pterin 2-amino-4-hydroxypteridine.
See alsoMOLYBDOPTERIN.
punctuated equilibrium A part of evolutionary
theory that states that evolution works by alternating
periods of spurts of rapid change followed by long
periods of stasis.
pupa The stage preceding the adult in a holometa-
bolous insect, for example, the stage in a butterfly or
moth when it is encased in a chrysalis and undergoing
metamorphosis.280 pterin