A comprehensive analysis of the genes of aquatic birds has revealed a family tree dramatically different from traditional relationship groupings based on the birds' body structure, according to a research report to be published in the current issue of the Proceedings of the Royal Society and featured on the cover of that issue.
The most startling and unexpected finding of the study is that the closest living relative of the elegant flamingo, with its long legs built for wading, is not another long-legged species of wading bird but the squat grebe, with its short legs built for diving. The two species, whose genes surprisingly are more similar to each other's than to those of any other bird, otherwise show no outward resemblance, according to Blair Hedges, an evolutionary biologist at Penn State.
Hedges leads one of the two research groups that collaborated on the study by separately performing two different kinds of genetic analyses using DNA samples obtained from separate sources. The other group is led by John A. W. Kirsch, professor of zoology and director of the Zoological Museum at the University of Wisconsin. "We knew people might have a hard time accepting these results so we decided to publish our two studies together in the same paper because the weight of the combined evidence is quite strong," Hedges says.
Another surprising implication of the study is that physical features like long legs and webbed feet--traditionally used to group birds of a feather into different flocks on the bird family tree--did not appear just once during the history of bird evolution, as had been the hallmark assumption of the traditional classification system. Instead, the study suggests such structures evolved repeatedly in the history of different aquatic bird species. Because many of the species in the study are located on the "twigs" at the ends of a branch of the bird family tree--not farther back in time on its "trunk"--the study also suggests that "evolutionary change in aquatic birds has proceeded at a faster pace than previously recognized," explains Marcel van Tuinen, a member of the Hedges research team.
The scientists say they feel the conclusions of their research are strengthened by their combination of two different analysis techniques, their use of genetic material from separate sources, and the comprehensiveness of both studies. The Kirsch lab used a technique called "DNA/DNA hybridization" and the Hedges lab used a technique called "DNA sequencing."
The DNA/DNA hybridization technique is a method of gauging the degree of genetic similarity between two species by comparing all the genetic material--the entire genome--contained in the DNA molecules of each species. "In birds, the entire genome comprises about 20 to 30 thousand or more genes," Hedges says.
In contrast, the DNA sequencing technique is a more targeted comparison of the composition of individual genes--specific sections of the DNA molecule that carry the codes for specific inherited traits. The sequencing technique compares the actual order, or sequence, of the innermost pairs of molecules in the gene, known as "base pairs," which are strung side by side along the core of the DNA molecule and which hold together the molecules that make up its two sides.
The hybridization studies in the Kirsch lab included 21 species representing the major families of aquatic birds. The sequencing studies in the Hedges lab included 6 genes from each of 28 species--the largest such study ever performed for aquatic birds.
"We never imagined the flamingo and the grebe would turn out to be closest relatives, and were so surprised by this outcome that we did additional examinations using different sources of flamingo and grebe genetic material and obtained the same results," Hedges says. "A lot of people may have trouble believing the results from these genetic studies for a while, but they carry a lot of weight because we have so much data from two different techniques, and it all paints the the same picture of the evolutionary history of aquatic birds." Hedges predicts the genetic evidence will continue to accumulate until its weight is convincing enough to be generally accepted by most scientists. He says, "What I like about the way science works is that eventually the truth will win."
High-resolution color photos and black-and-white drawings are available on the web at: http://www.
SUPPORT: This research was supported by NASA and the National Science Foundation.
MORE INFORMATION ABOUT THE ANALYSIS TECHNIQUES:
Researchers who, like Kirsch, use the DNA/DNA hybridization begin by splitting the two sides of the long DNA molecule by heating it almost to the boiling point, which breaks the bonds between the two sides at all the approximately 3 billion connecting points along its length. They next mix together the split DNA from two different species and then cool the mixture until the bonds re-form at points where the sequences are chemically compatible, creating hybrid DNA molecules built from the genetic material of the two different species. The last step in the process is to heat the strands again, this time measuring the temperature at which they separate. If the DNA sequences were a perfect match, the temperature at which they separate would be the same as for the DNA of a single species. Each degree lower in temperature corresponds to a percentage difference in the DNA sequence between the two species. A hybrid has spots where the two DNA strands don't bind because the sequences don't match there. If there are a lot of these mismatches, the hybrid molecule splits very easily at a lower temperature because fewer bonds have to be broken, requiring less energy.
Researchers who, like Hedges, use the DNA sequencing technique, begin by designing primers that bind to specific mitochondrial and nuclear genes. They then "sequence," or determine the base pairs of the DNA along those genes. Then they compare the genes of all the species they studied. The Hedges lab took the additional step of performing the technique separately with each side of the double-stranded DNA molecules they studied "as a double check just to make extra sure our data were accurate," he explains.