  Two processes can influence the evolution of protein interaction networks:
addition and elimination of interactions between proteins, and gene
duplications increasing the number of proteins and interactions. The rates of
these processes can be estimated from available Saccharomyces cerevisiae genome
data and are sufficiently high to affect network structure on short time
scales. For instance, more than 100 interactions may be added to the yeast
network every million years, a substantial fraction of which adds previously
unconnected proteins to the network. Highly connected proteins show a greater
rate of interaction turnover than proteins with few interactions. From these
observations one can explain ? without natural selection on global network
structure ? the evolutionary sustenance of the most prominent network feature,
the distribution of the frequency P(d) of proteins with d neighbors, which is a
broad-tailed distribution. This distribution is independent of the experimental
approach providing nformation on network structure.
