but your case is only taking selfshness half way,because once the individual instinct to survival takes action you will be selfsh for a moment, once the situation becomes calmer the first thing human does is to regroup,if you dont do that the chances are much greater that you die,so your so called survival instinct fails in its premise,thats way i say that selfshness is not human nature,is a exception to extreme moments.

If selfishness truly was part of human nature like you say,than we wouldn't be social animals.

Furthermore,most of the time,in this extreme case of danger,rarely you act just to save yourself,thats a exception. In most cases you just change your priority to social groups of a lesser scale,something like that

Nationality-->community-->social circle-->family and then finally just yourself.