  The analysis of gamma-ray burst (GRB) spectra often relies on empirical
models lacking a distinct physical explanation. Previous attempts to couple
physical models with observed data focus on individual burst studies, fitting
models to segmented spectra with independent physical parameters. However,
these approaches typically neglect to explain the time evolution of observed
spectra. In this study, we propose a novel approach by incorporating the
synchrotron radiation model to provide a self-consistent explanation for a
selection of single-pulse GRBs. Our study comprehensively tests the synchrotron
model under a unified physical condition, such as a single injection event of
electrons. By tracing the evolution of cooling electrons in a decaying magnetic
field, our model predicts time-dependent observed spectra that align well with
the data. Using a single set of physical parameters, our model successfully
fits all time-resolved spectra within each burst. Our model suggests that the
rising phase of the GRB light curve results from the increasing number of
radiating electrons, while the declining phase is attributed to the curvature
effect, electron cooling, and the decaying magnetic field. Our model provides a
straightforward interpretation of the peak energy's evolution, linked to the
decline of the magnetic field and electron cooling due to the expansion of the
GRB emission region. Our findings strongly support the notion that spectral and
temporal evolution in GRB pulses originates from the expansion of the GRB
emission region, with an initial radius of approximately $10^{15}$ cm, and
synchrotron radiation as the underlying emission mechanism.
