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Most students learn about Darwin's finches as a picture of different beaks in the NCERT evolution chapter. The picture is static. But on one small island, two biologists watched those beaks change from one generation to the next. This post explains what Peter and Rosemary Grant did, works through the logic of their key result, and shows what you can take from forty years of patient fieldwork.
Peter and Rosemary Grant at a glance
Both were born in Britain in 1936, according to their Kyoto Prize laureate pages. Peter Grant earned his PhD at the University of British Columbia in 1964 and joined Princeton University in 1985. Rosemary Grant's page lists her PhD from Uppsala University in 1985 and her later post as senior research scholar with the rank of professor at Princeton. Both are listed as professor emeritus at Princeton. They shared the 2009 Kyoto Prize in Basic Sciences, whose citation says they conducted their field study on Darwin's finches for more than thirty-five years from 1973.
What does NCERT say about Darwin's finches?
The NCERT Class 12 evolution chapter (2026-27 reprint) uses Darwin's finches as its main example of adaptive radiation: from an original seed-eating form, many kinds with altered beaks arose, some insect-eating and some plant-eating. That is evolution over long periods. The Grants showed the same process working on a time scale you can watch.
What happened in the 1977 drought?
In their 2009 Kyoto Prize lecture, the Grants describe the turning point. In 1977 almost no rain fell on Daphne Major, and 85% of the medium ground finch population died of starvation. Birds did not survive at random. Large birds, especially those with large beaks, survived better, because they could crack the large, hard seeds that were left after the small, soft seeds had been eaten.
When rain returned in 1978, the survivors' offspring were large like their parents. Beak size was inherited, so the population had changed. A 1981 paper by Peter Boag and Peter Grant in Science, Intense natural selection in a population of Darwin's finches, reported that the selection intensities were the highest then recorded for a vertebrate population.
A worked example: follow the logic with sample numbers
The numbers below are made up for practice. They are not the Grants' measurements.
Suppose a sample population of 1,000 medium ground finches has an average beak depth of 9.0 mm.
A drought kills 85% of birds: 1,000 × 0.85 = 850 die, so 150 survive.
Large-beaked birds survive more often, so suppose the 150 survivors average 9.5 mm. The difference between survivors and the whole population is 9.5 − 9.0 = 0.5 mm.
Offspring do not inherit all of that difference, because beak size depends partly on genes and partly on environment. If, say, 70% of the variation is inherited, the next generation's average shifts by about 0.7 × 0.5 = 0.35 mm, to roughly 9.35 mm.
This is how biologists predict a response to selection. The Grants' work mattered because they measured each of these quantities in a wild population rather than assuming them.
Did selection always push the same way?
No, and that is the most important lesson. The 1983 El Niño, which their lecture calls the strongest in 400 years, brought eight months of rain and changed the plants on the island to ones producing small seeds. In a drought two years later, small-beaked birds survived better.
Then, in 1983, a larger species, the large ground finch, settled on Daphne Major. By the 2003-04 drought there were more than 300 of them. Medium ground finches crashed again, with 90% dying. This time the medium ground finches with the largest beaks were outcompeted for the big seeds, and small-beaked birds survived better. The Grants' 2006 paper, Evolution of character displacement in Darwin's finches, reported the change 22 years after the competitor arrived, the strongest evolutionary response in 33 years of study. A 2016 genomic study found that a region containing the gene HMGA2 was involved, with a selection coefficient of 0.59 against large-beak genotypes in medium ground finches.
Looking back over 30 years, the Grants wrote in a 2002 Science paper that evolution can be predicted in the short term but not in the long term, because environments fluctuate unpredictably.
Common mistakes in exam answers
Saying birds grew bigger beaks to cope. Individuals did not change; survivors already had larger beaks.
Treating one direction as permanent. Selection reversed when conditions changed.
Mixing up Darwin's observation and the Grants' measurements. Darwin described the diversity; the Grants measured selection in action.
Our guide to evolution for NEET covers the rest of the chapter, and our post on Bernard Kettlewell's peppered moths gives a second worked example of selection.
What can a student learn from the Grants?
They went back to the same island year after year, measured the same traits, and let rare events such as droughts and floods do the experimenting. Long, steady effort let them see what a short study would miss. Your revision works the same way: small, regular sessions show you patterns in your own mistakes that a single cram never reveals. If you are not sure where to begin, Fermi's AI tutor works out what you already know and builds a plan for the session.
Try this: write the three conditions for natural selection, then show how the 1977 drought met each one.
