  We present the spectrum of the (normalized) graph Laplacian as a systematic
tool for the investigation of networks, and we describe basic properties of
eigenvalues and eigenfunctions. Processes of graph formation like motif joining
or duplication leave characteristic traces in the spectrum. This can suggest
hypotheses about the evolution of a graph representing biological data. To this
data, we analyze several biological networks in terms of rough qualitative data
of their spectra.
