Illustration of the two stages of shrinking in size of Canis lupus during wolf domestication and small dog development. The first size reduction occurred during evolution by natural selection of free-ranging, naturally self-supporting wolves into those dependent on scavenging around human settlements for food. This reduction was part and parcel of wolf domestication. A second size reduction, by artificial selection, occurred post-domestication, in one dog lineage, and resulted in the ancestor of the small breeds found today worldwide. The approximate timing of the development of this 'small dog haplotype' (SDH)  is shown (using the example of the New Guinea Singing Dog (NGSD), one of many 'ancient breeds').
Driscoll and Macdonald Journal of Biology 2010 9:10 doi:10.1186/jbiol226