Colony Critical Mass

I am trying to design a scenario for colonies trying to become
independant.
This led to a question on what is the smallest, sustainable,
self-sufficient
colony size. I realize that this will vary with the conditions of the
colony.

What is the minimum population needed to have a self-sustaining colony?
By self-sustaining, I mean enough people to avoid becoming inbred,
produce
food, shelter, purify water, and support light industry.
Please respond for these two conditions to give me a range:
1) Colony on the Moon
2) Colony on an Earth-like world (plenty of food, water, minerals, etc.)