Evolution is about how the town of Glen Canyon, Arizona copes with a meteor crash. The crash happens in the desert outside the town. Two of the local college professors, Ira and Harry decide to investigate the crash site. They meet Wayne on the way there. Wayne's car was damaged during the strike. Wayne leads them to the meteor crash. They discover a liquid substance coming out. Ira takes a sample of it to study later. Ira gets excited and interrupts a game that Harry is coaching. They go back down to the site to see how things are going. They discover that the liquid goo has rapidly evolved into fungi. They are kicked off the site and ordered to forget about the meteor.
Wayne discovers an amphibian alien that had eaten the owner of the country club that he worked at. He brings it into the college to get an opinion on it. Ira asks him if he had seen any more of these creatures and they go out on a hunt to find them. After they finish chasing one through the mall, the Governor of Arizona gets involved. Worried that the entire state will be invested in the creature, he orders that the military deals with the problem however they see fit. They decide to drop napalm on the site which will sanitize the area. Ira tries to explain that they are acting too quickly and that there may be consequences.
They evacuate the town and prepare to detonate the napalm in the morning. The team works tirelessly through the night in a desperate attempt at figuring out a way to safely and permanently kill the invading alien species. Harry tosses a match in the air which catches the only specimen that the team has to work with on fire. They realize that the napalm will act as a fertilizer. They save the town from destruction.