I don't understand what you are trying to say.
Evolve = change.
Culture = the way people do stuff.
Identity is a buch of different stuff that makes you you.
You ask how the Germans can remain German, and not get conquered or lose their identity.
Proper border control.
Perhaps I expressed myself improperly, cultures both evolve and fade, all evolution is not necessarily positive. ATM western culture is evolving from individualistic nationalist to globalist collectivist. In this evolution natural identities are as a temporary measure being replaced with synthetic identities which will later be replaced in as many cycles as required until we reach a single global identity.... of course the natural identities will still remain they will only be repressed and anywhere along the line where one rebels against this unnatural process they will be oppressed or killed, this was all prototyped by Russia and China but the failure did not kill the dream.
Culture is something that overlays identity, it is spontaneously created. You cannot change a culture without altering an identity. To have an American culture you MUST have an American identity, as the American identity is shifting atm the culture is shifting along with it, this can be clearly seen. What American meant both 100 and 50 years ago is completely different to what it means now...
because there is no actual American tribe it's a synthetic identity. Being an unnatural identity it's lifetime is finite and tied directly to the will of the inhabitants to keep it's original core culture going. Natural Identities do not suffer from this deficiency.
Again, Germans staying German by protecting German borders and keeping non-Germans out with force if necesary is exactly the same as the old tribal "killing over grazing fields". ATM German politicians are attempting to slowly kill the German identity by promoting ethnic dilution via mass immigration, if the current trends continue without oposition within 50 years there will be no Germans left, the old tribal identity will have shifted to a new one or be replaced by something else that will not be German. Germans have stopped killing over grazing fields.
EDIT: funny thing is I am kind of a heretic here, I would be fine with SA becoming a single tribe but I see no actual practical way that is actually possible without collapsing the current tribes or the victimization of the smaller one's by the larger one's. I would be fine with one big brownish enlightened populace but it's just not possible.