The unfortunate demise of billions of passenger pigeons, which led to its extinction, may be due to its loss of genetic diversity.

Passenger pigeons used to dominate the skies of North America, with an estimated population reaching about 5 billion. Also known as Ectopistes migratorius, these birds nested in flocks to drive off predators and keep their numbers growing.

Lack Of Genetic Diversity

Their effort to maintain their huge population eventually caused their extinction. With the birds banding together, they were easy target for hunters who catch and shoot the bird to trade in the market. Aside from commercialization, rapid extinction of passenger pigeons may be due to the loss of genetic diversity.

Through the analysis of DNAs of preserved birds, evolutionary biologists from University of California, Santa Cruz were able to compare the specimens of passenger pigeons coming from varied geographic regions with two band-tailed pigeons. They found that passenger pigeons showed evidence of natural selection. High genetic diversity was present in genomes that underwent mutations between generations, while regions that did not rearrange or mutate have low diversity. When this happens, harmful DNA will adjust with the good genes and result to the suppression of genetic variation.

"When we looked at rates of adaptive evolution and purifying selection in both species, we found evidence that natural selection had resulted in both a faster rate of adaptive evolution in passenger pigeons and a faster purging of deleterious mutations," said coauthor Gemma Murray. "That is exactly what you would expect to see if selection is causing the differences in genetic diversity."

Scientists deduced that while the pigeons adapted well to living in large flocks, they were not able to cope well when their numbers started to diminish.

Beth Shapiro, a coauthor of the study, said it was possible to avoid the extinction of passenger pigeons if it wasn't for overhunting and overexploitation. A constant reminder of poor legislation laws is Martha, the last known passenger pigeon that died on Sept. 1, 1914. The bird was the subject of genetic engineering to bring the extinct birds back to life.

"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," said Shapiro. "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."

Conservation Efforts For Rare Populations

National Museum of Natural History curator of birds Helen James suggests that there should be more intense conservation efforts focused on birds with rare population.

"We have to understand what's going on in populations, what's going on in the ecosystem at large," said James. "Because if a major factor that allows the species to survive suddenly changes, you absolutely can have sudden disappearances of species that are ecologically playing a really large role. And that can have cascading effects."

The study is published in Science on Nov. 17.

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