In contrast to long term evolution, short term evolution is independent of a specific DNA sequence and previous history.  It is genetically determined the same way for at least an entire species, rapid (takes few % of the life span), reversible, and related to a single environmental parameter according to the presented law, which yields phase transitions simultaneous for all ages.  The phases provide a new genetic specification.  The law is valid for species as remote as humans and flies.  If the nature of phases is established, one may direct rapid evolution.