How can I encourage alien expansions?

In 0.12.20:

1. In peaceful mode enemies never expand.
2. /c game.local_player.surface.set_multi_command{command={type=defines.command.build_base, destination={250, 100}, ignore_planner=1}, unit_count=10} Size of the base is selected by unit_count. The biters that are used for building are not spawned by this command, there needs to be a nearby spawner that provides the settlers.
3. /c game.local_player.surface.create_entity{name="biter-spawner", position={40, -40}}

Outside peaceful mode the biters select a candidate position, and send a group there once in a while. The interval depends on evolution factor and is between 60 minutes and 5 minutes (controlled in map-settings prototype in game data).

Positions selected needs to be at between 3 and 7 chunks from existing base and at least 3 chunks from player buildings (controlled in map-settings again). You can view chunks that are candidate for expansion wit debug view (F5 - F7 and F4) as green circles the map, the option name is called show_enemy_expansion_candidate_chunks.