  Metabolic networks are known to be scale free but the evolutionary origin of
this structural property is not clearly understood. One way of studying the
dynamical process is to compare the metabolic networks of species that have
arisen at different points in evolution and hence are related to each other to
varying extents. We have compared the reaction sets of each metabolite across
and within 15 groups of species. For a given pair of species and a given
metabolite, the number $\Delta k$ of reactions of the metabolite that appear in
the metabolic network of only one species and not the other is a measure of the
distance between the two networks. While $\Delta k$ is small within groups of
related species and large across groups, we find its probability distribution
to be $\sim (\Delta k)^{-\gamma'}$ where $\gamma'$ is a universal exponent that
is the same within and across groups. This exponent equals, upto statistical
uncertainties, the exponent $\gamma$ in the scale free degree distribution
$\sim k^{-\gamma}$. We argue that this, as well as our finding that $\Delta k$
is approximately linearly correlated with the degree $k$ of the metabolite, is
evidence of a `proportionate change' process in evolution. We also discuss some
molecular mechanisms that might be responsible for such an evolutionary
process.
