Back in the 1860s，a British scientist named Henry Walter Bates noticed something interesting in the animal world：a kind of butterfly，Common Mormon，can change the pattern on its wings so that it looks like another butterfly species，the Common Rose，which is poisonous for birds to eat．

Bates argued that animals develop this ability to protect themselves from being eaten．This theory had been widely accepted by scientists．But one question remains：how do these animals manage to do that？

After more than 150years，scientists are finally able to answer that question-it is all down to a gene （基因） called"doublesex"，according to a study published on March 6in the journal Nature．

In fact，scientists have long known that genes are responsible for this useful ability．But until the new study，scientists hadn't been able to find out which genes in particular were responsible．

Scientists at the University of Chicago compared the genetic structures of the Common Mormons that changed their wing patterns with those of ones that did not．And all the results pointed to a single gene，"doublesex"．

This was much to the scientists'surprise．They used to expect that something as complex as this would be controlled by many different genes．For example，one gene would control the color of the upper part of the wing，one would do so for the lower part．

However，the"doublesex"gene doesn't quite do everything by itself．Instead，it acts like a switch-it"tells"other genes to change the wing patterns．Isn't that clever？

Scientists believe that this special ability of the"doublesex"gene on Common Mormon was developed throughout the long history of evolution．"The harmless species gains an advantage by resembling something predators （捕食者） avoid，"Sean Carroll，a scientist told Nature．"it was obvious evidence for natural selection．"

33．How do scientists find the truth of bu