By day, Thomas Near studies the evolution of fish, wading through streams in Kentucky and Mississippi in search of new species. By night, Dr. Near, an assistant professor at Yale, is a heavy-duty gamer, steering tanks or playing football on his computer. This afternoon his two lives have come together. On his laptop swims a strange fishlike creature, with a jaw that snaps sideways and skin the color of green sea glass. As Dr. Near taps the keyboard, it wiggles and twists its way through a busy virtual ocean. It tries to eat other creatures and turns its quills toward predators that would make it a meal. The chairman of Dr. Near's department, Richard Prum, watches him play and worries about his reckless lunges. "You're just attacking them?" he asks as Dr. Near tries to eat a fat purple worm that looks too dangerous to bother.