High-redshift clusters of galaxies show an over-abundance of spirals by a factor of 2 - 3 , and the corresponding under-abundance of S0 galaxies , relative to the nearby clusters .
This morphological evolution can be explained by tidal interactions with neighboring galaxies and with the hierarchically growing cluster halo .
The efficiency of tidal interactions depends on the size and structure of the cluster , as well as on the epoch of its formation .
I simulate the formation and evolution of Virgo-type clusters in three cosmologies : a critical density model \Omega _ { 0 } = 1 , an open model \Omega _ { 0 } = 0.4 , and a flat model \Omega _ { 0 } = 0.4 with a cosmological constant .
The orbits of identified halos are traced with a high temporal resolution ( \sim 10 ^ { 7 } yr ) .
Halos with low relative velocities merge only shortly after entering the cluster ; after virialization mergers are suppressed .

The dynamical evolution of galaxies is determined by the tidal field along their trajectories .
The maxima of the tidal force do not always correspond to closest approach to the cluster center .
They are produced to a large extent by the local density structures , such as the massive galaxies and the unvirialized remnants of infalling groups of galaxies .
Collisions of galaxies are intensified by the substructure , with about 10 encounters within 10 kpc per galaxy in the Hubble time .
These very close encounters add an important amount ( 10–50 % ) of the total heating rate .
The integrated effect of tidal interactions is insufficient to transform a spiral galaxy into an elliptical , but can produce an S0 galaxy .
Overall , tidal heating is stronger in the low \Omega _ { 0 } clusters .