Selective animal breeding already has almost 300 years of history. A lot has been achieved since. Obvious results have been achieved in the field of dog breeding. Selective breeding has produced very tall dogs like the Irish Wolfhound (>71 cm), very heavy dogs, like the Boerboel (50-80 kg), very small dogs like the Chihuahua (20 cm), very fast dogs like the Greyhound (17.5 m/sec), and many more breeds with different looks and purposes. From 1960 onwards selective breeding in farm animals became very successful. E.g. in pigs between 1960 and 2010 the number of piglets weaned per sow per year increased from 14 to more than 30. The amount of lean meat per ton of feed increased in that period from 85 to 170 kgs due to a higher lean percentage and a better feed conversion. In layers in that period the amount of eggs per hen housed increased from 230 to 370 and the eggs produced per ton of feed from 5000 to 9000.
From 1990 onwards, they emphasis in the breeding goal gradually changed towards the ability to cope with these levels of production in the range of common farming systems. This includes animal health, survival of young stock, longevity of breeding stock, product quality, strong & functional body characteristics, etc. (see also chapter 3.18).
Animals reproducing in a farming system implies natural selection for these conditions, even in the absence of artificial selection. If farming systems change rapidly because of legislation or societal pressure, then natural selection cannot keep up and animal health, survival and longevity may be compromised for a while.