Because the drought reduced the number of seeds and finches with bigger beaks were able to eat the larger and harder seeds so more of them survived.
How did the population of medium ground finches on the island of Daphne Major change as a result of environmental changes? drought occurred-> decrease in food supply->competition between finches-> loss of 80% medium finches.
As different populations of finches occupied these niches, they evolved adaptations that enabled them to survive in the different habitats. Thus, in a relatively short period of time, many different species of finches evolved from a single ancestral population, a process called adaptive radiation.
1: Finches of Daphne Major: A drought on the Galápagos island of Daphne Major in 1977 reduced the number of small seeds available to finches, causing many of the small-beaked finches to die. This caused an increase in the finches' average beak size between 1976 and 1978.
During 1977 there was a major drought on Daphne Major and many of the plants on the island produced few or no seeds. The medium ground finch population, which depends on seeds for food, declined drastically from about 1400 individuals to a few hundred in just over two years.
Key factors in their evolutionary diversification are environmental change, natural selection, and cultural evolution. A long-term study of finch populations on the island of Daphne Major has revealed that evolution occurs by natural selection when the finches' food supply changes during droughts.
The Grants found that the offspring of the birds that survived the 1977 drought tended to be larger, with bigger beaks. So the adaptation to a changed environment led to a larger-beaked finch population in the following generation.
These four factors that affect evolution are natural selection, mutation, genetic drift and gene flow.
Ongoing field studies have documented rapid changes in these birds' beak sizes and shapes in response to sudden environmental variations -- drought, or human disturbances, for example -- yet very few genetic changes have been found that accompany those physical differences between finch species, nor between populations ...
Medium ground finches with smaller beaks, however, survived a severe two-year drought better than medium ground finches with larger beaks, the researchers report in the journal Science.
In the summer of 1976, there were 751 finches on Daphne Major when the Grants left the island. The 1976 medium ground finch population had an average beak depth of 9.65 mm and an average beak length of 10.71 mm.
On the Galapagos Islands, Darwin also saw several different types of finch, a different species on each island. He noticed that each finch species had a different type of beak, depending on the food available on its island. The finches that ate large nuts had strong beaks for breaking the nuts open.