Wiki science/Budding Effect
The budding effect is the phenomenon whereby one wiki produces another wiki, in analogy to a plant producing a bud. A large parent wiki can 'bud' several smaller child wikis, creating a kind of wiki family tree.
Issues involved are license(s) and wiki sociology.
Wiki can bud off slowly, or quickly. They can erupt from a battle on a parent wiki, or they may gradually form as the members of the parent wiki devote themselves to both parent and child. There are many technologies that can support the budding process, from the near link, to the page cluster to name spaces.
Wikimedia sister projects
Wikipedia is a wiki encyclopedia. It has a clearly defined purpose: creating a free encyclopedia. Entries that provide dictionary definitions are disapproved of. Because of this, some of the Wikipedia users founded Wiktionary. In this way, Wikipedia can be seen as the parent of Wiktionary.
Similarly, Wikibooks itself 'budded' off of Wikipedia, out of the need to provide more textbook-like content, where there is more room to leave the encyclopedic path: not as much need for NPOV; original research...
The budding effect becomes very clear when looking at the many languages that have their own Wikipedia. Existing articles from, mostly pivotal larger (on Wikipedia) languages like English, French and German are translated into other language Wikipedias. Also, many articles about topics that are more prevalent in less common languages have been translated into English.
Just like copylefted software that is forked, the use of a copyleft license for a wiki assures that the content can be fed back into the parent wiki.