Directional selection increased the frequency of individuals carrying the larger beaks and decreased the frequency of individuals carring small beaks.
What is directional selection?
Directional selection is a way in which natural selection acts by increasing the proportion of individuals with an extreme phenotypic trait.
This selection presents more frequently in those cases in which interactions between living organisms and the environment modify in the same direction.
How natural selection could cause the population of finches 1 to evolve over time to resemble the population of Finch 2?
In the exposed example, after the drought there were almost no seeds available for finch 1 to feed.
The introduction of a new allele to the population (by mutation or genetic flow) might have produced a few individuals with beaks similar to the one of Finch 2.
This beak was better for feeding. So individuals carrying this beak increased their fitness.
With time, the average size of the beaks was larger. The trait modification was related to the availability of only larger seeds with thick husks.
Eating large seeds with medium or small-sized beaks was impossible, so Finches needed to adapt, developing larger beaks to break bigger and harder seeds and eat the content.
Directional selection was responsible for the rapid change in the finches' beaks size after the drought.
The evolutive force modified the allelic frequencies, increasing the frequency of genetic variants that expressed the larger beak size and declining the frequency of the alleles that expressed smaller beak size.
You can learn more about directional selection at
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