Figures from "Ballou, J.D., 1992. Potential contribution of cryopreserved germ plasm to the preservation of genetic diversity and conservation of endangered species in captivity. Cryobiology 29: 19-25.

"Demographic and genetic objectives of captive propagation programs for endangered species focus on establishing demographically secure populations that maintain adequate levels of genetic diversity. Long-term storage and utilisation of cryopreserved germ plasm could extend the populations generation length and allow higher levels of genetic diversity to be maintained in smaller populations".

INBREEDING DEPRESSION

Inbreeding depression is a lack of ability to adapt to environmental change or to resist or adapt to disease due to the loss of genetic diversity.

The reduced fitness of animals due to inbreeding has been shown in both captive and wild populations.

The rate of loss of genetic diversity depends on the number of generations and the effective population size. The effective population size is the number of individuals of a species contributing to the genetic makeup of offspring. Thus it does not include juveniles, or non reproducing adults. Therefore the maximum effective populations size with a certain number of individuals is when there is an equal number of males and females and all are contributing equally to genetic makeup of the offspring.

As can be seen from Fig 1 a number of above 50 males and 50 females must be maintained to even begin to preserve genetic diversity for even a small number of generations.

Fig 2 shows that even over the short period of 25 years, with an effective population of 20 individuals and a generation length of 10 years, there is a considerable loss of genetic diversity.

If the generation length is very high, as it is in the case of cryopreserved sperm at 50,000+ years, genetic diversity can be maintained indefinitely.

As the generation length reaches 5 years the number of individuals needed to maintain 90% of the genetic diversity for 200 years reaches an asymptote of infinity.