The following curve shows the distribution of the masses of the individuals in
a population of gerbils.
Number of gerbile
Mass (grams)
Which of the curves below shows the same population after directional
selection has happened?

The graph D is the correct option.

Explanation:

In ecological studies, directional selection can be described as the event in which a single phenotype is favoured through the process of natural selection. Due to this, the allele frequency of the population shifts towards the particular extreme phenotype with time.

The Graph D shows how the population of gerbile was before and how directional selection favoured a single phenotype and caused it to become abundant.

"1. it should be, it has been for about 20 years, and it's really not much different in the end than breeding animals to have particular traits. there are always concerns about genetically engineered animals/plants affecting the environment in unexpected negative ways, but i can't think of an example where the fears didn't turn out to be baseless.  

first, as you point out, it would be a great advance in pharmacology, to be able to tailor drugs and their doses to me. the pros involve better medical treatment. the cons involve privacy issues like insurance companies denying coverage to people with genetic risk factors. looking at dna markers wouldn't affect the frequency of genetic diseases, but it would allow detection of them earlier. and ok, actually i suppose that if people used this information to avoid having children with genetic diseases, some might decrease in frequency."

what school and grade?