This really is an amusing comic.  There's not much depth to it but it's still very clever.  Evolution and creationism have always been topics of note especially with specific party affiliation.  Religious people tend to be more conservative so this comic is poking at conservatives for the most part.  The comic makes the joke that those who believe in creationism are the same people that actually have not evolved as much as others.  The artist uses the idea of evolution or lack of evolution to give the reason for people believing in creationism.