Peter e Rosemary Grant
2005 Balzan Prize for Population Biology
The 14 species of finches that inhabit the Galápagos Islands were amongst the key inspirations for Charles Darwin in his elucidation of the now universally accepted mechanism of evolutionary change: natural selection. On his journey on the HMS Beagle, Darwin visited the Galápagos archipelago and he inferred that the species of finches he observed had affinities with species on the mainland of South America. He hypothesised that the different species with different beaks had arisen through adaptation to local ecological conditions on different islands as a result of natural selection. Evidence from other species suggested that geographical separation of each island population was an important requirement for their divergence. This process of evolutionary change in the formation of new species became the central thesis of Darwin’s 1859 book On the Origin of Species.
The Grants have brought Darwin’s work into the 21st century, by an extraordinary long-term study that has combined the tools and intellectual frameworks of ecology, behaviour, genetics and evolution. Their findings have essentially confirmed and expanded Darwin’s hypothesis, but have placed it in a contemporary idiom by analysing the underlying mechanisms.
First, they have shown how rapid the action of natural selection can be. They discovered that when the food supply of the birds changes from year to year, as a result of climatic variation from El Niño events, there is intense natural selection resulting in changes in beak size and shape as well as body size. In years when the seeds are harder, birds with bigger, stronger beaks are at a selective advantage and in other years, the opposite is true. Two remarkable aspects of this finding are: a) the amount of genetic variation for these ecologically important traits (beak and body shape) in natural populations is substantial, and b) features of these populations can change very rapidly. Both of these discoveries were a major surprise to evolutionary biologists at the time.
Second, the Grants have shown the mechanism by which different populations of the Galápagos finch may become reproductively isolated, and hence eventually evolve into new species. They discovered that, in choosing a mate, female Galápagos finches discriminate between males on the basis of their songs. Both the song itself and the preference for a particular song are passed from parent to offspring through learning. Songs are culturally transmitted from father to son. This process ensures that females mate only with males of similar background, hence establishing reproductive isolation between groups, an essential ingredient of speciation. Reproductive isolation between Galápagos finch species is not, however, complete. Occasionally hybrids are produced and these are sometimes, but not always, at a selective disadvantage. When successful they help transfer genes among species, thus maintaining if not increasing genetic diversity among populations.
Third, the Grants have used variation in mitochondrial DNA and microsatellite regions of nuclear DNA to show that the 14 species of finch have indeed evolved from a common ancestor that arrived in the Galápagos 2-3 million years ago.
They have also been able to identify a gene, Bmp4, which influences the development of beak shape and therefore could have played an important role in the evolutionary divergence of the Galápagos finches.
The Grants’ work on Galápagos finches is universally recognised as perhaps the most significant field study of evolution in action of recent decades. It is cited in all the major textbooks on evolution and has been the subject of a popular science book The Beak of the Finch: A Story of Evolution in Our Time by Jonathan Weiner (1994, Alfred Knopf, New York).
The Grants are outstanding scientists in their field and worthy recipients of the Balzan Prize for Population Biology.