Six Incidents of Evolution Using Time Travel
ShortStory 2,656 words Recommended by 4 award/anthology sources
"The lancet liver fluke is a parasitic worm that needs a vector to carry it from its snail host to its mammalian host, so the larval parasite is excreted by the snail to be eaten by an ant. Once inside the ant, the parasite moves to the brain and changes the host’s behavior, forcing the ant to climb to the top of a blade of grass, so that it will be eaten by a grazing mammal."
Quasar links out; the story lives on Asimov's, where the author gets the click.