Final answer:
Directional selection can be positive or negative, depending on the favored phenotype. In Galapagos finches, positive directional selection is demonstrated by the shift towards smaller bill sizes in response to an abundance of small soft seeds, while negative directional selection is demonstrated by the shift towards larger bill sizes in response to a scarcity of large hard seeds.
Step-by-step explanation:
Directional selection occurs when one extreme phenotype is favored over the other in response to a changing environment. Positive directional selection refers to the shift towards a phenotype at one end of the spectrum, while negative directional selection refers to the shift towards a phenotype at the opposite end.
An example of positive directional selection associated with Galapagos finches is when smaller-billed birds had higher survival and reproduction rates during a period of abundance of small soft seeds. This led to a shift in the population towards smaller bill sizes. On the other hand, an example of negative directional selection is when larger-billed birds had higher survival and reproduction rates during a period of scarcity of large hard seeds, leading to a shift towards larger bill sizes.