  We formulate the equivalence problem, in the sense of E. Cartan, for families
of minimal rational curves on uniruled projective manifolds. An important
invariant of this equivalence problem is the variety of minimal rational
tangents. We study the case when varieties of minimal rational tangents at
general points form an isotrivial family. The main question in this case is for
which projective variety $Z$, a family of minimal rational curves with
$Z$-isotrivial varieties of minimal rational tangents is locally equivalent to
the flat model. We show that this is the case when $Z$ satisfies certain
projective-geometric conditions, which hold for a non-singular hypersurface of
degree $\geq 4$.
