In the 11 January issue of Nature, Jeffrey Podos describes how changes in the shape and size of the beaks of Darwin's finches, driven by fluctuations in which foods are available, also affect how well they sing.

Podos conducted the study while a postdoctoral researcher at the University of Arizona 1998-2000. He just returned from field work in Ecuador to his faculty position at the University of Massachusetts.

Podos found that large, thick beaks designed for crushing hard seeds are not as "tuneful" as slender beaks adapted for picking up insects. Because many birds rely on each others' songs as a way of identifying suitable partners, changes in song may affect their ability to recognize potential mates, which in turn could lead to evolution of new species.

Biologists have long recognized the importance of courtship signals in determining who mates with whom, potentially affecting whether a species splits into two or remains as one. But until now it has not been clear exactly how a changing environment could affect these courtship signals.

The key that Podos found to unlocking this mystery is the beak of the bird, which is used in two very different jobs: feeding and singing. Changes in the food environment do not directly cause new bird songs, but beaks that evolve in response to changing foods can't necessarily make the same sounds they used to.

In particular, a bird with a large, thick seed-crushing beak can't make the rapid and precise beak movements that a bird with a slender beak can.

Building on previous research that showed how beak shape and size affects bird song, Podos devised an ingenious way to measure the "skill" of a song:

A song's trill rate (syllables per second) and frequency bandwidth (range of high and low tones) are inversely related -- as one goes up, the other goes down. But within this limitation, some birds are able to create songs that maximize both qualities.

Podos measured vocal performance by how far a particular song deviates from this maximum. By measuring beaks and recording songs of different finch species, and different individuals within a species, Podos was able to show that as beaks grow larger, song performance worsens.

In keeping with the theme of the UA program under which Podos did his research, an NSF Research Training Grant in the Analysis of Biological Diversification, Podos was able to incorporate the degree of relatedness among the species he studied into his results.

In the past, researchers comparing species treated each individual species as a separate entity. But because species are interrelated, their characteristics are not independent of each other.

Using a method called independent contrasts analysis, Podos tracked the relationship between beak size and vocal skill along the branches of the evolutionary tree of the finches, strengthening the validity of his results.

The Galápagos finches are one of the most striking and best-known cases of adaptive radiation, wherein an ancestral species quickly split into many descendant species, each occupying its own niche.

Previous studies of these finches, who have inspired generations of biologists going back all the way to Darwin, have shown that they have the ability to adapt rapidly to fluctuations in food availability.

In response to the sudden changes in food sources that plague these harsh islands, the finches change the size and shape of their beaks from one generation to the next. This has led to a strikingly diverse group of finches which, although they are all closely related, have very different sizes, shapes, and habits.

But until now, there have been few clues as to possible mechanisms by which they split into distinct species.

There are two ways different species remain separate: either the offspring of hybrid matings are adversely affected, as with the sterility of mules (the offspring of horse-donkey matings); or the separate species never mate in the first place. In Darwin's finches, hybrids are fertile, suggesting that the main way these species stay separate is by not mating outside their species.

Although Podos's research does not directly address the role of song recognition in mate choice in these birds, in many other songbirds females recognize males of the same species by their songs.

If song recognition breaks down because of changes in the beaks of Darwin's finches, new species can be formed. Changes in beak size and shape, driven by extreme fluctuations in the abundance and diversity of foods available, thus provide a possible cause for the rapid speciation and high diversity of these finches.

Perhaps the most striking aspect of Podos' findings is that they illustrate how different aspects of an organism -- aspects that are usually studied separately (in this case, ecological niche, morphological change, and mate-recognition signals) -- are tightly interlinked.

A change in one aspect can indirectly affect the others in unexpected ways, and with far-reaching consequences. Fittingly, Darwin's finches thus continue to provide evolutionary biologists with new insights into the origin of species. - By Margrit McIntosh

Related websites:

Jeffrey Podos

Research Training Group in the Analysis of Biological Diversification at the University of Arizona

[Contact: Jeffrey Podos]

16-Mar-2001