Directional selection increased the frequency of individuals carrying the larger bill depths and decreased the frequency of individuals carring small and medium bill depths.
What is directional selection?
Natural selection is an evolutive force that favors beneficial alleles and increase their frequency in the population.
Directional selection is the way in which natural selection acts on a population 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.
Data on the chart
- Before the drought ⇒ Bar chart.
The chart indicates that before the drought, the average size of the bill depth was 9.5 mm. This is an intermediate phenotype between the shortest (7.5 mm) and the largest (11 mm).
So, most individuals in the population expressed the intermediate phenotype of the bill depth.
- After the drought ⇒ Line chart
The line chart indicates that, after the drought,
→ only individuals with the largest bill depth (one of the extreme phenotypes) expressed the highest survival rates.
→ Individuals with the shortest bill depth (the other extreme phenotype) expressed the lowest survival rates.
→ Individuals with the intermediate phenotype expressed an intermediate survival rate.
Directional selection favored one of the extreme phenotype -largest bill depths- over the other extreme phenotype and over the intermediate phenotype.
The frequency of individuals carrying the largest bill depths increased.
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