  Does galaxy evolution proceed through the green valley via multiple pathways
or as a single population? Motivated by recent results highlighting radically
different evolutionary pathways between early- and late-type galaxies, we
present results from a simple Bayesian approach to this problem wherein we
model the star formation history (SFH) of a galaxy with two parameters, [t,
\tau] and compare the predicted and observed optical and near-ultraviolet
colours. We use a novel method to investigate the morphological differences
between the most probable SFHs for both disc-like and smooth-like populations
of galaxies, by using a sample of 126,316 galaxies (0.01 < z < 0.25) with
probabilistic estimates of morphology from Galaxy Zoo. We find a clear
difference between the quenching timescales preferred by smooth- and disc-like
galaxies, with three possible routes through the green valley dominated by
smooth- (rapid timescales, attributed to major mergers), intermediate-
(intermediate timescales, attributed to minor mergers and galaxy interactions)
and disc-like (slow timescales, attributed to secular evolution) galaxies. We
hypothesise that morphological changes occur in systems which have undergone
quenching with an exponential timescale \tau < 1.5 Gyr, in order for the
evolution of galaxies in the green valley to match the ratio of smooth to disc
galaxies observed in the red sequence. These rapid timescales are instrumental
in the formation of the red sequence at earlier times; however we find that
galaxies currently passing through the green valley typically do so at
intermediate timescales.
