Evolutionary anthems: the songs of Darwin finches might be responsible for the group rapid speciation

Natural History, Oct, 2003 by Adam Summers

On Santa Cruz Island, in the eastern Pacific Ocean, the morning sounds of songbirds foraging and courting are reassuringly familiar in the otherwise outlandish landscape of the Galapagos Islands. The dry washes here partly conceal tortoises the size of refrigerators; iguanas as long as your arm sprawl in the baking sun. Darwin's finches, one of the best-studied examples of rapid speciation, are the source of the early morning's whistles and trills. But the birds are far more than mere pleasant diversions that remind homesick biologists of their own territorial origins. Rather, already famous as the subjects of longterm studies on feeding adaptations and the origins of species, the birds are proving to biomechanists that their calls represent a mechanical link between foraging abilities and song production. The co-variation of song and beak size may have been the driving force behind the rapid evolutionary development of finch species in the small island chain, a process that took less than 3 million years.

Speciation--how one species gives rise to another--is easiest to grasp for populations that become isolated. Imagine that an earthquake upends enough rock to create rapids in a formerly sluggish stream (a common event in South America). The new stretch of rapids could keep fish upstream of the rocks from mating with their downstream counterparts. Inevitably, over the generations, the two groups will have to contend with differences between the two habitats--whether in dissolved oxygen levels, water temperature, food availability, or the presence of parasites. Those selection pressures--as well as the simple accumulation off diverse mutations--may be enough to genetically isolate the upstream from the downstream population.

Speciation without physical separation, however, is a trickier concept. Species arising by such a process are known as "sympatric," a term whose Greek roots mean "of the same country." The finches of the Galapagos present a textbook example of sympatric speciation. One common ancestor gave rise to fourteen distinct species, even though members of the ancestral population were within easy flight of one another--in other words, even though there was no geographical barrier to interbreeding.

Thirty years of fieldwork, molecular biology research, and morphological study have led to a good understanding of the evolutionary history of Darwin's finches, particularly the link between their food habits and the shapes of their beaks. Today's birds descend from a generalist ancestral finch that invaded the islands from mainland Ecuador. Galapagos species now inhabit a variety of ecological niches, and each species has a beak suited to finding food in its niche: insect-eating species, for example, have narrow, warbler-style beaks, useful for nabbing insects from leaves and bark; seedeaters have robust bills, tough enough and strong enough to crack the hard seeds they favor. Now the variations among Darwin's finches have enabled Jeffrey Podos, a behavioral ecologist at the University of Massachusetts in Amherst, and colleagues at Duke University to prove that a bird's beak is as vital to its song as to its supper.

The clear tones of birdsong emerge from internal air sacs that can inflate and deflate, much like a bagpipe's bladder. Muscles surrounding the sacs force air through a part of the bird's respiratory tract called the syrinx, a thin-walled region of muscle and cartilage roughly analogous to human vocal chords. As it passes through the syrinx, air vibrates at several dominant frequencies and many overtones, blatting as though it were blown through the mouthpiece of a trumpet. And, just as in a trumpet, the tone of the sound is profoundly affected by the length and shape of the resonating chamber "downstream" of the original vibrations. In the bird's case, the vocal tract acts as a long, fleshy resonating chamber, damping out many of the overtones. By rapidly opening and closing its beak a bird can alter the damping characteristics of the vocal tract.

Podos and his colleagues demonstrated in the laboratory that when sparrows sing, their beaks partly determine the tone of their call. The slower the beaks move, the simpler is the melody of the call, both in tonal range and rhythm. The next step was to study birdsong in the field, focusing on several species of closely related birds with varying beak shapes and sizes.

Mechanical systems usually have to sacrifice force for speed. That constraint is particularly telling in biological mechanical systems--where, for instance, jaws that can move rapidly cannot close with a lot of force. Podos realized that among Darwin's finches, the varieties of this trade-off and the natural variability of beak shape could enable him to test whether a bird's song could indicate the bird's ability to eat hard seeds. With Joel Southall, also at the University of Massachusetts, and Marcos R. Rossi-Santos of Projeto Baleia Jubarte in Caravelas, Brazil, Podos filmed the calls of seven species of Galapagos finches. The birds ranged from the warbler finch (Certhidea olivacea), with a pointy, narrow beak [see illustration on opposite page] to the large ground finch (Geospiza magnirostris), with a broad, heavy bill [see illustration below]. The results meshed well with the laboratory data. The heavy-billed birds had simpler calls, presumably because the bill, more suited for closing forcefully on a tough seed, was not able to move as rapidly as the more delicate beak of the insect eaters.