What is the bottleneck effect?

The bottleneck effect refers to a significant reduction in a population's size, which often occurs due to events such as natural disasters, habitat destruction, or other catastrophic events. This sharp decrease in population not only leads to a dramatic loss of individuals but also results in a smaller gene pool since only a limited number of individuals are left to repopulate. As a consequence, the genetic diversity of the population is reduced, making it more vulnerable to diseases and reducing its ability to adapt to environmental changes.

In contrast, the other options do not accurately describe the bottleneck effect. For instance, introducing new genes into a population relates to gene flow or genetic mixing, which is not a result of a population size reduction. An increase in effective population size after a disaster would contradict the very nature of the bottleneck effect, which is characterized by a decrease in population size. Lastly, the shift in gene frequencies due to migration is associated with gene flow, where individuals move between populations, facilitating genetic variability rather than experiencing the genetic limitation typical of the bottleneck effect.