A new study on passenger pigeon (Ectopistes migratorius) genomics suggests that even species with large and stable populations can be at risk of extinction if there's a sudden environmental change. The passenger pigeon was once the most abundant bird in North America, numbering between 3 billion and 5 billion. While populations this large are usually associated with high genetic diversity, recent studies have shown that of the passenger pigeon to be surprisingly low. Theory suggests larger populations will be more greatly impacted by natural selection. Here, to investigate a role for natural selection in the reduced genetic diversity of these birds, Gemma Murray et al. analyzed nuclear genomes of four passenger pigeons from different locations, and compared them with those of their closest living relatives, band-tailed pigeons. They found that the passenger pigeons' large population allowed for faster adaptive evolution; namely, high-diversity regions of their genetic codes underwent stronger and faster selection to remove harmful mutations and keep advantageous genes - compared to the genomes of band-tailed pigeons. While this may sound like good news, strong selection for specific, advantageous genes also drove a huge loss in genetic diversity, the authors say. Thus, after exposure to unexpected environmental changes, including changes that may have required survival traits that weren't maintained, the birds were more vulnerable to extinction, the authors suggest.