> If a chicken was born with some form of defence, its likely to be killed before it could spread its DNA. Exactly. For some trait to evolve, individuals with

> If a chicken was born with some form of defence, its likely to be killed before it could spread its DNA. Exactly. For some trait to evolve, individuals with it must be more likely to reproduce than others without it. In a hunting scenario, this in fact leads to evolution of traits that prevent chicken from getting eaten (because chicken that aren't eaten can have more offspring in the wild than those that are). However, in farming, all reproduction is controlled. An inedible chicken wouldn't be allowed to reproduce. Hence any trait that is not directly favorable to humans is culled from the population before it can spread.