  Photometric redshifts are commonly used to measure the distribution of
galaxies in large surveys. However, the demands of ongoing and future
large-scale cosmology surveys place very stringent limits on the redshift
performance that are difficult to meet. A new approach to meet this precision
need is forward modelling, which is underpinned by realistic simulations. In
the work presented here, we use simulations to study the sensitivity of
redshift distributions to the underlying galaxy population demographics. We do
this by varying the redshift evolving parameters of the Schechter function for
two galaxy populations, star-forming and quenched galaxies. Each population is
characterised by eight parameters. We find that the redshift distribution of
shallow surveys, such as SDSS, is mainly sensitive to the parameters for
quenched galaxies. However, for deeper surveys such as DES and HSC, the
star-forming parameters have a stronger impact on the redshift distribution.
Specifically, the slope of the characteristic magnitude, $a_\mathrm{M}$, for
star-forming galaxies has overall the strongest impact on the redshift
distribution. Decreasing $a_\mathrm{M}$ by 148 per cent (its given uncertainty)
shifts the mean redshift by ${\sim} 45$ per cent. We explore which combination
of colour and magnitude measurements are most sensitive to $a_\mathrm{M}$ and
we find that each colour-magnitude pair studied is similarly affected by a
modification of $a_\mathrm{M}$.
