Birds, DNA, and Evolutionary Convergence
|Tracing genealogies fascinates many people, and reconstructing the genealogies (which they call "phylogenies") of groups of organisms is a favorite sport of biologists. A persisting mystery has been the evolutionary relationships of various groups of birds. Which birds are similar because they are descended from relatively recent common ancestors (true evolutionary relationship), and which are similar because, although coming from different recent ancestors, they have evolved similar structures in response to similar ways of life (evolutionary convergence). This mystery is exemplified by a long debate over who are the relatives of the Wrentit. Confusion is indicated by its name: does it share recent common ancestors with wrens, or titmice, or members of some entirely different group? At one time or another, the Wrentit has been declared a near relative of wrens, bushtits, titmice, mockingbirds, Old World warblers (which include Dusky and Arctic Warblers which stray into North America), and babblers (Eastern Hemisphere insect eaters).|
|Normally evolutionary family
trees are constructed by carefully comparing details of
structural features, because taxonomists known that overall
similarity in form can be misleading. In spite of their
fishlike shapes, whales have long been recognized as
phylogenetically much more closely related to people than to
fishes, because the presence of mammary glands and hair
(scanty as it is) and the structure of their brains, hearts,
and many other features show them to be mammals. The
superficial similarity of fishes and whales is an example of
convergence. The whale-fish convergence indicates that
streamlining is the evolutionary solution to minimizing drag
on large creatures moving rapidly through water.
Since most birds have also had to solve the problem of moving rapidly through a fluid, air, they tend to be very convergent in shape. There also is great potential for convergence in bill structure (if two unrelated birds have the same diets) and leg structure (if, say, they both perch, wade, or paddle). All birds are also much more recently and closely related to one another than are whales and fishes, making structural differences among them relatively slight. Not surprisingly, taxonomists have had considerable problems reconstructing the phylogeny of the birds. A classic example of avian convergence is that between swallows and swifts. Both are specialized for scooping up flying insects, and early ornithologists grouped them together. But detailed analysis of their anatomy revealed swifts to be distant relatives of hummingbirds and swallows to be songbirds.
New techniques, however, are coming to the rescue of bird taxonomy. It is now possible to compare directly the DNAs in different organisms, and evolutionists Charles Sibley and Jon Ahlquist have been diligently assaying the similarities of these molecules (which encode the genetic information) of diverse groups of birds.
Before the reality of continental drift had been established in the 1960s, it had been argued by some bird taxonomists that each of the flightless ratites -- the African Ostrich, South American rheas, Australian Emu, New Guinea cassowaries, and New Zealand Kiwis -- was more closely related to various flying birds than to one another. After all, how could a group of flightless birds spread across ocean barriers? But when it became clear that the southern homelands of these birds were once united, the idea that they were all more closely related to one another than to flying birds gained popularity. Sibley and Ahlquist made comparisons of the DNAs of ratites which show them to be, indeed, each other's closest relatives, with South American tinamous (ground-dwelling, short-winged, partridge-like birds) their nearest flying relatives.
Continental drift has, in turn, permitted Sibley and Ahlquist to estimate an absolute time scale of divergence for all groups of birds. The scale calibrates relative rates of DNA divergence against the time of known geological events. For example, the genetic distance between the Ostrich of Africa and the rheas of South America represent about 80 million years of DNA evolution, since it was roughly 80 million years ago that Africa and South America had drifted far enough apart to make the Atlantic Ocean a barrier for flightless animals. Using this DNA "clock," it seems that the last common ancestor of finches and mockers-thrashers lived about 50 million years ago, and the split between the mockers and thrashers took place about 10 million years ago.
Sibley and Ahlquist have also shown that a large number of Australian passerines, often thought to be relatives of robins, wrens, nuthatches, and so on, are rather like the marsupials, the product of an independent "radiation" (evolution of great diversity) on the island continent. Thus, the beautiful Australian fairy wrens are not related to our wrens at all, the nuthatch-like sittellas and tree-creepers are not related to nuthatches and creepers, and the red- and yellow-breasted Australian robins are not even thrushes. In fact, all share more recent common ancestors with crows and shrikes than with their American and Eurasian namesakes.
Interestingly the Australian birds converge in more than appearance. The fairy wrens characteristically cock their tails, and many of their calls are often very wren-like trills. The sittellas often forage head-downward, and the tree-creepers climb up tree trunks seeking prey under the bark. The robins, however, have a mode of hunting not found in the North American bird fauna. They are "pounce predators," often clinging to the sides of tree trunks 3 to 6 feet high and pouncing on insects on the ground. All in all, the Australian bird convergence is even more spectacular than the convergence of various marsupials with the placental mammals that dominate on all other continents -- after all the mammalian convergence was recognized early on, while the birds fooled biologists until very recently.
Closer to home, another problem that long bedeviled ornithologists was the identity of the birds that had colonized the Hawaiian Islands and evolved into the group known as Hawaiian honeycreepers. Although obviously closely related to one another, the honeycreepers have radiated to fill a wide variety of habitats. Many have evolved remarkably different bills -- some finch-like, some long and down-curved, some parrot-like, etc. Comparisons of honeycreeper DNA with that of other groups has confirmed the suspicion of ornithologists that honeycreepers are most closely related to the cardueline finches: goldfinches, crossbills, grosbeaks, siskins, etc. Sibley and Ahlquist estimate that an ancestral finch reached the Hawaiian area some 15-20 million years ago -- long before the current islands emerged from the sea. That finch colonized one of the pre-Hawaiian islands that has long since been worn away by the sea. That island was produced by the same volcanic "hot spot" that has been producing the present group. The remains of that first home of the honeycreepers, now represented by one of the submerged volcanic remnants called the Emperor Seamounts, are being carried northward away from the hot spot from which it emerged by the drifting Pacific tectonic plate. Such drifting is the same kind of motion that causes continents to move with their tectonic plates.
Now what about the Wrentit? DNA comparisons show it to be genetically similar to the babblers and the Old World warblers -- two groups that are closely related. Ecologically and behaviorally it most resembles babblers. Wrentits and babblers build similar nests, and many babblers, like Wrentits, inhabit semiarid regions, sing a great deal, and dine on insects and small fruits. Interestingly, however, Australian "babblers" turn out to be unrelated to Asian and African babblers but, like Australian robins, are instead relatives of crows.
Temperature Regulation and Behavior;
Copyright ® 1988 by Paul R. Ehrlich, David S. Dobkin, and Darryl Wheye.