But passenger pigeons were so plentiful and so mobile that beneficial genetic mutations spread and detrimental ones disappeared very quickly throughout their population. This caused a loss in overall genetic diversity, which meant less raw material for adapting to human-induced change.
It’s “totally the opposite of what you would expect,” Dr. Fritz said.
In the new study, a team of evolutionary biologists compared the genomes of four passenger pigeon specimens from different geographic regions with those of two band-tailed pigeons, a close living relative, and saw signatures of remarkably efficient natural selection in the passenger pigeons.
The researchers found typically high genetic diversity in regions of the genome that tend to get chopped up and rearranged between generations, but extremely low diversity in regions that don’t. Under strong natural selection, when beneficial mutations occur in these latter regions that don’t get scrambled as much, large swaths of neutral or even slightly harmful DNA get fixed along with the good genes, suppressing genetic variation, said Beth Shapiro, a professor at the University of California, Santa Cruz, and an author of the study.
The researchers also surveyed a smaller amount of DNA in the mitochondria of 41 passenger pigeons to estimate the population size of the species before it died out.
This analysis revealed regions of high genetic diversity, which “wouldn’t exist unless the passenger pigeon population was really large, for a really long amount of time,” said Gemma Murray, a postdoctoral researcher at the University of California, Santa Cruz, and another author of the study.
Her team inferred that passenger pigeons had a huge, stable population for at least 20,000 years before extinction. This finding challenges a popular hypothesis that suggested that the low genetic diversity in passenger pigeons was a result of demographic fluctuations, with population crashes causing bottlenecks in genetic variation.
“We were astounded to learn that the passenger pigeon population had been enormous for at least 20,000 years,” Dr. Shapiro said. “That meant they were really big during the last ice age, and they remained big even as the climate changed dramatically during the warming period after.”
Altogether, the researchers’ results suggest that passenger pigeons may have been hyper-adapted to a gregarious lifestyle. They might, for instance, have evolved predator defenses, foraging strategies and mating behaviors tailored to living in large, social colonies, Dr. Murray speculated.
These traits may not have worked so well when the birds were hunted en masse and had to survive in smaller clusters.
The findings also have implications for how conservation scientists think about vulnerability to extinction, Dr. Shapiro added. The passenger pigeon illustrates that even species with colossal population sizes are not safe from disappearing.
“Perhaps we should step back and think more holistically about how species have adapted and evolved as we try to come up with ways to protect them,” she said, citing the example of fish species that have historically lived in extremely large populations.
“We think now of restoring them by creating patches of protected habitat, but we don’t know if the way they’ve evolved through their entire history means that they’re not fit for living in small populations.”