Final answer:
The two scenarios where a population is sharply reduced in size are the bottleneck effect, often caused by natural disasters affecting the genetic structure, and the founder effect, where a new colony is founded by a small group from a larger population with a different genetic makeup.
Step-by-step explanation:
The scenarios in which a population is sharply reduced in size are the bottleneck effect and the founder effect. The bottleneck effect occurs when a large portion of a population is wiped out due to a natural disaster or other catastrophic event, resulting in a sudden change in the population's genetic structure. For instance, a natural disaster like an earthquake or a hurricane could randomly kill a significant portion of a population, leaving a random genetic sample as survivors. This trimmed-down genetic pool becomes representative of the entire population, which might be considerably different from the pre-disaster genetic structure. The founder effect, on the other hand, arises when a small group of individuals splits off from a larger population to start a new colony. The genetic makeup of this small group may differ considerably from the larger population they left, significantly influencing the genetic structure of the new colony. An example of this is the genetic history of the Afrikaner population of Dutch settlers in South Africa, where frequencies of certain mutations are higher due to the genes present in the founding individuals.