Survival In Harsh Surroundings
The first animal settlers probably did not find ideal living conditions on the islands, and most of them did not survive. They either arrived before their food supply was established or were wiped out during periods of severe weather. Survival into succeeding generations was equally chancy. A male and a female tortoise, for example, had to land on the same island and find each other for offspring to follow. For much of the life on the islands, it has been “survival of the lucky.”
This harsh environment was an ideal place for Darwin to study evolution in isolation. By the time the Beagle reached the Galapagos in 1835, near the end of a mapping expedition of South America and selected Pacific Islands, Darwin had become familiar with many South American plant and animal species. More than the similarities, it was the differences in related species on the Galapagos that puzzled him.
Darwin was particularly interested in the Galapagos finches. He noted thirteen distinct species on the four islands that he visited. The species varied in size, but otherwise were very much alike, with one notable exception—their beak structure. The size and shape of a bird's beak depends on what it eats. For example, finches with long pointed beaks feed on cactus flowers. Those with shorter but heavier beaks use them to break up seeds. Other shapes serve finches that dine on insect larva. Over time, the finches' beaks adapted to their food needs through the process of natural selection and evolution.
Since Darwin's time, many scientists have visited the Galapagos to study its wealth of plant and animal life and observe natural selection in action. One of the most exhaustive studies of Galapagos finches (which everyone now calls Darwin's finches) was made by biologists Peter and Rosemary Grant, of Princeton University in New Jersey, and their associates. Beginning in 1973, the British-born Grants followed more than 20 generations of the birds. By the mid-1990's, the research team had caught, measured, and leg-banded about 20,000 finches.
Through these long-range studies, the scientists observed the effects of extreme climatic changes on the birds. For example, in 1977, the normal wet season turned out to be a period of drought. By the end of the year, some 1,400 of one of the most common species of finches on the island of Daphne Major had been reduced to fewer than 300.
During the drought, the seed-eating birds ate all of the small, soft seeds that were available, until only larger, harder ones were left. The biggest finches with the largest, strongest beaks were the only ones able to crack open and eat those seeds. Consequently, more large finches than small ones survived the drought. In addition, many more males than females survived. The females who lost their mates, for the most part, chose males with larger bodies and larger beaks. So succeeding generations of finches were larger and had larger beaks. Rarely had such a dramatic change in a species been observed in so short a time.