The cow genome has been published and the results show changes due to millions of years of natural selection but also to the thousands of years of selective breeding by humans.
Both types of cattle show evidence of natural selection in genes that appear to be involved in making the animals -- large, horned and potentially dangerous -- docile. In some breeds, specific variants of behavior-related genes are "fixed," or seen in essentially every animal. Curiously, some of those genes are in regions that in the human genome seem to be involved in autism, brain development and mental retardation.
So...by "docile", you really mean "mentally retarded". (via long now)