Which scenario describes the bottleneck effect?

The bottleneck effect specifically refers to a reduction in the size of a population due to a significant event, such as a natural disaster or other catastrophic occurrence. When such an event drastically reduces the population, the genetic diversity is also greatly diminished because only a small number of individuals are left to repopulate. This limited gene pool can lead to decreased genetic variability and may increase the chances of inbreeding and its associated problems.

In the context of this scenario, a population that is drastically reduced will largely reflect the genetic traits of only the surviving individuals, which may not represent the full diversity of the original population. The resultant population after the bottleneck is often less resilient to environmental changes and other stressors due to the lost genetic variation.

The other options describe different concepts in population genetics and evolutionary processes. For instance, the establishment of a new population by a small group relates to the founder effect, while migration refers to the movement of individuals that can increase genetic variation in a population. Natural selection involves the preservation of advantageous traits rather than a reduction of population size.