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Natural and Artificial Selection, Genetic Drift and Mutation

APBIO-RJ1VYC

Arabidopsis thaliana is a plant that serves as a model organism in plant biology and genetics studies. Like many plants, A. thaliana has hair-like outgrowths on its leaves and stems, called trichomes, that prevent herbivory by secreting inflammatory chemicals. Like any phenotype, trichome number is inherited and vulnerable to selective pressures from the environment.

"Vernalization." Wikipedia. Wikimedia Foundation, 24 Feb. 2004. Web. 21 Aug. 2016. Hair-like projections called trichomes are visible on each leaf of Arabidopsis thaliana.

A pair of biology students grew 175 A. thaliana wild-type plants, starting from seeds. They observed several variations in the number of trichomes on each plant, and plotted their data in Figure 1 below:

Trichome Number in Generation 1. Gray Mitchell, C. Created for Albert.io. Copyright 2016. All rights reserved.

They set out to examine how the phenotype distribution could change in a second generation if the trait for trichome number was selected for. They used a cotton swab to brush pollen from the top 10% of hairiest plants (greatest number of trichomes) and cross-pollinated the flowers in this selected population. At the end of their six-week life cycle, all the fertilized A. thaliana plants developed seeds, which the students planted, keeping the seeds from the selected population separate from the control population. When all the new seedlings were about two-weeks old, the students counted the number of trichomes on each plant and graphed their data in Figure 2 below:

Trichome Number in Generation 2. Gray Mitchell, C. Created for Albert.io. Copyright 2016. All rights reserved.

Which of the following is a plausible explanation for their results?

A

Due to pressure from natural selection, the second generation had more trichomes than the first generation.

B

Artificial selection produced a directional shift in the gene pool toward an increased number of trichomes in the second generation.

C

Genetic drift resulted in offspring that had a greater number of trichomes than their parents.

D

Spontaneous mutation created a genotype with a greater number of trichomes in the second generation, and artificial selection selected for the 'hairiest' population.