There was a great mini series on TV recently with Neil Oliver and Chris Packham (amongst others) looking at the ancient history of the Orkney Islands. On one of the small islands they vsisited there were still cattle that have been left to their own devices for 40 years, since the last inhabitants left. I wouldn't have thought that was a viable gene pool but they are still there however many generations later - needless to say the team left them well alone; they were definitely behaving like wild animals.