Absolute r-band magnitudes of the first (filled green circles) and the second (open blue circles) brightest galaxies as a function of the magnitude gap for the COSMOS+AEGIS+XMM-LSS sample of groups, having spectroscopic members. The solid black and dashed red lines show linear fits for the spectroscopic and red sequence selections of group membership, respectively. The upper panel and lower panel show the butterfly diagrams for high-z and low-z groups, respectively.