What is the bottleneck effect?

The bottleneck effect refers to a significant reduction in the size of a population due to environmental events or human activities (such as natural disasters, habitat loss, or overhunting), leading to a decrease in genetic diversity. When a population is dramatically reduced, only a small number of individuals contribute their genes to the next generation. This can result in a change in allele frequencies because the genetic makeup of the surviving population may not reflect the genetic diversity of the original population. The loss of individuals not only reduces the population size but also limits the genetic pool, which can affect the population's ability to adapt to environmental changes and can lead to increased inbreeding and susceptibility to diseases.

In contrast, the other options present scenarios that do not align with the specific definition of the bottleneck effect. Population expansion typically leads to an increase in genetic variation if the growth occurs from a diverse gene pool. Random mating involves the mixing of alleles in a population without external influences that alter gene frequencies in one direction or another. Changes in allele frequency due to immigration relate to the introduction of new alleles from outside populations, rather than the loss of alleles that characterizes the bottleneck effect. This emphasis on a sharp reduction in population size is what distinguishes the bottleneck