Why do some species look so much alike? So devilishly alike, in the case of Empidonax flycatchers. Since 1859 and the publication of Darwin's On the Origin of Species, evolutionists have answered this question with one of two evolutionary hypotheses, depending upon the circumstances of the case. One is called convergence, the other evolutionary stasis. Skip the rest of this page.

Convergence. Convergence results when lineages that are not closely related come to resemble each other, because natural selection has favored similar adaptations. A classic example is the similarity in shape of dolphins and sharks. Even though dolphins last shared a common ancestor with sharks several hundred million years ago, they look a lot more like sharks than they do terrestrial mammals, to which they are much more closely related. Why? Because catching fish requires speed, and speed underwater is enhanced when the swimmer has the general shape we see in both sharks and dolphins.

Another example of convergence, also related to swimming fast underwater, is the body form and short wings of auks and penguins. Not very closely related, these two orders of birds have the same ecological "niche." Both fly underwater in pursuit of fish, auks in the Northern Hemisphere, penguins in the Southern Hemisphere. Water is a more viscous medium than air, and the optimal wing length for flying in water is shorter than for flying in air. Over evolutionary time, penguins became so specialized for underwater flight that they gave up aerial flight. Auks have not gone that far, but their wings are so short that they must flap very rapidly to stay aloft, and they have difficulty maneuvering. Both groups of birds, by the way, are convergent in body form on sharks and dolphins, too.

Sibling Species. The extreme similarity of the species of Empidonax flycatchers is not due to convergence. All indications are that the group is monophyletic, which means that they are all descended from a common ancestor, and that that ancestor has no other living descendants. Here and there in the bird world, as in other parts of the tree of life, are clusters of species that are unusually similar in appearance, much more so than other monophyletic groups. The edible-nest swiftlets (Collocalia) of Southeast Asia, the leaf-warblers (Phylloscopus) of Eurasia, and our own empids are among the most notorious of these sibling species. Although we do not know the reason for this unusual level of morphological stasis, careful study has shown that these 15 are "good" species, i.e., they don't interbreed (with the possible exception of the recently split Cordilleran and Pacific-slope Flycatchers). That being the case, they must have some way of telling each other apart. In empids, as in most other clusters of avian sibling species, it appears that they tell each other apart by sound. If we wish to tell them apart, we must follow their lead and use sound as well.

The great evolutionary biologist Ernst Mayr, who coined the term "sibling species," pointed out that once we learn to tell sibling species apart, they often turn out to have numerous subtle differences that we had not previously appreciated. That is the case with Empidonax. In the pages that follow, you will learn about the major differences that have evolved within the genus. Not all of these are useful as field marks, but they will show you how distinct are the four major branches of the genus. Sorting the 11 North American species into four major groups makes it much easier to organize all the information you will be receiving. Even the vocalizations are more similar within branch that among branches. We would not have this advantage if the evolutionary relationships within the genus had not been worked out by evolutionary biologists.

Fortunately for us, the late Dr. Ned K. Johnson, of the University of California, Berkeley, was interested in the question of sibling species, and he spent close to five decades studying Empidonax as a means of understanding sibling speciation. He started in the 1950s with the standard tools of avian systematics, dial calipers and his own eyes and ears. All through his career he also worked to enlarge the toolkit of the biosystematist. His first monograph on empid systematics, published in 1963, made extensive use of sound spectrograms. When techniques from molecular biology became available in the late 1960s, Johnson was one of the first to incorporate them in his research program. These techniques make it possible for scientists to measure biochemical (i.e., genetic) similarity, and then use it to estimate the evolutionary similarity of groups of species and other taxa. The techniques and technology have improved markedly over that period, so that it is now common practice for a student to sequence DNA as part of a project for a graduate degree. As a result of Ned Johnson's determination, we now have a complete geneaology, called a "phylogenetic tree," of the genus. Click here to see this tree, and learn about the evolutionary relationships within the genus Empidonax.