Bottleneck effect occurs when a population suddenly gets much smaller.
Genetic variation can be reduced by random genetic drift, population bottlenecks, founder effect.
Genetic variation can be reduced when a population suddenly gets much smaller.
What can be reduced when a population suddenly gets much smaller?

[A] Pollution
[B] The size of the Earth
[C] Greenhouse gasses
[D] The size of bottlenecks
[E] The size of the environment
[F] Genetic variation
[G] Plant growth is reduced
[H] Quality of life