  Spectra of real world networks exhibit properties which are different from
the random networks. One such property is the existence of a very high
degeneracy at zero eigenvalues. In this work, we provide possible reasons
behind occurrence of the zero degeneracy in various networks spectra.
Comparison of zero degeneracy in protein-protein interaction networks of six
different species and in their corresponding model networks sheds light in
understanding the evolution of complex biological systems.
