  The functioning of many biochemical networks is often robust -- remarkably
stable under changes in external conditions and internal reaction parameters.
Much recent work on robustness and evolvability has focused on the structure of
neutral spaces, in which system behavior remains invariant to mutations.
Recently we have shown that the collective behavior of multiparameter models is
most often 'sloppy': insensitive to changes except along a few 'stiff'
combinations of parameters, with an enormous sloppy neutral subspace.
Robustness is often assumed to be an emergent evolved property, but the
sloppiness natural to biochemical networks offers an alternative non-adaptive
explanation. Conversely, ideas developed to study evolvability in robust
systems can be usefully extended to characterize sloppy systems.
