Evolution of Evolution — Text-only | Flash Special Report
Rethinking the Bird Tree of Life
Modern genetic studies reveal Darwin's finches are not finches at all
By Sushma Reddy and Shannon Hackett
Charles Darwin referred to birds often when making his case in “On the Origin of Species.” Well before Darwin’s time and beyond, birds were among the most loved and well-studied organisms. Studies of these conspicuous and often beautiful animals have shaped biological thought—from natural history to behavior, ecology, reproduction, speciation, vocalization, cognition, evolution and more. Given all this attention, it is surprising that avian evolutionary relationships—the bird tree of life—have remained so unresolved.
The high-species diversity of birds, compared to some other vertebrate groups, has long inspired speculation about their origin. Since Darwin’s time, theories have been put forward to explain this diversity, but research demonstrates there is no simple explanation. For instance, there are few common features shared by major groups of birds to suggest their relationships—for example, parrots look like other parrots, yet have little in common, morphologically, with other birds; the same is true for owls or pelicans and so forth. Researchers used what little evidence was available from observation of morphology, behavior and ecology, to assess how birds were interrelated. More recently, genetic data began to piece together parts of the puzzle, but results from early studies were varied, often inconclusive, and even inconsistent.
Scientists now believe the problem stems from the shape of the bird evolutionary tree. From fossil and genetic evidence, researchers think modern groups of birds diverged into their different forms within a few million years of one another, about 65-100 million years ago. This makes the matter of deciphering the evolutionary history of birds a complex problem requiring far more data than has ever been collected to address the problem.
For more than five years, we have been part of a collaborative project to study this big mystery regarding birds. We set out to study the deep evolutionary history of birds using massive amounts of DNA from all the major groups of birds. Our study benefited from technological advances that allowed us to collect large amounts of sequence data, as well as data from recent genome projects such as those for human and chicken, which have greatly contributed to our knowledge of genomes. Using 19 different genes, or more than 32,000 DNA base-pairs for each of the 169 bird species, we deciphered a robust “family tree” of birds. Our study revealed several unexpected yet strongly supported relationships, and showed that much of conventional wisdom on the evolution of birds is wrong.
One important lesson from this study is that appearances can be misleading. Many different looking bird groups are actually closely related. Some of these unlikely “cousins” include parrots and songbirds, flamingos and grebes, and hummingbirds and nightjars. And, many of the groups that do look and act similar turn out not to be closely related. For example, birds of prey or raptors, such as falcons and hawks, independently evolved similar lifestyles. Likewise, owls and nightjars, both nocturnal, cryptically colored groups, are unrelated. Interpretation of many adaptations to different lifestyles, environments and behaviors can now be assessed with this new historical perspective.
Our study, along with other recent genetic studies, demonstrates that much of bird classification is wrong. Many of the traditional orders, families, genera and even species are not natural groups, indicating that many of the traditional features used to unite these groups do not reflect the evolutionary history of the birds, as previously assumed.
Another example of complicated bird evolution, particularly relevant as we celebrate Darwin and his life and accomplishments, comes from one of the most studied and influential group of birds—Darwin’s finches of the Galapagos, which have long been considered a classic case of adaptive radiation. Some of the most elegant and detailed studies demonstrating evolution by means of natural selection have been done on these finches. However, genetic studies suggest we have a lot to learn, even about this famous group of birds. Despite their morphological diversity, birds classified in different species based on distinct bill size have identical or nearly identical DNA. In addition, the use of DNA sequence data to find the finches’ place in the avian tree of life shows that Darwin’s finches are not finches at all, but instead, belong to the family of tanagers. New data and approaches can reinterpret even the most familiar of textbook examples.
As we continue to explore and piece together the puzzle of the bird tree of life, we will undoubtedly discover and reinterpret other fascinating facets of avian evolution. Birds exhibit incredible diversity and using this "family tree," we can begin to grasp how these varieties originated. By better understanding how birds evolved, we also better understand the features that have fascinated so many scientists and amateurs for centuries.
Sushma Reddy is currently a postdoctoral fellow at the Field Museum of Natural History in Chicago, Ill., working on the Early Bird project, part of the National Science Foundation-supported Assembling the Tree of Life (AToL), program. She became an assistant professor of biology at Loyola University Chicago in the fall of 2009. Her research covers topics in evolution, biogeography and genetics, primarily using birds but also other vertebrates.
Shannon Hackett is the associate curator and head of the Bird Division at the Field Museum of Natural History in Chicago, Ill., and co-chair of the Committee on Evolutionary Biology at the University of Chicago. She is also a fellow of the American Ornithologist’ Union. She is researching the evolution of birds as part of the Early Bird project, part of the National Science Foundation-supported Assembling the Tree of Life (AToL), program.
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