Reduction of genetic diversity is a much higher risk to small populations than it is for big ones. Inbreeding and genetic drift are lurking beneath the surface and will in the case of their occurrence weaken the population’s adaptability to fluctuating environmental conditions and debilitate its chances of survival as a species in the long-term. A population consisting of many individuals has a better chance to withstand climatic calamities.
An example: takhi, that are in good condition and have thick healthy coats are most likely to survive a severe winter season than those without such a protective cover against the freezing cold. To put it bluntly: only those animals that dispose of certain inborn qualities will be fit to pass these qualities on to their progeny. Yet there are other factors which may put a small sub population to the test in the wild. Disease, drought and other calamities will affect a part of the population.
Diversity of the gene pool must guarantee the survival of at least a sufficient number of individuals for preserving the population. Genetic diversity withers away in case of inbreeding. Inbreeding is likely to occur sooner in sub populations which parentage can be retraced to only a few ancestors.
The publications of Jan Bouman and Hil Bos (1977 and 1978) show us that inbreeding can have an unfavourable effect on the fertility of individual Przewalski horses. Above that, the practice of inbreeding will also stimulate the proliferation of certain hereditary defects, such as underdeveloped ovaries.
In order to keep the genetic risks at a more or less acceptable low, the Foundation carried out a breeding strategy in its semireserves starting from the principle that the various breeding groups should consist of individuals with low inbreeding coefficients and bearing no close relationship whatsoever. The Przewalski horses that were sent to Hustai National Park have a broad genetic basis (I. Bouman, 1998).
Now that they all have been released into the wild, they succeed in managing their own affairs, which also means that they take the partners of their own choice. Gradually the number of free roaming harems increases. Several wild born stallions did manage to seize the harems of some imported stallions. Twenty years after their reintroduction only 25% of the original founder population of the 84 imported horses was still alive.
The inbreeding is low and the genetic variety broad in this building phase of the population.
A good illustration of what the founder effect is, can be found in the fox population in the dune reserve of the Dutch province of North Holland. A considerable part of the population has a very characteristic bodily feature: rather short mandibles. Before 1968 foxes had not been endemic in these western parts of the Low Countries. Most probably these foxes have been artificially introduced into this area. One even suspects that the entire sub population in the area above the North Sea Channel, which connects the inland port of Amsterdam with the sea, can be retraced to four foxes from one litter that were released near Heemskerk. Evidently an occurrence, that did result in a high grade of inbreeding, due to which a physical anomaly had been able to dominate many generations of foxes.