  We present semi-analytic dynamical models for giant molecular clouds evolving
under the influence of HII regions launched by newborn star clusters. In
contrast to previous work, we neither assume that clouds are in virial or
energetic equilibrium, nor do we ignore the effects of star formation feedback.
The clouds, which we treat as spherical, can expand and contract homologously.
Photoionization drives mass ejection; the recoil of cloud material both stirs
turbulent motions and leads to an effective confining pressure. The balance
between these effects and the decay of turbulent motions through isothermal
shocks determines clouds' dynamical and energetic evolution. We find that for
realistic values of the rates of turbulent dissipation, photoevaporation, and
energy injection by HII regions, the massive clouds where most molecular gas in
the Galaxy resides live for a few crossing times, in good agreement with recent
observational estimates that large clouds in local group galaxies survive
roughly 20-30 Myr. During this time clouds remain close to equilibrium, with
virial parameters of 1-3 and column densities near 10^22 H atoms cm^-2, also in
agreement with observed cloud properties. Over their lives they convert 5-10%
of their mass into stars, after which point most clouds are destroyed when a
large HII region unbinds them. In contrast, small clouds like those found in
the solar neighborhood only survive ~1 crossing time before being destroyed.
