"A single population of Canada goose became resident in the Hawaiian islands and gave rise to the diverse geese of the islands," the researchers wrote last month in Proceedings of the National Academy of Sciences. "The endangered nene is the only surviving member of [that evolutionary] radiation."
To study the geese's ancestry, James and her colleagues used clues coded in genetic material obtained from fossils and from living populations of geese. Tracking the path of the feathered Hawaiian immigrants, the researchers traced them back to a branch within the family of Canada geese.
The researchers' sleuthing began with a comparison of genetic sequences from the nene, several types of living Canada geese, and fossils of several extinct geese from Hawaii. Thirteen fossil geese that died between 500 and about 5,000 years ago provided ancient DNA samples for Hawaiian birds.
"I've been astonished at the degree of the Hawaiian fossils' preservation," said James. "The birds died in lava-tube caves, where they were less exposed to ultraviolet radiation." UV light breaks down DNA.
To salvage as much DNA as possible, the scientists looked at an abundant type of DNA that exists inside the tiny energy-producing organelles of cells, called mitochondria.
The more genetic differences, or mutations, that are in the mitochondrial DNA of two related animals, the greater the length of time that has passed since they descended from a common ancestor. And because mutations accumulate at a predictable rate, scientists can estimate how much time has passed since two populations diverged from a common ancestral band.
Paxinos is now doing a similar analysis of the geese using DNA from the nuclei of cells from fossils and living geese in the hope of supporting the current finding.
All in the Family
The research team found that the nene and two extinct species of Hawaiian geese were all closely related to one other genetically. Surprisingly, all the Hawaiian geese were almost as closely related to two subspecies of Canada geese as they were to each other.
Still more surprising, those two Canadian varietiesknown as the dusky Canada goose and the giant Canada goose, respectivelyshare more genetic similarities with the nene and its extinct relatives than they do with other Canada geese. This makes Hawaii's geese a genetic subset of the Canadian species.
Based on the genetic analysis, the researchers said the common ancestor of all Hawaiian geese must have settled the islands within about the past 500,000 years. That happens to have been about the time volcanoes first created the big island of Hawaii, which suggests that the birds showed up when that island was still young.
From the traits of these two related Canadian populations, the common ancestor of the Hawaiian birds must have been larger than the modern nene, James and her colleagues inferred. The largest of the Hawaiian geese, called the giant Hawaii goose, was flightless and is now extinct.
From fossil evidence and modern data, the researchers also compared characteristics of the different species of geese, such as bone structure and body shape. The results showed that different subspecies of Canada geese have a predictable relationship between body size, wing length, and skull shape. Larger birds in this group have consistently longer wings and narrower heads.
The Hawaiian geesethe nene and its extinct cousinsdon't follow this pattern. The giant Hawaii goose had relatively short wings, while the nene has the longest wings of the three species, despite having the smallest body size. Even so, the nene's wings are much shorter than the wings of similar-size populations of Canada geese.
The different patterns of body shape also suggest that the larger birds in Hawaii evolved to fill ecological niches that geese in Canada don't occupy, such as grasslands, rain forests, and open lava fields. And it indicates that the larger Hawaiian geese were poor fliers or flightless.
Part of the reason for the diverse forms and unique biology of the Hawaiian geese, the researchers said, is that Hawaii had no mammalian herbivores when the geese arrived. Their relatively rapid evolution would have enabled the birds to occupy many ecological niches that, in their native Canada, were already full and therefore off limits to them.
In addition, the new avian settlers had no land-based predators in their adopted homeland. Therefore, an ability to fly would be less essential to survival for the largest of the birds, the researchers speculated.
Because the island geese don't migrate south during winter months like their Canadian cousins, they also didn't need the same long wings that Canada geese use to save energy during extended flights.
In the long run, the inability to fly might have been the giant Hawaii goose's undoing: The species became extinct after humans arrived on the island hundreds of years ago, probably as a result of habitat destruction and hunting.
The National Geographic Society supported the team's research through a grant to Robert C. Fleischer, who was the senior author of the published paper.
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