According to Wikipedia [1], cattle have a population of 1.3 billion with a biomass of 156 million tons. Chickens have a population of 24 billion with a biomass of 14.4 million tons.

For reference, humans have a population of 7 billion, with a biomass of 105 million tons.

[1] http://en.wikipedia.org/wiki/Biomass_%28ecology%29#Terrestri...

Natural selection would favor those organisms who live long enough to reproduce.

Living into old age isn't a factor.

There wouldn't be nearly as many cows, chickens or pigs on this planet if they weren't delicious. Especially pigs, humans would kill them on sight due to the tendencies of feral ones to destroy crops.

Of course. Animals that serve as food are much more plentiful than those similar to them that aren't. Of course, they may or may not be happy about it (compared to their wild relatives, who also live unimaginably cruel lives by human standards), but nature could care less about it.