New genetic research suggests that the alleles causing small size in certain dogs may have evolved early in the process of domestication and be derived from the wolf of West Asia (Canis lupus pallipes), of smaller size and tamer behavior than its more common relative Canis lupus lupus.
Melyssa M. Gray et al., The IGF1 small dog haplotype is derived from Middle Eastern grey wolves. BMC Biology, 2010.
See also the commentary by C.A. Driscoll and B. Macdonald: Top dogs: wolf domestication and wealth. Journal of Biology, 2010, arguing for a true origin of the dog in West Asian Mesolithic and later introgression of large size genes from the common wolf.