Keats' Telescope posts a link to a study on the origin of domestic animals. The low level of mtDNA diversity in many domestic animals indicates that these are descended from only a few domestication events. However, modern domestic animals seem to also possess some evidence of genes of "wild" origin, indicating that after they were domesticated they did not become isolated genetically, but continued to be crossed with wild animals, either intentionally (by humans), or by accident.
Trends in Genetics
Volume 21, Issue 4 , April 2005, Pages 214-218
Genes of domestic mammals augmented by backcrossing with wild ancestors
Carles Vilà et al.
Both archaeological data and the presence of few mitochondrial DNA lineages suggest that most widespread domestic mammals (cattle, sheep, goats, pigs and dogs) derive from only a handful of domestication events. However, each of these species shows a high level of diversity at the nuclear genes of the major histocompatibility complex (MHC). Through simulations incorporating various degrees of population subdivision, growth rate and selection, we demonstrate that the numerous MHC DRB alleles that are present in modern domestic mammals implies that substantial backcrossing with wild ancestors, either accidental or intentional, has been important in shaping the genetic diversity of our domesticates. These results support the view that, contrary to common assumption, domestic and wild lineages might not have been clearly separated throughout their history.