Final answer:
Peter and Rosemary Grant demonstrated natural selection in finches on Daphne Major by studying changes in beak sizes that corresponded to environmental changes. During drought, large-billed birds survived better and reproduced, leading to increased average beak sizes. Conversely, during periods of abundant small seeds, smaller-beaked birds flourished, leading to a decrease in average beak sizes over time.
Step-by-step explanation:
How Peter and Rosemary Grant Showed Natural Selection in Finch Beak Size
Peter and Rosemary Grant demonstrated the impacts of natural selection on the beak size of finches on Daphne Major in the Galápagos by conducting long-term studies that documented changes in beak sizes across generations. They found that medium ground finches exhibited inherited variation in bill shape with some having wide, deep bills and others having thinner bills. During a severe drought in 1977, the island's vegetation changed, resulting in a decline in the number of small, soft seeds but less so in large, hard seeds. This environmental pressure meant that large-billed birds could survive better by efficiently feeding on the available large seeds. Subsequently, the Grants observed an increase in average beak size as these large-billed birds had a survival advantage and produced offspring with similarly larger bills.
In contrast, a period with abnormally high rainfall, associated with an El Niño event, led to the abundance of small, soft seeds. As a result, the population of small-billed birds grew, thereby over time, the average beak size in the population shifted towards smaller dimensions. This naturally occurring experiment confirmed that beak size evolution is driven by the available food sources and that the characteristics of surviving birds get passed on to future generations, fitting the Darwinian model of evolution by natural selection.
Furthermore, they documented similar patterns of natural selection and evolution in finch populations responding to changing environmental conditions, showing that beak size adaptations are not unidirectional but can evolve in response to the nature of food resources. Such insights underscore the dynamic process of evolution as a response to environmental pressures over time.