  Explaining the formation and evolution of galaxies is one of the most
challenging problems in observational cosmology. Many observations suggest that
galaxies we see today could have evolved from the merging of smaller
subsystems. Evolution of galaxies tells us how the mass or number density of
the lens varies with cosmic time. Merging between the galaxies and the infall
of surrounding mass into galaxies are two possible processes that can change
the comoving number density of galaxies and/or their mass. We consider five
different evolutionary models of galaxies .These models are: Non evolutionary
model, Guiderdoni and Volmerange model, fast merging, slow merging and mass
accretion model. We study the gravitational lens image separation distribution
function for these models of evolving galaxies. A comparison with data for
lensed quasars taken from the HST Snapshot Survey rules out the fast merging
model completely as this model produces a large number of small-separation
lenses. It is possible that the mass accretion model and the non evolutionary
model of galaxies may be able to explain the small angle separations.
