Individuals within a population vary in their traits.

Supported by:
Figure 1: Histogram of distribution of beak depth of medium ground finches

Figure 2: Relationship between beak depth of offspring and their parents

Some of these variable traits are heritable -- passed on to offspring.

Supported by:
Figure 2: Relationship between beak depth of offspring and their parents

More offspring are produced than can survive because of limited resources such as food and nesting sites.

Supported by:
Figure 3: Changes in Geospiza fortis population and seed abundance on Daphne Major, before and after the drought of 1977

Individuals with advantageous traits will survive and reproduce.

Supported by:
Figure 1: Histogram of distribution of beak depth of medium ground finches

How do you know that finches' beak depth is heritable?
You can see from Figure 2 that there is a correlation between the parents' and offsprings' beak size.

How did the finch population change from before the drought to after?
According to Figure 1, the average beak depth increased in size and the finch population had more finches with greater beak depths in 1978 than before the drought.

Why do you think the average beak depth of the birds increased?Because the drought reduced the number of seeds and finches with bigger beaks were able to eat the larger and harder seeds so more of them survived.