November 30, 2011
Several million years ago, a progenitor of the group of songbirds known as the honeycreepers arrived in the Hawaiian Islands. The birds diverged into different species to fill a variety of niches, subsisting on everything from grubs to tree sap to nectar from tropical flowers. But until recently, scientists didn’t know exactly how the various honeycreeper species currently in existence were related to each other—or what bird from the mainland was their closest ancestor.
A new study by a team of Smithsonian scientists, published in Current Biology, has pieced together this puzzle and resolved the mystery. “This radiation is one of the natural scientific treasures that the [Hawaiian] archipelago offers out in the middle of the Pacific,” says Dr. Heather Lerner, a professor at Earlham College, who worked on the study as a postdoctoral researcher at the Smithsonian Conservation Biology Institute with Rob Fleischer and Helen James.
Most species of honeycreeper are brightly colored and sing a canary-like song. But beyond that, they are remarkably diverse. ”You have honeycreeper species that are adapted for nectarivory, while others eat seeds, fruit, or even snails,” Lerner says. “You have some bill types that are just unique among birds.”
Like the famous finches Darwin studied in the Galápagos Islands, scientists have long considered the honeycreepers a textbook example of adaptive radiation—a group of species that evolved to fill a variety of niches upon arriving in a new habitat. “In trying to understand all of this diversity, if you don’t understand how and when it evolved, you really can’t test a lot of hypotheses,” says Lerner. “The phylogeny—the individual relationships between species—are fundamental.”
To begin sorting out this mystery, the research team gathered DNA samples from a wide variety of birds. In addition to sampling all 18 living honeycreeper species—and one recently extinct group—they took DNA from 28 bird species that appeared to share physical characteristics with honeycreepers or have similar ranges.
The researchers then used cutting-edge DNA sequencing techniques, some of which have been developed over the past few years. These methods were crucial, because sorting out the tangled relationships among various species required a massive quantity of DNA in order to find commonalities and differences in the genetic codes. “We would have been in the lab forever if we hadn’t used some new technologies,” Lerner says. “So what we did is, instead of copying maybe 500 DNA base pairs at a time, or 1000, we did 10,000 to 12,000 at a time. It’s a complete revolution in terms of DNA sequencing.”
The scientists examined a series of locations in the DNA to look for variations among the species. The degree of variation providing information about when the various species had diverged from each other, because DNA tends to mutate at a set rate over time. The more differences that exist between species, the longer ago their evolutionary paths diverged.
The team’s findings were somewhat surprising: As it turned out, an ancestor of the rosefinches, a group of Eurasian bird species, was the closest relative shared by all Hawaiian honeycreeper species. The founders finches likely immigrated to the Hawaiian Islands sometime between 7.2 million and 5.8 million years ago.
Hawaii’s unusual geology played a role in the rapid evolution of many honeycreeper species that followed. The volcanic islands have formed one by one over time, as the Pacific tectonic plate is dragged across a “hot spot” of magma, and each new island provided a new opportunity for colonization.
‘The timing that we get from our calibration suggests that they got there about the time that Kaua’i was forming,” Fleischer says. “But they didn’t really start to speed up the process of splitting into different lineages until the island of Oahu formed, when you now suddenly had a blank slate of open habitat.” Between 4 million and 2.5 million years ago, the DNA analysis indicates, the honeycreepers underwent a rapid period of speciation, with various species evolving new bill shapes and other features to take advantage of the many new niches available.
In recent years, honeycreeper species have suffered greatly from habitat loss and other problems posed by human development, with 38 species going extinct. The research team plans to use these DNA analysis techniques with samples taken from extinct museum specimens to see where the species fit into the evolutionary family tree.
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