What causes many endangered species that now found only in zoos to have very little genetic variation?

The primary cause of reduced genetic variation in endangered species now found only in zoos is the phenomenon known as the founder effect. The founder effect occurs when a small group of individuals becomes isolated from a larger population and establishes a new population. This small group may not represent the full genetic diversity of the original population, leading to a loss of genetic variation in the new population.

Endangered species that are now found only in zoos often originate from small, isolated populations that have been reduced to critically low numbers due to various factors such as habitat loss, hunting, and over-exploitation. When these species are brought into zoos for conservation and breeding purposes, they may carry only a fraction of the genetic diversity that existed in the wild population.

As these species reproduce in captivity, the limited genetic variation becomes further concentrated, resulting in a loss of alleles and reduced genetic diversity. Over time, this can have detrimental effects on the species' ability to adapt to changing environmental conditions and increased susceptibility to diseases, making them more vulnerable to extinction.

To mitigate the effects of the founder effect and preserve genetic diversity, conservation programs in zoos often implement strategies such as selective breeding, population management, and genetic rescue. These measures aim to introduce new genetic material into captive populations, maintain genetic diversity, and reduce the risk of inbreeding depression and other genetic problems that can arise from low genetic variation.