The human race has never experienced an evolutionary event even remotely comparable to the flood story. The Toba catastrophe theory posits a reduction of human forbearers to thousands, not 6
Reduce a mammal species to only three closely interrelated breeding pairs and it will die out ---period. Again, there have been more than enough captive breeding programs of endangered species in the last century to know this from direct observation. Even in programs of as much as 25 breeding pairs you still have to introduce at least two animals from outside the group a minimum of every 10 generations. If you don't do this the entire program will be consumed by sterility and stillbirth
Humans seen to know this intuitively and this is reflected in marriage customs almost everywhere. Small tribes of humans invariably set up either a moiety system for endogamous unions and/or practice some form of exogamy with other tribes.
Take the Australian Aranda for example. Two unamed moieties divided the tribe in half. Each moiety was divided into two sections and each section was subdivided into two subsections. Although the tribe as whole was endogomous, the moieties were strictly exogamous. In othe words, marriages must cross moiety lines. Spouses had to be chosen from the proper subsection of the opposite moiety.
In other cultures exogamy has taken the form of either buying the bride from friendly tribes and/or stealing a bride from hostile tribes.
T he obvious purpose of all such systems is to preserve genetic diversity. Without it, small tribes die out.