Evolutionary Escalation

• WWizards of the Coast2016-11-08
  Official Ruling
  You choose two targets for Evolutionary Escalation’s ability as it’s put onto the stack. No player may cast spells or activate abilities during your turn before this happens.
• WWizards of the Coast2016-11-08
  Official Ruling
  If one target becomes illegal before Evolutionary Escalation’s ability resolves, you’ll put three +1/+1 counters on the target that’s still legal.
• WWizards of the Coast2016-11-08
  Official Ruling
  If there aren’t two legal targets for Evolutionary Escalation’s ability as it’s put onto the stack, it’s taken off the stack and no creature gets +1/+1 counters.

Rulings via Scryfall