For the Birds: Bird Phylogeny Revised

The Swedish naturalist, Carl Linnaeus (1707-1778), developed the system of classification that biologists use today to order the rich diversity of life on earth. Linnaeus gave every species a two-part name, denoting the genus and the species of that organism. A genus contains a number of similar but distinct species. Genera (the plural of genus) are combined into families, families into orders, orders into classes and classes into divisions or phyla.

Linnaeus classified organisms based on their morphological similarity. Species belonging to the same genus are generally more similar to each other than they are to members of their own family that are in different genera.

A classification based on similarity of structure runs into two problems. First, two closely related species may diverge from each other over time and appear to be quite different. Second, distantly related species may converge on similar morphologies over time and thus be mistakenly placed in the same genus or family. As one example, Linnaeus placed the barnacles and the mollusks (snails, clams and their relatives) in the same group because all have a hard shell made of calcium. By looking at the early development of these organisms, biologists came to realize that barnacles are crustaceans and therefore more closely related to shrimp, lobsters and crabs.

Charles Darwin held the view that our classification system of life on earth should be a huge family tree, reflecting the relatedness of organisms. Understanding the relationships of the higher groups (orders, classes and phyla) is difficult based solely on morphology. Spirited arguments among biologists in the scientific literature about the relationships of these higher groups are common.

The analysis of DNA gives us a new way to develop the tree of life in the way Darwin envisioned. DNA is the molecule that ultimately determines the way an organism looks and behaves. The idea behind DNA comparisons is straightforward. More closely related species should have very similar DNA while more distantly related species should have more differences in their DNA.

For birds, the Yale biologist Charles Sibley, working in the 1970’s and 1980’s with his colleague Jon Ahlquist, were the first to use DNA comparisons to examine how the higher groups of birds were related. Their results were groundbreaking.

Sibley and Ahlquist used a technique called DNA hybridization. DNA is a double-stranded molecule with one strand being the mirror image of the other. By heating DNA, the double strands separate. Sibley and Ahlquist recombined single strands of DNA from two species to make hybrid DNA and determined how closely the two strands fit together. Species that made tight DNA hybrids were considered to be closely related.

Their many experiments produced a number of surprising results that are reflected in the current classification scheme used by the American Ornithologists Union and in the organization of field guides. For instance, Sibley and Ahlquist showed that the New World vultures are properly classified with the herons, not with the hawks and falcons. Vireos are not closely related to warblers as formerly thought but rather closer to the crows and ravens.

It is now possible to analyze DNA in much greater detail than Sibley and Ahlquist could do with their hybridization studies. Biologists can determine the sequence of the four molecules (called nucleotides) in DNA. Human DNA has three billion nucleotides; that’s a lot of information to compare!

As an example of the power of this approach, humans and chimpanzees have DNA that differs by less than 1%. Our DNA is slightly less similar to that of gorillas and even less similar to orangutans. So using DNA comparisons, we know that chimpanzees are the closest living relatives of humans and gorillas are the next closest of the great apes to us.

A recently published paper by Shannon Hackett and colleagues in the journal Science presents the results of DNA sequence comparisons for 169 species of birds, representing all of the major groups of birds. Like the work of Sibley and Ahlquist, many surprising relationships were found. We can expect the order of birds in our field guides and checklists to change to reflect his new work.

Here are some of their major findings. The perching birds (the passerines) are most closely related to the parrots! Falcons are closely related to these two groups of birds but not to the hawks and eagles. Thus, the falcon family and the hawk family have converged. The closest relatives of the penguins are the albatrosses. Unlike Sibley and Ahlquist, the new papers shows New World vultures are closest to the hawks and eagles. You can see further results of this important paper at: http://whozoo.org/birds/birdphylogeny.html

[Originally published on August 30, 2008]