The Bottleneck effect occurs when a population is greatly reduced for one or more generations, followed by a rapid and massive population increase. When the population shrinks, genetic diversity is lost as alleles are missing and misrepresented in the small populations. As the population rebounds, it becomes homogenous as few individuals are producing offspring, lowering gene representation and genetic diversity. Population bottlenecks are included in genetic drift. Hawaiian flies are a common example of the bottleneck effect.
The bottleneck effect has a negative effect on biodiversity because of its quick onset. Gradual population size changes often increase biodiversity, contrarily.
Nei, M., Maruyama, T., & Chakraborty, R. (1975). The bottleneck effect and genetic variability in populations. Evolution, 29(1).