By measuring many birds every year, the Grants were able to assemble a detailed portrait of natural selection in action. In the study shown in Figure 3, they measured beak depth from 1976 to 1978. In the wet year of 1976, the plants of Daphne Major produced an abundance of the small, tender seeds that these finches could easily eat. However, a severe drought occurred in 1977. During this year, the plants on Daphne Major tended to produce few of the small seeds, which the finches rapidly consumed. Therefore, the finches resorted to eating larger, drier seeds, which were harder to crush. As a result, birds with larger beaks were more likely to survive and reproduce, because they were better at breaking open these large seeds. As shown in the data, the average beak depth of birds in the population increased substantially, from 8.8 mm in pre-drought offspring to 9.8 mm in post-drought offspring. How did we explain these results? According to evolutionary theory, birds with larger beaks were more likely to survive and pass this trait on to their offsprings. Overall, these results illustrate the power of natural selection to alter the features of a trait - in this case, beak depth-in a given population.