We study formation and evolution of bar-disk systems in fully self-consistent cosmological simulations of galaxy formation in the \Lambda CDM WMAP3 Universe .
In a representative model we find that the first generation of bars form in response to the asymmetric dark matter ( DM ) distribution ( i.e. , DM filament ) and quickly decay .
Subsequent bar generations form and are destroyed during the major merger epoch permeated by interactions with a DM substructure ( subhalos ) .
A long-lived bar is triggered by a tide from a subhalo and survives for \sim 10 Gyr .
The evolution of this bar is followed during the subsequent numerous minor mergers and interactions with the substructure .
Together with intrinsic factors , these interactions largely determine the stellar bar evolution .
The bar strength and its pattern speed anticorrelate , except during interactions and when the secondary ( nuclear ) bar is present .
For about 5 Gyr bar pattern speed increases substantially despite the loss of angular momentum to stars and cuspy DM halo .
We analyze the evolution of stellar populations in the bar-disk and relate them to the underlying dynamics .
While the bar is made mainly of an intermediate age , \sim 5 - 6 Gyr , disk stars at z = 0 , a secondary nuclear bar which surfaces at z \sim 0.1 is made of younger , \sim 1 - 3 Gyr stars .