Phenotypic plasticity?

[GnnS]

Hi its my first post.I am not a Biologist(so forgive me if my knowledge is limited on some subjects).There is a phenomenon that is called phenotypic plasticity that helps organisms adapt on their environment.My question is how this benefitial adaptation occurs?Which mechanism triggers the change?

[Darby]

What genes turn on and off, and how strongly they're expressed, is part of a cascade that starts with external influences on the internal environment.