We present a new analysis of galaxy colours and star formation rates as a function of environment, from the Sloan Digital Sky Survey and the 2dF Galaxy Redshift Survey. The galaxy distribution is well characterised by only two distinct populations: one that is blue and star–forming, and one that is red and quiescent. The colour and H$\alpha$ distribution of each of these populations is approximately independent of environment, while their relative abundances depend strongly on local galaxy density. Even in the lowest-density environments, a substantial ($\sim 30$ per cent) population of galaxies have red colours and exhibt no detectable H$\alpha$ emission. We compare these data with theoretical predictions of galaxy formation, properly accounting for observational selection biasses and uncertainties. We conclude that galaxy transformations must either be rapid (with a timescale $\tau\lesssim0.5$ Gyr), or must occur preferentially at high redshift.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html