary in size, shape of the skull and modifications of the postcranial skeleton in particular the limb bones to a degree that would suggest species-level, if not generic-level, differences, if their remains were discovered in the wild.

  • But, Hare notes, the physical changes that appeared in dogs over time, including splotchy coats, curly tails, and floppy ears, follow a pattern of a process known as self-domestication;
  • The theory about multiple origins and timing was out there for some time, but this is the first evidence for it genetically;
  • I hear another sound;
  • Veeramah in a release accompanying the study.

Darwin's [ 5 ] entire theory of evolution by means of natural selection provides a material explanation for diversity of form in nature. He used examples from domesticated plants and animals as analogies for how adaptation can arise from selection acting on variations that cause differences in reproductive success across generations.

As Gregory [ 6 ] indicates, we can learn many lessons about evolution in natural systems through detailed studies of our domesticated species, and variation in the domestic dog raises a number of questions commonly asked about wild species of mammals: As with the studies of human origins over the past two decades [ 7 - 10 ], many of the above questions related to the domestic dog are being addressed in considerable detail with the use of phylogenetics, population genetics, molecular biology and comparative genomics [ 11 - 19 ].

This commentary was prompted by a recent paper in BMC Biology [ 20 ] that addressed differences in size among breeds of dogs as well as the timing and origin of small-sized dogs. The foundation for this paper originated with the work of Sutter et al.

In particular, IGF1 insulin-like growth factor 1 was suggested as a candidate gene for body size variation in domestic dogs and variation at 116 SNPs single nucleotide polymorphisms for 526 dogs clustered into two major groups, small and large breeds.

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As shown by their study, most small breeds of dog have two unique markers SINE element insertion in intron 2 of the IGF1 gene and a SNP allele not found in either wolves or large breeds. Based on the phylogenetic and geographic distribution of sequence variants associated with the 'small dog haplotype', these authors concluded that small dogs originated in the Middle East, as they share a relationship with wolves from this region.

Furthermore, they suggest that changes unique to small size occurred early in the evolution of domestic dogs. As a result of these findings and more recent molecular-based studies on the evolution of the domestic dog, I will provide an update on how close we are to resolving the above questions.

Ancestry of the domestic dog Darwin [ 5 ] stated that 'I do not believe, as we shall presently see, that all our dogs have descended from any one wild species'. Rather, he suggested that domestic dogs 'descended from several wild species'. The unresolved issue relates to whether or not all lineages of dogs originated from a single wolf stock or multiple stocks of wolves. Most studies of variation at the mitochondrial control region suggest that patterns of relationship among dog and wolf mitochondrial lineages is the result of multiple origins of dogs from different wolf stocks followed by introgressive hybridization between dogs and wolves [ 112223 ].

Unraveling the mysteries of dog evolution

A recent study of variation at the Mhc major histocompatibility locus also suggested that the high level of variation observed at this locus is best explained by continued backcrossing between dogs and wolves subsequent to domestication [ 16 ].

These results contrast with a recent study [ 19 ] based on mitochondrial DNA mtDNA that implies an origin for the domestic dog from a 'single gene pool', rather than multiple domestication events and continued hybridization with wolf stocks. There is one reason why I doubt the conclusions from Pang et al.

Their analysis compared patterns of variation in 1576 dog mtDNA to 40 wolf sequences. Such asymmetry in sampling of the wolf population is likely to bias any conclusions about origin.

Given the fact that wolves, dogs and other members of the genus Canis are inter-fertile [ 24 - 26 ], there is a high likelihood that dogs and wolves interbred subsequent to hybridization, thus complicating the derivation of the number of founders for dog lineages.