These Butterflies Evolved to Eat Poison. How Could That Have Happened?

By Carl Zimmer Monarch butterflies eat only milkweed, a poisonous plant that should kill them. The butterflies thrive on it, even storing milkweed toxins in their bodies as a defense against hungry birds. For decades, scientists have marveled at this adaptation. On Thursday, a team of researchers announced they had pinpointed the key evolutionary steps …