September 25, 2005

Darwin's Finches

A few years ago I took a trip to the Galapagos, the enchanting archipelago that had captured Charles Darwin’s imagination. Having looked for a tour that specialized in birding, I joined other birders who thought a trek to birder’s paradise was a required once-in-a-lifetime journey for any real birder who could afford the trip. And what a paradise the Galapagos was with the insanely goofy and cute blue-footed boobies, the magnificent frigate birds, the Galapagos penguins and even those fantastic iguanas: both land and marine.

What an incredible experience to sail from isle to isle following in Darwin’s footsteps as he described the variations he observed: such as how the prickly pear cactus has adapted to the predators on different islands. On one island the lack of predators has caused the prickly pear cactus to have thorns as soft as cat whiskers and on others, the cactus encases its lower trunk with a scab of hardened armor and raises its first arms to four or more feet off the ground to stymie the huge land tortoises and the ravenous land iguanas. And then there is the incredible variety of short-tailed finches bearing beaks that tell of subtle and not so subtle differences in their lives. It was a journey that I’d prepared for by reading The Beak of the Finch: A Story of Evolution in Our Times, by Jonathan Weiner.

Jonathan Weiner won the 1995 Pulitzer Prize for Non-Fiction for this remarkable work. He told the story of Rosemary and Peter Grant who chose the Galapagos Islands to study the finches that fascinated Darwin and which provided that 19th century scientist with clues leading to his definitive work: Origin of the Species. This is also a story about Charles Darwin, who as a young man boarded the Beagle as the ship’s naturalist and who then collects enough observations on the journey around the world to spend the rest of his life filling out the science.

Weiner uses the story of the Grants and their studies in the Galapagos to discuss Darwin and his work and he uses their story to round out what we know about evolution with the surprising discoveries the Grants made during their decades long studies there. At Galapagos, Darwin had found 13 different types of finches which obviously shared a common root. When he got back to England, he found that all of these finches were distinct species, not just sub-populations. And he wondered, how did they become separate species? This was a key piece in his formulating the theory of evolution.

The Grants have been studying the Galapagos finches for 30 years now, having started their first field observations in the early 1970s. What they wanted to find out was why the beaks of the medium beaked finches on one island were a different size than those of the medium beaked finches on other nearby islands. They setup a grand experiment to see why this was so and to see if they could explain the variations within species between islands.

GalapagosIs.gifThey chose Daphne Major, an uninhabited island made from a single volcanic cone covered with desert scrub with no fresh water source beyond the rain. Yet, it characterized the Galapagos terrain. It is a wind-swept island with no natural landing spot, but only a rough jump and scramble up the rocks from a bouncing boat. On my trip to Daphne Major, I wondered at the fortitude of the scientists who spent months on that spec of land.

The Grants and their assistants captured and banded practically every finch on the small island of Daphne Major and hundreds on the nearby island of Santa Cruz. For each breeding season they banded the nestlings, so they created a historical record of the finches over generations. And they measured what the environment was like: the types of plant cover, the seed abundance and seed diversity, the rainfall, etc.

For seven years, while the Grants observed the finches the seasons were mostly dry with a brief rainy season which marked the breeding season for the birds. Year after subsequent year, the sparsity of the rainfall caused the plant cover to change as plants bearing the seeds the birds depended on started to fail. Eventually, only the hardest seeds that had been ignored when other seeds were more plentiful were left. And only the birds with the largest beaks could eat those hard seeds.

What the Grants observed was that the average size of the beaks got steadily bigger every year. And they watched the females select for their mates the birds with the biggest beaks in order to pass down that characteristic to their offspring. Eventually, the Grants thought they might have gathered enough data to go back to their labs, after one last breeding season.

daphne-major.jpgBut that year, the rains never came and so no breeding season happened. The lack of rain put tremendous pressure on the population. And as the drought dragged on, only a few of the hardiest birds with the largest beaks survived. Meanwhile the scientists stayed and watched the decimation of the birds they had come to know.

But this was not the end of the story. As Weiner wrote,

“Evolution discloses a meaning in death, although the meaning is like some of the berries that Darwin tasted in the Galapagos, ‘acid & Austere.’ There is a special providence in the fall of a sparrow. Even Drought bears fruit. Even death is a seed.”

The drought ended in a sudden abundance of rain. The la nina which had caused the drought gave way to the monumental el nino of 1981-82 with huge storms resulting in an enormous spurt of growth on the once-parched land. And the birds responded in a frenzy of mating and nesting to make up for lost time. In fact, what the scientists observed during that year was that some of the birds produced up to seven successful nests in what would normally be a single breeding period.

Shortly there after, the beaks of the finches began to shrink. The adaptation that had made the large beak so important became a detriment as the seeds that were so plentiful now were better eaten by smaller beaked birds. The female birds began selecting mates with smaller beaks leading to offspring also with smaller beaks. This was natural selection in action.

What the Grants discovered was that evolution isn’t something that only happens over long periods of time, but it is something that can vary over even a short period of years. In fact, they found that the changes that forced the speciation of the finches was actually an oscillating cycle that only over time makes enough of a difference to register in the fossil record. They’ve shown that evolution is something that can be observed. It is a remarkable story and Weiner’s book is a wonderful way to learn more about this fascinating science.

[Ed: this is another of my articles which was first published in the August 2005 issue of the Vox Populi Nebraska ezine.]

Posted by Mary at September 25, 2005 10:03 PM | Environment | Technorati links |
Comments

Amen, brother...em, sister.

Posted by: degustibus at September 26, 2005 01:53 PM