Due to their volcanic origin and isolation, the islands originally had no creatures at all. Whatever creatures eventually arrived divided up the resources among themselves. Some 500,000 years ago a tropical storm blew small birds more than 1,000 kilometers across the Pacific Ocean from mainland South America to the Galapagos Islands. Under typical conditions, the differences between birds in, for example, bill size and shape have little effect on their ability to survive. But during difficult or unusual times, such as during a drought, the differences in birds' seed-cracking ability and in the number of seeds they can get per day can be the difference between death and survival.
Evolution refers to effects of selection over generations of individuals.
For instance, if finches of a certain bill size survive and produce more young than do individuals with a different bill size, then those with the first bill size will become more common than those with the second. As with most explorations in the curriculum, the students' work is based on real data collected by professional researchers. Students discover what happened to one species of finch on the island during the drought of 1977. They learn that food availability plays a crucial role in the survival of organisms and that survival may depend on very small heritable differences in size or shape. In the case of this finch species, the difference between survival and death during the drought is a difference of 0.5 mm in bill depth.