Final answer:
In an environment with a scarcity of small seeds and an abundance of large tough seeds, natural selection would favor sparrows with large, broad beaks. Over time, the population would evolve with more individuals having these traits, similar to the Galápagos finches' beak size changes observed by the Grants.
Step-by-step explanation:
The question regarding the sparrow population pertains to the subject of Biology, specifically within the field of evolutionary biology and natural selection. Based on the provided information, we can make predictions about the sparrow population in response to environmental changes. During years of drought, where large seeds with hard cases become the main available food source, natural selection would favor birds with large, broad beaks that are capable of eating these tough seeds. Over time, we would expect the population to evolve such that a higher percentage of sparrows have these larger beaks.
This pattern of natural selection is similar to the well-known case of the finches studied by the Grants in the Galápagos. In that scenario, a drought led to birds with bigger beaks surviving and reproducing at higher rates, causing an increase in average beak size within just a couple of years. The principle at work here is that individuals with traits better suited to the environment will survive and reproduce more effectively, leading to a change in the genetic makeup of the population over generations. In the case of the sparrows, if the drought continues and the population remains isolated, we would expect a rise in the number of birds with traits favorable for surviving these conditions, resulting in evolutionary changes to the population's beak size distribution.