(A) Shown is the Z score for the probability *P*(*k*_{1},*k*_{2}) of the two nodes with degree *k _{1},k_{2}* being connected to each other in the natural PDUG. Unlike in other networks, nodes of similar degree tend to be connected.

(B) The Z score plot of *P*(*k*_{1},*k*_{2}) for the structure similarity graph obtained in the evolution model is remarkably similar to the actual one.