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ally, 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.
But 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 niņa that caused the drought gave way to the monumental el niņo 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 now plentiful seeds were better eaten by smaller beaked birds. The female birds began selecting mates with smaller beaks leading to smaller beaked offspring. This was natural selection in action and a reverse of what had happened before.
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
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