We measure the diversity of galaxy groups and clusters with mass M > 10 ^ { 13 } h ^ { -1 } M _ { \odot } , in terms of the star formation history of their galaxy populations , for the purpose of constraining the mass scale at which environmentally-important processes play a role in galaxy evolution .
We consider three different group catalogues , selected in different ways , with photometry and spectroscopy from the Sloan Digital Sky Survey .
For each system we measure the fraction of passively-evolving galaxies within R _ { 200 } and brighter than either M _ { r } = -18 ( and with z < 0.05 ) or M _ { r } = -20 ( and z < 0.1 ) .
We use the ( u - g ) and ( r - i ) galaxy colours to distinguish between star-forming and passively-evolving galaxies .
By considering the binomial distribution expected from the observed number of members in each cluster , we are able to either recover the intrinsic scatter in this fraction , or put robust 95 % confidence upper-limits on its value .
The intrinsic standard deviation in the fraction of passive galaxies is consistent with a small value of \mathrel { \raise 1.505 pt \hbox { $ \scriptstyle < $ } \kern - 6.0 pt \lower 1.72 pt \hbox { { $% \scriptstyle \sim$ } } } 0.1 in most mass bins for all three samples .
There is no strong trend with mass ; even groups with M \sim 10 ^ { 13 } h ^ { -1 } M _ { \odot } are consistent with such a small , intrinsic distribution .
We compare these results with theoretical models of the accretion history to show that , if environment plays a role in transforming galaxies , such effects must occur first at mass scales far below that of rich clusters , at most M \sim 10 ^ { 13 } M _ { \odot } .