The Imperium itself is comprised of political groups called Families. Each Family is made up of representatives from various planets and colonies with members all vying for power and influence over the Imperium council. Anyone can choose to join a Family and work their way up the ranks in hopes of reaching the Imperium council. Representatives remain in power for as long as their Family continues to support them.
The Scientia are the only group that bypass the Family structure having their own elected group that represent them in the Imperium directly.