According to TPM, Anakin was concieved by the midichlorians. In RotS it was alluded to that perhaps Anakin was concieved at the direction of Plagus or Palpatine. Is their a reason or "back story" as to why the Skywalker lineage was chosen? I recall hearing once that the Skywalker line had ties into some ancient group similar to Christ's 12 apostles. And because Shmi was the last surviving person from that ancient clan, she was chosen.