What would decrease genetic variation in a population? What are some examples?

A population bottleneck decreases genetic variation in a species or subspecies. A founder effect is similar. The Cheetah is an example. All cheetahs share a small number of alleles. They have less than 1% genetic variation. They are almost like twins. They had two bottlenecks, one about 10,000 years ago and again in the last 100 years.Dating the genetic bottleneck of the African cheetah. Humans are another example. Elephant seals were hunted to 20 individuals in 1890. There is no variation in 24 genes.. There are 30,000 today. California Condors. Pandas. Inbreeding of dog or cat breeds does this too.