Barbara Kingsolver weaves the theme of each girls' different approaches to life all throughout the novel. She leaves a trail of symbolic references. Take for example Rachel's reaction to the ant infestation; she says she
"stuck my elbows very hard into the ribs of the people who were crushing in around me, and kind of wedged myself in...instead of getting trampled I simply floated like a stick in a river, carried along by everyone else's power."
This is indicative of her entire approach to life--don't ruffle feathers, let everyone else do the work, and take the easiest, most convenient route to comfort and prosperity. Later, she describes herself as "the Fourth of July," to reflect her rather sparkly, noisy personality. Earlier, in listing all of the girls, Kingsolver uses the phrase "carry us" to describe Rachel; she lets Axelroot and a series of other marriages and circumstances carry her about and determine her destiny, just as she did with the ant situation. She is also the "lock" of lock stock and barrel, because she locks on a way out, and takes it. It's just easier that way.
For Leah, she is described as "marry us," to symbolize how she takes Africa into her heart, and essentially, marries Africa and its causes, making them forever her own. She is also described as "day," because she is straightforward, bright, and proactive in her approach to life. Later, she is described as the "barrel" of "lock, stock and barrel," beause she "barrels forward, setting everything straight." She is aggressive, takes on other people's problems, and fights to fix them. She is a bright force for positive change, and takes it all on herself.
Adah is an interesting one; she is "night," because of her dark cynicism and inward thoughts. She is a bit cryptic and depressing, and plays the role of cynical judge for everyone else's behavior. As she says of herself, she has great flavor because she is "spiced with the flavors of vice," or bad thoughts. She is the "stock" in lock, stock and barrel, because she "quietly takes stock" of every situation and analyzes it. She also is ferried out of Africa, helped out, and eventually comes into her own. She is a deep thinker, an analyzer, and approaches life slowly and with forethought.
I hope that those thoughts helped; good luck!