How did the finches on the Galapagos Islands evolve?
|
Print Collector / Getty Images Charles Darwin is known as the father of
evolution. When he was a young man, Darwin set out on a voyage on the HMS Beagle. The ship sailed from England in
late December of 1831 with Charles Darwin aboard as the crew's naturalist. The voyage was to take the ship around South America with many stops along the way. It was Darwin's job to study the local flora and fauna, collecting samples and making observations he could take back to Europe with him of such a diverse and tropical location. The crew made it to South America in a few short months, after a brief stop in the Canary Islands. Darwin spent most of his time
on land collecting data. They stayed for more than three years on the continent of South America before venturing on to other locations. The next celebrated stop for the HMS Beagle was the Galapagos Islands off the coast of Ecuador. Charles Darwin and the rest of the HMS Beagle crew spent only five weeks in the Galapagos Islands, but the research performed there and the species Darwin brought back to England were instrumental in the formation of a core part of the original theory of evolution and Darwin's ideas on
natural selection which he published in his first book . Darwin studied the geology of the region along with giant tortoises that were indigenous to the area. Perhaps the best known of Darwin's species he collected while on the Galapagos Islands were what are now called "Darwin's Finches".
In reality, these birds are not really part of the finch family and are thought to probably actually be some sort of blackbird or mockingbird. However, Darwin was not very familiar with birds, so he killed and preserved the specimens to take back to England with him where he could collaborate with an ornithologist. The HMS Beagle continued to sail on to as far away lands as New
Zealand before returning to England in 1836. It was back in Europe when he enlisted in the help of John Gould, a celebrated ornithologist in England. Gould was surprised to see the differences in the beaks of the birds and identified the 14 different specimens as actual different species - 12 of which were brand new species. He had not seen these species anywhere else before and concluded they were unique to the Galapagos Islands. The other, similar, birds Darwin had brought back from the South
American mainland were much more common but different than the new Galapagos species. Charles Darwin did not come up with the Theory of Evolution on this voyage. As a matter of fact, his grandfather Erasmus Darwin had already instilled the idea that species change through time in Charles. However, the Galapagos finches helped Darwin solidify his idea of natural selection. The favorable adaptations of Darwin's Finches' beaks were selected for over generations until they all branched out to make new species. These birds, although nearly identical in all other ways to mainland finches, had different beaks. Their beaks had adapted to the type of food they ate in order to fill different niches on the Galapagos Islands. Their isolation on the islands over long periods of time made them undergo speciation. Charles Darwin then began to disregard the previous thoughts on evolution put forth by Jean Baptiste Lamarck who claimed species spontaneously generated from nothingness. Darwin wrote about his travels in the book The Voyage of the Beagle and fully explored the information he gained from the Galapagos Finches in his most famous book On the Origin of Species. It was in that publication that he first discussed how species changed over time, including divergent evolution, or adaptive radiation, of the Galapagos finches. Watch Now: Profile of Charles DarwinThe differences in shape and size of beaks in Darwin’s finches illustrate ongoing evolutionary change. Key Points
Key Terms
Visible Evidence of Ongoing Evolution: Darwin’s FinchesFrom 1831 to 1836, Darwin traveled around the world, observing animals on different continents and islands. On the Galapagos Islands, Darwin observed several species of finches with unique beak shapes. He observed these finches closely resembled another finch species on the mainland of South America and that the group of species in the Galápagos formed a graded series of beak sizes and shapes, with very small differences between the most similar. Darwin imagined that the island species might be all species modified from one original mainland species. In 1860, he wrote, “seeing this gradation and diversity of structure in one small, intimately related group of birds, one might really fancy that from an original paucity of birds in this archipelago, one species had been taken and modified for different ends.” Natural SelectionDarwin called this mechanism of change natural selection. Natural selection, Darwin argued, was an inevitable outcome of three principles that operated in nature. First, the characteristics of organisms are inherited, or passed from parent to offspring. Second, more offspring are produced than are able to survive; in other words, resources for survival and reproduction are limited. The capacity for reproduction in all organisms exceeds the availability of resources to support their numbers. Thus, there is a competition for those resources in each generation. Third, offspring vary among each other in regard to their characteristics and those variations are inherited. Out of these three principles, Darwin reasoned that offspring with inherited characteristics that allow them to best compete for limited resources will survive and have more offspring than those individuals with variations that are less able to compete. Because characteristics are inherited, these traits will be better represented in the next generation. This will lead to change in populations over generations in a process that Darwin called “descent with modification,” or evolution. Studies of Natural Selection After DarwinDemonstrations of evolution by natural selection can be time consuming. Peter and Rosemary Grant and their colleagues have studied Galápagos finch populations every year since 1976 and have provided important demonstrations of the operation of natural selection. The Grants found changes from one generation to the next in the beak shapes of the medium ground finches on the Galápagos island of Daphne Major. The medium ground finch feeds on seeds. The birds have inherited variation in the bill shape with some individuals having wide, deep bills and others having thinner bills. Large-billed birds feed more efficiently on large, hard seeds, whereas smaller billed birds feed more efficiently on small, soft seeds. During 1977, a drought period altered vegetation on the island. After this period, the number of seeds declined dramatically; the decline in small, soft seeds was greater than the decline in large, hard seeds. The large-billed birds were able to survive better than the small-billed birds the following year. The year following the drought when the Grants measured beak sizes in the much-reduced population, they found that the average bill size was larger. This was clear evidence for natural selection of bill size caused by the availability of seeds. The Grants had studied the inheritance of bill sizes and knew that the surviving large-billed birds would tend to produce offspring with larger bills, so the selection would lead to evolution of bill size. Subsequent studies by the Grants have demonstrated selection on and evolution of bill size in this species in response to other changing conditions on the island. The evolution has occurred both to larger bills, as in this case, and to smaller bills when large seeds became rare.  How did Darwin's finches evolve?Evolution in Darwin's finches is characterized by rapid adaptation to an unstable and challenging environment leading to ecological diversification and speciation. This has resulted in striking diversity in their phenotypes (for instance, beak types, body size, plumage, feeding behavior and song types).
How did the finches in the Galapagos become different species?It is thought that their ancestor, and closest known relative, is the dull-coloured grassquit, which is found on mainland South America. Once the original grassquits arrived at Galapagos, they diversified and adapted to the different environments found on the Islands, eventually becoming different species.
How did finches adapted to their environment?Long, pointed beaks made some of them more fit for picking seeds out of cactus fruits. Shorter, stouter beaks served best for eating seeds found on the ground. Eventually, the immigrants evolved into 14 separate species, each with its own song, food preferences, and beak shapes.
|
