The integration of proteins into the yeast protein-protein interaction network by age and origin. Each pane gives the average (with standard errors) of a statistic that reflects the integration of proteins in each age/origin group into the yeast physical interaction network: (a) degree, (b) betweenness centrality, (c) degree divided by protein length, and (d) betweenness divided by protein length. Similar trends are seen in each plot. Young proteins are less integrated than older proteins, and duplicate proteins are more connected than novel proteins. Overall, the young novel proteins are significantly less integrated than all other groups.