Final answer:
The graph in question shows the changes in beak sizes of Galápagos finches during a period from 1975 to 1978, highlighting the increase in average beak size due to natural selection following a drought that reduced small seeds.
Step-by-step explanation:
The subject matter of this question concerns the population size and evolution of Galápagos finches over a period between 1975 and 1978, more specifically changes in beak size as a result of natural selection. This is shown through graphs that express the shift in beak size from the entire population pre-drought in 1976 to the survivors post-drought in 1978. The information indicates that during the drought of 1977, the availability of small seeds decreased, leading to the death of many small-beaked finches and an increase in the population's average beak size since larger-beaked finches could eat both small and large seeds. Therefore, the graph is most accurately described as showing the population size of finches between 1975-1978 as it relates to evolutionary changes in beak size resulting from the drought on the Galápagos island.