This paper reports a two-dimensional hybrid simulation study which utilizes an expanding box model to represent the slow compression of the plasma as it flows through the magnetosheath. In the code we model the compression as an external force: The physical sizes of the simulation box decrease with time. We present results of a simulation which starts in a parameter region of low beta where the plasma is stable with respect to both the Alfv¿n ion cyclotron (AIC) and mirror instabilities. In this stable region the plasma behaves double-adiabatically and an important proton temperature anisotropy appears. When the plasma becomes unstable to AIC instability, the adiabatic behavior is broken and the AIC waves keep the system close to marginal stability, the theoretical growth rate being about constant, small and positive. The AIC waves are continuously generated and the proton parallel beta increases with time. This marginal stability behavior is slightly disrupted for high proton parallel beta, where the mirror mode becomes unstable. The mirror waves rapidly grow and coexist with AIC wave, in later times the growth of AIC waves is inhibited and mirror waves become dominant. During the stages dominated by AIC and mirror waves, anticorrelation between anisotropy and proton parallel beta is observed. The hybrid expanding box simulation directly verifies the marginal stability evolution of the magnetosheath plasma.