All forms of life on earth, whether microbes, plants, animals, or human beings, contain genes. Genetic diversity is the sum of genetic information contained in the genes of individual plants, animals, and micro-organisms. Each species is the storehouse of an immense amount of genetic information in the form of traits, characteristics, etc. The number of genes ranges from about 1000 in bacteria to more than 400 000 in many flowering plants. Each species consists of many organisms and virtually no two members of the same species are genetically identical.

An important conservation consequence of this is that even if an endangered species is saved from extinction it has probably lost some of its internal diversity. Consequently, when populations expand again, they become more genetically uniform than their ancestors. There are mathematical formulas to express a genetically effective population size that explain the genetic effects on populations that have gone through a bottleneck before expanding again such as the African cheetah or the North American bison. Subsequent inbreeding in small populations may result in:

1. reduced fertility and
2. increased susceptibility to disease.