The Role of Interactions and Mergers
  Galaxy evolution depends strongly on the environment. Dynamical interactions and subsequent evolution make galaxies more concentrated, with higher surface densities, and also trigger star-formation, which consumes the available cold gas. Already at z=0, a large variety of galaxy types are observed, with different evolution stages, from the unevolved gas rich dwarf irregulars, or low surface brightness galaxies, to early-type concentrated galaxies, with no remaining gas.   The dynamical processes of galaxy interactions, including internal evolution, are reviewed and evidence is shown of much larger interaction/merging rate at high redshift. 
F. Combes