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Threats on little populations.

Smaller populations can suffer two kinds of hazards

Drop in Fertility below the expected one.
As stated above, the observed rate may significantly differ form the theoretical one. Pushing it to a limit, a population reduced to one couple may well have an expected Fertility of 2.1 but of course will end with something quite different, often 2, rarely 8 but sometime 0 children. This last figure would put en end to the simulation.

Random changes in sex ration
The software (like nature does) chooses randomly the sex of offspring. This figure will very rarely be exactly 0.5. So each generation will appear to have a small excess of one gender. Lechery as well as any other forms of polygamy are no part of this simulation (this can appear as a crude approximation). This results in a lessening of the fertility.
Here again the effect will be strongly emphasized in small populations.