Up to about 200 years ago, the passenger pigeon was one of the most numerous vertebrate animals on Earth. The species had an estimated 3 to 5 billion individuals at the peak of its existence and had such big flock sizes that they were known to block out the sun. However, this changed during the early 1800s when they were commercialized as a cheap food source. Over the course of only 100 years, a combination of hunting and deforestation killed millions of pigeons a year, until finally, the birds were completely extinct. So how did these birds, that existed for tens of thousands of years, die off in little over one lifetime? This is the question Dr. Gemma Murray and her team sought to answer by looking at the genetics of some well-preserved passenger pigeons.
To understand what could have led to the rapid demise of the passenger pigeon, the team first compiled samples of DNA from various pigeons that had been preserved in museums. Through a series of statistical analyses, the team was able to determine that the passenger pigeon population had remarkably low diversity. Each individual was not too different, at least genetically, from any other individual. This came as a surprise, as large populations normally have higher chances of new mutations, and therefore individuals should have more diversity.
Yet as the team looked at the entire genetic code, it became apparent why this was the case — natural selection was acting very strongly on the population. Natural selection allows for beneficial genes to be kept in a population and harmful ones to be weeded out. Normally this helps the population, but it also serves to lower diversity, keeping many genes identical in a population. Without genetic variation, it can be hard for a population to adapt to a quickly changing environment.
When the team looked at these genes, it became apparent why they were so conserved. The team identified 32 genes that appeared to be heavily beneficial in passenger pigeons. Among them were genes that helped improve the immune system, consume high-energy foods for long travels, and help to control stress. All of these genes were important to passenger pigeons living in large populations..
Most genes will have different versions. For example, one gene might give you straight hair but the other version might give you curly hair. With either version of a gene, you normally have a 50/50 chance of it getting passed on to your kids. However, because each gene has a specific location on a chromosome, the closer two genes are to each other the more likely they are to be inherited together. And when one gene is beneficial and becomes prevalent in a population, other genes near it can become just as prevalent, even if they aren’t beneficial.
This appeared to be the case in the passenger pigeon, whose important genes had an abundant amount of less important genes “linked” with it. This created a problem for the passenger pigeons. As their important genes became more prevalent due to natural selection, so did the linked genes. By the time of the late 19th century, when their numbers were dwindling, the remaining population did not have enough diversity to adapt to smaller flocks and as a result, collapsed completely.
If something as numerous as the passenger pigeon can be brought to extinction in just under a century due to a lack of diversity, then it could happen to other organisms. Many endangered animals are now suffering due to a lack of diversity, and rehabilitation programs that focus on breeding often fail to take this into account. Many of the crops we enjoy today, such as bananas and wheat, are almost genetically identical, which could also pose a problem in the near future if they can’t adapt fast enough to a virus or a pest. Ultimately, this investigation doesn’t help us to bring back the passenger pigeon, but it can help us to prepare for the future and maybe keep something like this from happening again.