The pattern of Hox gene expression can be divided up into three phases throughout limb bud development, which corresponds to three key boundaries in proximal-distal limb development. The transition from the first phase to the second phase is marked by the introduction of Shh signals from the ZPA. The transition into the third phase is then marked by changes in how the limb bud mesenchymal cells responds to Shh signals. This means that although Shh signaling is required, its effects change over time as the mesoderm is primed to respond to it differently. These three phases of regulation reveal a mechanism by which natural selection can independently modify each of the three limb segments – the stylopod, the zeugopod, and the autopod.