Abstract
The traditional view in population synthesis and evolutionary synthesis models assumes that galaxies can be considered closed systems. Thus, the evolution of the stellar population in galaxies which do not interact with the environment (including other galaxies) is described by the so called Pure Luminosity Evolution (PLE) model. As we have heard several times during this conference (and before), galaxies do interact and very few galaxies are expected to evolve passively. Early-type galaxies seem to behave closer to the PLE model than later types, but E galaxies in the bright phase predicted by this model at the galaxy formation epoch have not been observed. Most likely E galaxies do not form all their stars in short lived initial bursts (several talks in this conference). Number, size, and luminosity evolution is required to understand late-type and Irr galaxies. The PLE model is unable to reproduce the number counts, and color and redshift distributions of the galaxies in the HDF (Pozzetti et al. 1997).
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