Groups: The rich, the poor and the destitute

I discuss recent observational results that suggest an evolutionary path for groups in which galaxy tidal interactions and mergers strip stars, dark matter, and gas from galaxies. As the group evolves, its velocity dispersion increases, the brightest group galaxy grows and moves toward the center of the potential, other giant galaxies merge, concentrating and/or creating their dwarf satellite populations, group members evolve from late to early types, and intragroup stars are drawn out of their host galaxies. Dark matter and gas are stripped from the galaxies' outer halos and dispersed into a common group halo. The ultimate evolution of this group may be an isolated elliptical or "fossil group". In this process, some evolved groups acquire the characteristics of rich clusters. Given that clusters themselves evolve via the accretion of poor groups from the field, there is evidence that galaxy evolution in groups dominates the evolution of the galaxies that ultimately end up as cluster members and that evolutionary mechanisms associated with clusters, such as ram pressure stripping or strangulation, are relatively unimportant.