Darwin's (and Grants') Finches

Written byRicki Lewis

| 3 min read

This morning came the talk that everyone had been waiting for - Princeton professors Peter and Rosemary Grant presented their 33-year project on the adaptive radiation of Darwin's finches on the Galapagos. When they took the stage, the local media surged forward as attendees packed the room. Peter Grant began at the beginning: ?Two to 3 million years ago, an ancestral group of finches flew from the mainland to the islands at a time of great volcanic activity. They encountered an environment very different from the present one.? There were only 5 islands; others would arise from the volcanic ?hot spot? in the western part of the archipelago. The climate was warmer and wetter than today. As the birds colonized different islands, they effectively used different resources, so that, over time, the ancestral stock gave rise to two major subgroups, with 5 species occupying trees and 6 that live mainly on the ground and eat seeds. The same species inhabit different islands, but with slightly different adaptations. The Grants' life work has been to meticulously reconstruct how the original species diverged. Peter Grant vividly described and illustrated some of the remarkable adaptations of these special birds. Those with tough, large beaks can penetrate the toughest seeds, whereas their smaller-beaked cousins eat the softer prickly pear cactus. ?On one island, finches exploit the eggs of boobies for moisture and protein,? said Grant, as he showed a slide of a bird doing a backflip of sorts to kick apart an egg, which the group then devoured. ?Even more bizarre is the vampire finch. It stands on the backs of sitting boobies and pecks at the base of developing wing feathers, drinking the blood,? he said as he showed a rather graphic slide. The Grants scrutinize every aspect of being a finch on the Galapagos, atmany levels. Peter Grant described their classic studies of tracking changing beak morphology as El Ninos waxed and waned, but also talked about an ongoing collaboration with investigators at Harvard Medical School using DNA microarrays to identify candidate genes that contribute to beak development. They've homed in on one - bone morphogenetic protein 4. A well-characterized signaling molecule, BMP4 seems to be quite important in the finch. ?BMP4 is expressed earlier than in other species, at a higher level than in other species, and also over a broader spatial domain with greater differences between finches,? he said. Even though a focus on molecules can explain the developmental genetics of beak variation, it is behavior that seems to drive the evolution of the finches - in particular, birdsong. Male birds learn their species-specific songs from their fathers during a critical period in the nest. Rosemary Grant recalled one telling example of the power of song. ?In 1975, a cactus finch got a cactus spine in its throat. Instead of the normal cha cha cha song, it sang jar jar jar. The sons copied their father's song, and last year, their great great grandsons were still singing it.? She then described rare instances of misimprinting (hearing the wrong song), such as when orphan birds learn the song of a different-species neighbor by mistake. Using 14 microsatellite markers, the Grants have tracked, from 1982 to 2004, parallel morphological (beak shape) and genetic change in two species on Daphne minor, Geospiza Fortis and G. Scandens, as the former has picked up genes from the latter due to misimprintings, a very rare event. ?Since 1983, G. scandens has converged morphologically and genetically with G. fortis, but is kept apart by song. It still acts like a pure species, mating according to song type. Song is a learned, culturally transmitted trait that acts as a reproductive barrier between species,? Rosemary Grant explained. In the accompanying commentary, Trevor Price of the University of Chicago, who uses mtDNA sequences to trace the time of the finches' arrival, placed the famous tale into perspective by comparing the birds to warblers in North America, which underwent little if any divergence over thesame time period. His conclusion: The finches provide a rare window into evolution, thanks to the isolation ofthe Galapagos. ?I think the important lesson of Darwin's finches that has been neglectedis that virtually all genera on continents show signatures of adaptive radiation ofa lot of early speciation that slowed down toward the present. The only place we can study this is in Darwin's finches.?

Darwin's (and Grants') Finches

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