  It is of prime importance to recognize evolution and extinction effects in
supernovae results as a function of redshift, for SN Ia to be considered as
distance indicators. This review surveys all observational data searching for
an evolution and/or extinction, according to host morphology. For instance, it
has been observed that high-z SNe Ia have bluer colours than the local ones:
although this goes against extinction to explain why SN are dimmer with
redshift until z ~ 1, supporting a decelerating universe, it also demonstrates
intrinsic evolution effects. -- SNe Ia could evolve because the age and
metallicity of their progenitors evolve. The main parameter is carbon
abundance. Smaller C leads to a dimmer SN Ia and also less scatter on peak
brightness, as it is the case in elliptical galaxy today. Age of the progenitor
is an important factor: young populations lead to brighter SNe Ia, as in spiral
galaxies, and a spread in ages lead to a larger scatter, explaining the
observed lower scatter at high z. -- Selection biases also play a role, like
the Malmquist bias; high-z SNe Ia are found at larger distance from their host
center: there is more obscuration in the center, and also detection is easier
with no contamination from the center. This might be one of the reason why less
obscuration has been found for SNe Ia at high z. -- There is clearly a sample
evolution with z: currently only the less bright SNe Ia are detected at high z,
with less scatter. The brightest objects have a slowly declining light-curve,
and at high z, no slow decline has been observed. This may be interpreted as an
age effect, high-z SN having younger progenitors.
