Galaxy environments are roughly divided into the `field' and clusters. Field galaxies are separated by large distances, so encounters are rare. Cluster environments are much denser, but the galaxies are bound and viralized such that their typical velocities are quite high (greater than 1000 km/sec relative to each other). A high encounter velocity would make for short impulse timescales and very little change in the shape or nature of the galaxies.
Despite the cluster environment being an apparently poor place for dynamical evolution, there came the gradual acknowledgment that there existed a special supergiant galaxy in the core of most clusters. These galaxies, called cD galaxies, are located at the geometric and velocity center of the clusters, have extreme luminosities (i.e. mass) and sizes and are surrounded by numerous smaller galaxies.
If the velocities are too high in the cores of clusters for mergers, then what is the origin of these supergiant galaxies? The answer lies with the concept of dynamical friction, outlined below.