Peter and Rosemary Grant, the celebrated Princeton University biologists who have spent decades expanding on Charles Darwin's work in the Galápagos Islands, have won a lucrative scientific award called the Kyoto Prize.

The honor, worth about $500,000, is to be announced today by the Inamori Foundation, a 25-year-old philanthropic organization founded by Japanese businessman Kazuo Inamori. As of yesterday, not even the Grants knew they had won, according to a university official who was sworn to secrecy.

The husband-and-wife duo are renowned in the scientific community for their work in the field of evolution, an area of science much in the news this year on the occasion of Darwin's 200th birthday. To the general public, the Grants are perhaps best known as the main characters in The Beak of the Finch, the Pulitzer Prize-winning book by Jonathan Weiner.

Both 72, the Grants have been traveling regularly since 1973 to the Galápagos, the remote islands west of Ecuador. There, they have painstakingly recorded the characteristics of numerous varieties of finches - including, as the book title suggests, the dimensions of the birds' beaks.

From these measurements they have told the story of how the process of evolution unfolds, publishing in the world's leading scientific journals.

As the weather varied from year to year, causing sharp changes in the types of seeds and other food available, birds with different beak sizes were more or less able to eat the food at hand.

Those that could eat survived to reproduce - and over the centuries, the natural variations in beaks became some of the characteristics that distinguish one species from another.

In an Inquirer interview in 2005, the Grants said they had at times engaged in spirited debates about their work, but that the differences had grown less frequent.

"Over the years," Peter Grant said then, "you get into thinking in a similar way."

The Kyoto Prize is awarded in three categories. The Grants have won for basic science. Isamu Akasaki, a semiconductor researcher, is winning in the advanced-technology category. Composer-conductor Pierre Boulez is being honored in the arts and philosophy category. The prizes will be presented in Japan in November.

Contact staff writer Tom Avril

at 215-854-2430 or tavril@phillynews.com.