Mutation and Genetic Variation
The Galapagos finches evolved from a common ancestor millions of years ago. The evolution into many different species with varying beak sizes, body sizes, songs, and feeding behaviors occurred due to genetic variation introduced through mutations. The shape and size of the finches' beaks are continuously evolving, helping the birds to adapt to their environments. The diagram shows the different species of Galapagos finches.
Peter and Rosemary Grant studied medium ground finches (Geospiza fortis) on the Galapagos island of Daphne Major for over 40 years. Daphne Major was chosen to study the medium ground finches because it had not been inhabited by humans, and contained few predators and little competition with other species of finch. Medium ground finches eat mainly small seeds, insects, and fruits. Each year, the Grants returned to the island and made observations of the finches' beak size, weather, and food supply. In 1977, a drought killed many plants, causing a shortage of small, tender seeds. After all of the tender seeds were gone, large, tough seeds were the only available food source. In 1982, a population of a large ground finch species came to the island and began to colonize. Large ground finches eat mainly large seeds, fruits, caterpillars, and large insects. In 1984, heavy rains on the island resulted in an abundance of small, soft seeds and a shortage of large, tough seeds. A drought in 2003 created another shortage of large seeds. The diagram shows the average beak size of the medium ground finch from 1973 to 2012.
The line-graph shows beak depth in millimeters along the vertical axis from 8 to 10.5, in increments of 0.5. The horizontal axis shows years from 1970 to 2010, in increments of 5. The line starts at 9.5 millimeters around 1973 and drops to slightly more than 9 millimeters in 1975. The line then rises sharply to more than 9.5 millimeters around 1978 and continues until just after 1985 when it drops to 9.5 millimeters. The line continues to drop to a little less than 9.5 millimeters around 1988 and drops again to around 9 millimeters in 1995. The line then remains level until around 2003 when it rises to about 9.25 millimeters. Just before 2005, the line sharply declines to just above 8.5 millimeters and remains level from then on. Significant periods are shown on the graph at 1977 to show a drought, at 1984 to show heavy rainfall, and at 2003 to show a drought.
