Indeed, that’s why its genome is so weird. Genomes can evolve either through drift, in which DNA changes randomly, or through natural selection, in which genes become more or less common because they affect their owner’s ability to survive and reproduce. Typically, both forces are important. But the passenger pigeon was so absurdly abundant that natural selection dominated. “There was almost no portion of the passenger pigeon’s genome that was evolving neutrally,” says Shapiro.
This might all sound pretty wonky, but it matters when thinking about why passenger pigeons died.
In 2014, Chih-Ming Hung and colleagues from National Taiwan Normal University used the genomes of passenger pigeons to reconstruct their historical population size. They concluded that the pigeon was a boom-and-bust bird, whose numbers cycled dramatically between incredible highs and stark lows. That created a natural vulnerability, which humans inadvertently exploited by persecuting the pigeons during a bust phase.
But Shapiro says that Hung’s team made a mistake. They used a technique which assumes that genomes are evolving neutrally—and the passenger pigeon’s largely isn’t. (Hung stands by the conclusions of his study.) Instead, it had been crafted by natural selection to an extent that most other species are not. “Passenger pigeons were not a naturally vulnerable species,” as has been repeatedly suggested over the last five decades, says Novak. “It was a superspecies in its natural element.”
So, why did this superspecies die out? Shapiro thinks it’s because the bird specifically evolved to live in mega-flocks, and developed adaptations that became costly when their numbers suddenly shrank at human hands. “Maybe they were simply not adapted to being in a small population, and didn’t have time to recover,” she says. Maybe they hit a tipping point when there were just too few of them to survive, regardless of whether they were being hunted.
That’s a bit of a leap from the data, but it’s an idea that’s worth entertaining, says Erich Jarvis from Rockefeller University, who studies bird genomes. “It makes one think: Just because humans leave a small population behind without killing off the rest of the species, it does not mean that the species will survive. It would be good to see if there are other species like this.” Shapiro agrees, and wants to see if the same patterns exist in the genomes of other animals that live in massive groups—perhaps herring, bristlemouth fish, or red-billed queleas.
Meanwhile, Novak cautions that we don’t know if the pigeon actually did suffer at low densities, or the minimum number that would have been necessary to prevent extinction. Indeed, he says that there’s evidence the passenger pigeon would have fared reasonably well at low numbers, if we had just left them alone. Records show that they were breeding just as efficiently when there were only a few hundred left as when there were billions.