  The environment is known to affect the formation and evolution of galaxies
considerably best visible through the well-known morphology-density
relationship. In this paper we study the effect of environment on the evolution
of early-type galaxies by analysing the stellar population properties of 3,360
galaxies morphologically selected by visual inspection from the SDSS in the
redshift range 0.05<z<0.06. We find that the distribution of ages is bimodal
with a strong peak at old ages and a secondary peak at young ages around ~
2.5Gyr containing about 10 per cent of the objects. This is analogue to 'red
sequence' and 'blue cloud' identified in galaxy populations usually containing
both early and late type galaxies. The fraction of the young, rejuvenated
galaxies increases with both decreasing galaxy mass and decreasing
environmental density up to about 45 per cent. The rejuvenated galaxies have
lower alpha/Fe ratios than the average and most of them show signs of ongoing
star formation through their emission line spectra. All objects that host AGN
in their centres without star formation are part of the red sequence
population. We confirm and statistically strengthen earlier results that
luminosity weighted ages, metallicities, and alpha/Fe element ratios of the red
sequence population correlate well with velocity dispersion and galaxy mass.
Most interestingly, however, these scaling relations are not sensitive to
environmental densities and are only driven by galaxy mass. We infer that
early-type galaxy formation has undergone a phase transition a few billion
years ago around z~0.2. A self-regulated formation phase without environmental
dependence has recently been superseded by a rejuvenation phase, in which the
environment plays a decisive role possibly through galaxy interactions.
