  We study the nucleation dynamics of a model solid state transformation and
the criterion for microstructure selection using a molecular dynamics (MD)
simulation. Our simulations show a range of microstructures depending on the
depth of quench. We closely follow the dynamics of the solid and find that
transient {\em non-affine zones} (NAZ) are created at and evolve with the
rapidly moving transformation front. The dynamics of these plastic regions
determines the selection of microstructure. We formulate an {\it elastoplastic
model} which couples the elastic strain to the non-affine deformation, and
recover all the qualitative features of the MD simulation. Using this model, we
construct a dynamical phase diagram for microstructure selection, in addition
to making definite testable predictions.
