Evolution of the comoving number density of faint IR emitting galaxies (10⁷<L_IR/L_⊙< 10¹¹, black symbols), LIRGs (10¹¹ ≤ L_IR/L_⊙< 10¹², red symbols), and ULIRGs (L_IR/L_⊙ ≥ 10¹², magenta symbols) of the whole galaxy population (filled points) and of the group galaxy population (stars). The comoving number density of the total population is taken from Magnelli et al. (2013) at z> 0.1 and from Sanders et al. (2003) at z ~ 0. The dashed lines show the best fit relation between comoving number density and redshift, (1 + z)^α (best-fit values of α are given in the text).