  We investigate the environmental dependence of galaxy population properties
in a complete volume-limited sample of 91566 galaxies in the redshift range
0.05 <= z <= 0.095 and with M_r <= -20.0, selected from the Sloan Digital Sky
Survey (SDSS) Data Release 4. Our aim is to search for systematic variations in
the properties of galaxies with the local galaxy density. In particular, we
analize how the (u - r) color index and the morphological type of galaxies (the
latter evaluated through the SDSS Eclass and FracDev parameters) are related to
the environment and to the luminosity of galaxies, in order to find hints that
can be related to the presence of a ''void'' galaxy population. Void galaxies
are identified through a highly selective criterion, which takes also into
account redshift and allows us to exclude from the sample all the galaxies that
are not really close to the center of underdense regions. We study the (u - r)
color distribution for galaxies in different luminosity bins, and we look for
correlations of color with the environment, the luminosity, and the
morphological type of the galaxies. We find that galaxies in underdense regions
(voids) have lower luminosity (M_r > -21) and are bluer than cluster galaxies.
The transition from overdense to underdense environments is smooth, the
fraction of late-type galaxies decreases while the fraction of early-type
galaxies increases smoothly from underdense to dense environments. We do not
find any sudden transition in the galaxy properties with density, which,
according to a suggestion by Peebles (2001), should mark the transition to a
population of "void" galaxies in LCDM models. On the contrary, our results
suggest a continuity of galaxy properties, from voids to clusters.
