For centuries, biologists have compared the looks and behaviors of closely related species to get a sense of their evolutionary history. Now, researchers are tracing that history in their genomes as well. They find that just a few regions of the genome drive the separation of incipient species early on, but that over time, lots of the genome gets involved in distinguishing the two. To study genome evolution, scientists sequenced 10 genomes from each of three species of a colorful tropical butterfly called Heliconius (pictured). They assessed the differences in the bases, the letters that make up DNA, keeping tabs on where those differences appeared in the genome and how many existed among the three species. They found that the two closest relatives, H. cydno—which has a white band on its wings—and H. pachinus—which has yellow bands—had just 12 small regions where they were different, for a total of 165,000 bases. They split apart 450,000 years ago. Eight of those regions coincided with genes important for setting the color pattern, according to a study published today in Cell Reports. The third, H. melpomene, with its red and yellow stripes, is 1 million years older and in that time, hundreds of differences had accumulated, spread across tens of millions of bases. It seems that evolution starts out slowly, with just a few key differences appearing, but then snowballs with differences accumulating at a faster rate over time.