  Gamma-ray bursts (GRBs) are highly variable and exhibit strong spectral
evolution. In particular, the emission properties vary from pulse to pulse in
multipulse bursts. Here we present a time-resolved Bayesian spectral analysis
of a compilation of GRB pulses observed by the {\it Fermi}/Gamma-ray Burst
Monitor. The pulses are selected to have at least four timebins with a high
statistical significance, which ensures that the spectral fits are well
determined and that spectral correlations can be established. The sample
consists of 39 bursts, 117 pulses, and 1228 spectra. We confirm the general
trend that pulses become softer over time, with mainly the low-energy power-law
index $\alpha$ becoming smaller. A few exceptions to this trend exist, with the
hardest pulse occurring at late times. The first pulse in a burst is clearly
different from the later pulses; three-fourths of them violate the synchrotron
line of death, while around half of them significantly prefer photospheric
emission. These fractions decrease for subsequent pulses. We also find that in
two-thirds of the pulses, the spectral parameters ($\alpha$ and peak energy)
track the light-curve variations. This is a larger fraction compared to what is
found in previous samples. In conclusion, emission compatible with the GRB
photosphere is typically found close to the trigger time, while the chance of
detecting synchrotron emission is greatest at late times. This allows for the
coexistence of emission mechanisms at late times.
