Hidden reaction norms, cryptic genetic variation, and evolvability.

There is reason to suspect that hidden reaction norms (variation that is phenotypically expressed only after changes in internal or external environmental conditions) may store an evolutionarily significant pool of cryptic genetic variation upon which selection may act. I review mechanisms that may hide variation and the processes that can release this variation. I discuss the potential significance of cryptic genetic variation and elaborate on an example where the release of such variation initiated the evolution of the genetic architecture of multicellular organisms.