We present results from two-dimensional (2-D) numerical experiments on mantle flow in subduction zones and the thermal and compositional evolution of the mantle wedge. Experiments build upon previous modeling efforts by including melting processes and back arc plate motion relative to the trench. The thermal structure of the wedge is calculated for various plate motion conditions, and a particle technique is used to track melt production and extraction in the back arc mantle and the arc-wedge corner. Numerical experiments test models developed from geochemical data that suggest the mantle is partially melted, or preconditioned, beneath the back arc prior to entering the arc-wedge corner. Results show wedge temperatures increase with increasing back arc plate rates and higher degrees of retrograde subduction. Decompression melting is recorded for material in the wedge. Mean degrees of melting are generally smaller than beneath ridges (1--7%) and depend on back arc plate rate and, in particular, the choice of ambient mantle potential temperature (Tm). Results show that preconditioning of mantle by partial melting at the back arc is optimized for intermediate back arc plate rates. Implications of these models include (1) arc melts should have components with characteristics of a low degree, decompression melting process, (2) arc melts may be depleted in incompatible elements, or preconditioned, prior to entering the wedge, and (3) models for slab-influenced arc magmagenesis within the wedge should consider the melt history of material just above the slab-wedge interface.