I read Brahmans evolved from only 8 rishi's (have 8 gothrs only). This means , there would be atleast one same gothra marriage will happen in every 8-sequence of generations. But this effect is negligible over mutations in 8 generations(Hindus prefer atleast 2 generations gap for same gothra marriage).
My final line is ,The community with less variations in gothra will be the weakest group in future.
This shows , the big communities will become stronger and more stronger and the weak will die soon to single numbers or disappearance in long run.
P.S: Inter-cast marriages are taken negligible in this view.