system
Given the following post, respond as if you're commenting on Reddit:
human
[ELI5]: Why do pigs go through a big transformation if they're introduced into the wild?

How does a domesticated pig turn into a wild boar when released to the wild?
gpt
A creatures phenotype (physical manifestation) is dictated by its genetic code. There are actually different ways in which genetic code will be expressed depending on the environment that the creature finds itself; this phenomenon is called epigenetics. There is another phenomenon called neoteny in which creatures retain juvenile characteristics depending on their environment, and will quickly mature given certain conditions. 

So just as an example to illustrate the point, say that a pig is in a farm and is fed and shielded from predators. The chemical profile of this pig might show low levels of testosterone because there had not been any circumstances that would have precipitated the production of excess testosterone. When the pig is let out into the wild, it is suddenly in danger of predators and starved of nutrients, so the relevant chemical cascades kick in which will be conducive to its survival, and these may actually change the way it physically appears (testosterone --> greater hair production, etc).