The shifting songs of Darwin's finches have given new insight into processes that shape the course of evolution, preventing newly forked branches on life's tree from growing back together.

Even though it's biologically possible for Geospiza fortis and Geospiza scandens – the original residents of the Galapagos island of Daphne Major – to interbreed with newly arrived Geospiza magnirostris, the species have stayed separate.

The birds learned to sing new tunes, setting off a behavioral cascade that swept the island in just a few decades: Evolution in action, audible to the naked ear.

The findings, published October 31 in the Proceedings of the National Academy of Science, "throw light on what happens at a crucial stage in speciation," wrote Princeton biologists Rosemary and Peter Grant.

Since the late 1970s, the Grants have worked on Daphne Major, studying descendants of some of the same finches that inspired Charles Darwin's evolutionary theories.

Scattered on isolated islands, Galapagos finch species have diverged from a common ancestor over the last several million years. Enough time has passed for species to become physically distinct, adapted to the unique niches of their home islands.

Not enough time has passed, however, for interbreeding to become impossible. Yet finch species often keep to themselves, even when winds or migratory impulses carry them between islands.

That reproductive separation between geographically overlapping but biologically compatible species – technically known as allopatric speciation – is considered an important phase in species divergence. It keeps species apart long enough for their differences to become absolute. It's happened many times in the tree of life's divergence, but at time scales lost to prehistory.

In their new study, the Grants describe it in real-time.

When they arrived in 1978, G. fortis and G. scandens were Daphne Major's sole finch inhabitants. Five years later, G. magnirostris, arrived on the island. After several decades, a few of the original finches interbred, producing a hybrid that appears destined to become its own species. Yet neither bred with with G. magnirostris. According to the Grants, G. fortis and *G. scandens *maintained separation through song.

For the finches, as for so many birds, songs – sung by males, learned from their fathers – are a central form of communication. They enable individuals to recognize others of their species, advertising the possibility of reproduction. Each species' song is distinctive.

As it happened, the song of G. magnirostris originally overlapped with the tunes of G. fortis and G. scandens. That's no longer true. Since 1983, their trill rates, frequency and bandwidth have all changed drastically.

What's most intriguing about the change is that it doesn't seem to have a physical origin. The shapes of G. fortis and G. scandens beaks haven't changed, as might be expected. (After all, it was variations in finch beak form that so inspired Darwin.) Nor did their bodies change.

Instead the change was behavioral. The finches quickly learned new songs, demonstrating what researchers call "peak shift." Male finches first sing in imitation of their fathers; as they mature, they add new riffs, and teach those songs to their own sons – who, in turn, riff even further.

Peak shift represents a much more active, dynamic evolutionary mechanism than random genetic mutation. In just a few decades, all across the island, both G. fortis and G. scandens occupied a completely new acoustic space.

The Grants think peak shift may be a common evolutionary phenomenon, responsible for separating similar species for the millions of years necessary to become genetically incompatible.

"Behavioral modification of mate-signal learning, through a peak shift mechanism, without genetic change, may be widespread," they wrote.

Images: Top row, the three finch species on Daphne Major; below, the change since 1983 in the acoustic space occupied by their songs. G. fortis and G. scandens (red and green) now occupy very different ranges from G. magnirostris (blue)./Peter and Rosemary Grant.

See Also:

Citation: "Songs of Darwin's finches diverge when a new species enters the community: Implications for speciation." By B. Rosemary Grant, Peter R. Grant. Proceedings of the National Academy of Sciences, Vol. 107 No. 44, November 1, 2010.

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