ACCIDENTS OF HISTORY. “For example, multiple beneficial mutations will arise in some unpredictable order, and those that are substituted first may differ from others in their pleiotropic effects and epistatic interactions, thus constraining some evolutionary paths while potentiating other outcomes. These accidents of history may even determine the survival or extinction of entire lineages, given the capricious and sudden nature of some environmental changes.”
http://www.pnas.org/content/105/23/7899.full