Meaning of hopeful monster

hope'ful mon'ster

Pronunciation: [key]
— Biol. Biol.
  1. a hypothetical individual organism that, by means of a fortuitous macromutation permitting an adaptive shift to a new mode of life, becomes the founder of a new type of organism and a vehicle of macroevolution.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.