How does survival of the fittest demonstrate Lucy to be the strongest character in the novel?

Part of
Lucy's ability to outmaneuver Faber is a survival of the fittest. In being able to deny Faber
the ability to use electrical means of conveying his message to Germany as well as being able to
kill someone towards whom she had feelings represents a survival technique in which survival of
the fittest can be seen.  At such a point, Faber realizes either he will be successful or she
will be.  Yet, I think that Follett constructs her...