Snake Oil Series by Parke Godwin

Imagine two brothers off on an intergalatic Spring Break. When their friends leave them behind on Earth, they've got a few millenia to kill before they'll manage to get back to school. So, as an experiment, mind you, they decide to give evolution a bit of a nudge... And that's when all hell breaks loose... a little more literally than either of them planned...

Barion and Coyul are at it again... This time, they've been discovered toying with evolution. Barion has been summoned back to answer to the Council for their transgressions and Coyul is left behind to clean up the mess...