In Darwin's evolutionary footsteps

In Darwin's evolutionary footsteps

Peter and Rosemary Grant landed in 1973 on the tiny uninhabited island of Daphne Major, the cinder cone of an extinct volcano. Daphne is as steep as a roof, with cliffs running all around the base, and just one small spot on the outer slope flat enough to pitch a tent.

Their goal, as they relate in their new book, 40 Years of Evolution, was to study finches in the genus Geospiza – the birds that gave Darwin some of his first inklings of evolution by natural selection – and to try to reconstruct part of their evolutionary history. Instead, they made an amazing discovery.

After several years of meticulous measurements, the Grants and their students realised that the finches’ dimensions were changing before their eyes. Their beaks and bodies were evolving and adapting from year to year, sometimes slowly, sometimes strikingly, generation after generation. The researchers were watching evolution in real time, evolution in the flesh.

Darwin never dreamed that was possible. In the first chapter of On the Origin of Species, he writes that while natural selection is at work everywhere and always, “we see nothing of these slow changes in progress, until the hand of time has marked the lapse of ages.”

The Grants discovered that Darwin’s process was more powerful than he thought. So they stayed on. Daphne Major turned out to be a perfect theatre for watching evolution in action — big enough to support many hundreds of finches, but small enough that the Grants and their students and assistants could band and recognise and measure almost every bird.

The Grants kept up their watch during years of downpours and years of drought — seasons of feast and famine for the finches. And Darwin’s process unfolded before their eyes in intense episodes that illustrated better than anything in the Origin the struggle for existence, and the ways that life adapts and emerges fitter from the struggle.

Strange finch

Its own origins date to 1981, when a strange finch landed on the island. He was a hybrid of the medium-beaked ground finch and the cactus finch. He had the sort of proportions that touch our protective feelings: a big head on a stout body. In other words, he was cute. They called him Big Bird.

Hybrids are not unknown among Darwin’s 13 species of finches, but they are rare. Because they evolved so recently, birds of these different species can mate but ordinarily choose not to.

Big Bird had a strange song that none of the finch watchers had ever heard. His feathers were a rich, extra-glossy black. He had more tricks in his repertory than his neighbours: He could crack the spiky, troublesome seeds of the Tribulus plant, normally the speciality of the big-beaked ground finch, as well as small seeds favoured by the small-beaked ground finch. He could dine on the nectar, pollen and seeds of the cactus, which belongs to the cactus finch.

Big Bird mated with a medium-beak on Daphne. Their offspring sang the new song of Big Bird. And slowly, Big Bird became a patriarch. He lived 13 years, a long time for one of Darwin’s finches. His children, grandchildren and great-grandchildren all sang his song, and they were clannish. They roosted in hearing distance of one another on the slopes of Daphne Major. What’s more, they bred only among their kind, generation after generation.

Long-lasting species

Big Bird’s lineage has now lasted for 30 years and seven generations. The Grants are cautious about its prospects – “It is highly unlikely that we have witnessed the origin of a long-lasting species, but not impossible,” they write. The Grants say that if the Big Birds are, in fact, a species, they would call them Geospiza strenuirostris, from the Latin word meaning strong, exceptionally vigorous and active.

But alas, there are rough times ahead in the Galapagos, as in every other place affected by global warming. In the coming decades, as the climate of these islands grows warmer and wetter, Daphne may lose its cactus. And Big Bird can’t live without cactus.

Even in 40 years, of course, you can’t expect to witness the whole story of evolution, which has been going on for nearly four billion. Still, it turns out that if you know what to look for, put yourself in the right place and keep your eyes open, you can see a lot. The Grants have won just about every award in their field, including the prestigious 2009 Kyoto Prize in Basic Science. 

Recently, I called the Grants to talk about Big Bird. They sounded the same as ever, still fascinated by the finches. I asked when the significance of the story had dawned on them. “Maybe in 2007, we really grasped what was going on,” Peter said. 

“But it was a very gradual process,” Rosemary added. “No eureka moment.” If they hadn’t stayed on, hadn’t kept watching all those years, they would never have witnessed this surprise ending. “But then,” Rosemary said, “you could almost say that anything we would see on Daphne, we didn’t expect.”

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