Home: Scrying: For the love of species I: Meet the finches

For the love of species I: Meet the finches

February 12, 2006

Tomorrow, I’ll offer my definition of “species” and go into more detail on the processes that cause a species to happen, but for now, here is a (the) classic example: The finches of the Galapagos Islands. Even since Darwin published The Origin of Species, these little finches have been adapting their way into the hearts of professional and amateur biologists across the world. Here, I show how speciation occurred among the finches, as Darwin described. Also, I look at a study by Peter and Rosemary Grant in the 1970s that supported Darwin’s theory, providing evidence of natural selection and possibly speciation.

The woodpecker finch of the Galapagos island uses a twig or cactus spine as a tool to fish for bugs.Speciation and Darwin’s Finches

The Galapagos Islands, situated far from any continent and thus free of any large predators, were the perfect setting for speciation. At some point, a single species of finch flew to the relatively fresh volcanic islands, and settled in. Without any competition from other birds or other animals, the finches prospered. Throughout the islands, a variety of food sources offered a diversity of habitats for the finches. Through adaptive radiation, these niches were soon filled with finches.

The original species of bird most likely ate small insects in trees. When random mutations led to differences in beaks, some finches were able to sample new diets. Quickly (as finches will happily produce four babies a month in pleasant tropical conditions) different finches had found ways to eat fruit and buds from trees, seeds on the ground, or even pry bugs from a cactus with a stick.

Information on the geology of the Galapagos IslandsAs different groups of finches filled different niches, their offspring became increasingly different. Eventually, they were unable to reproduce with each other, and distinct groups became distinct species. It could be that now, after millennia of isolated habitats and evolved differences, that the cactus-spine wielding Cactospiza pallida would be horrified to know that he was a distant cousin of the broad, black-billed Geospiza magnirostris.

Darwin theorized in the 19th century that environmental pressures led to diversity and distinct species of finches in the Galapagos Islands. Later, in 1973, Peter and Rosemary Grant of Princeton University sought to prove Darwin’s idea, by measuring the correlation between beak size in the ground finch Geospiza fortis and the level of precipitation.

The Grants hypothesized that during dry weather, larger, tougher seeds (left over from previous seasons) would be more readily available, and so finches with larger, tougher beaks would be more likely to survive. When the drought ended, smaller-beaked finches would again have an opportunity to survive. If the finches were to pass their beak size on to their offspring (as the Grants found was the case) several dry years could lead to deeper beaks among greater numbers of finches.

The Grants’ experiments supported their hypothesis well. This provided an excellent example of natural selection for the classic finches—but does it indicate speciation? The Grants showed that beak size in finches related to the environment, but the changes occurred amongst a single species of finch. Without any isolation or separation among the group, the experiment shows only directional selection.

If further environmental change were to lead to disruptive selection, where both the small-beaked finches and large-beaked finches could survive successfully, and meanwhile lose preference for those different, they might eventually experience further speciation.

Notes: Galapagos volcano photo from Volcano World; Woodpecker finch (Cactospiza pallida) photo from PBS.org’s The Life of Birds; Geologic information via Galapagos Geology and Dive and Discover: Expeditions to the Sea Floor; Expedition 6; Galapagos Rift Valley