  We use Monte Carlo realizations of halo formation histories and a spherical
accretion model to calculate the expected scatter in the velocity dispersions
of galactic halos of a given mass due to differences in their formation times.
Assuming that the rotational velocity of a spiral galaxy is determined by the
velocity dispersion of its halo and that its luminosity is related to its total
baryonic mass, this scatter translates to a minimum intrinsic scatter in the
Tully-Fisher relation. For popular cosmological models we find that the scatter
due to variations in formation histories is by itself greater than allowed by
observations. Unless halos of spiral galaxies formed at high redshift (z>1) and
did not later accrete any significant amount of mass, the Tully-Fisher relation
is not likely to be the direct result of cosmological initial conditions but
rather a consequence of a subsequent feedback process.
