D: Yaël, the monarch is my favorite butterfly. I like its beautiful orange and black colors.
Y: Don, those colors evolved to warn predators. The monarch contains toxic compounds that disrupt a sodium ion pumping mechanism in cells. If a predator eats it, the toxins cause it to get sick, or even suffer cardiac arrest. The monarch gets the toxin from the food that its caterpillar eats, the milkweed plant. The plant makes the toxin to protect itself from predators.
D: But Yaël, how does the monarch itself keep from getting poisoned?
Y: Don, scientists know that the monarch has mutations at three different locations in the gene that encodes the instructions for its sodium ion pump protein. These protect it from the toxin and allow it to eat a food that no competitor can, while gaining protection from predators.
D: Okay, but if it takes three separate mutations to make the ion pump protein resistant to the poison, that sounds awfully improbable. How could it happen?
Y: To find out, an international team of molecular biologists used genetic engineering techniques to cause each of the three mutations in a common laboratory insect that can’t eat milkweed; the fruit fly. They put milkweed toxin into the engineered flies’ food, to test whether they could survive. The researchers found two of the three mutations conferred some protection from the toxin all by themselves. But, they also caused the flies to suffer occasional seizures. The other mutation protected against seizures. This one seems to have come second in monarch evolution. Because the mutations came in the right order, they incrementally benefited monarchs and were favored by evolution.