The term "Lisp" (or "Lisp-like") is an umbrella for lots of different languages, such as Common Lisp, Scheme, and Arc. There is similar fragmentation in other language communities, like in ML.

However, Ruby and Python have both managed to avoid this fate, where innovation occurred more on the implementation (like PyPy or YARV) instead of making changes to the language itself.

Did the Ruby and Python communities do something special to prevent language fragmentation?

Ruby and Python both have benevolent dictators at their helm.  They are languages deeply rooted in pragmatic concerns.  Those are  probably the most significant factors inhibiting fragmentation.   Lisp and ML, on the other hand, are more like "design by committee" languages, conceived in academia, for theoretical purposes. 

Lisp was originally designed by John McCarthy as a practical mathematical notation for computer programs.  He never implemented it as an actual programming language; the first implementation was developed by Steve Russell, but he was not a benevolent dictator.  Over time, many different implementations of Lisp appeared; Common Lisp was an attempt to standardize them.

Lisp is more of a "family" of languages.  So is ML, which followed a similar evolutionary path to Lisp.
