Cute idea! As far as critique: 3 and 4 don't look like they quite match the evolutionary chain started by 1 and 2 if 5 is the end one. And their colors are all much the same until suddenly 5 is brighter.
They still look like they could be from the eveolutionary to me, but I can see were there could be a missing link. Pass 2 they all get a little brighter , but your right the last one is most noticeable. I appreciate the feed back^^