In life as we knew it, tensions are starting to flare between miranda and her mom. Miranda is punished for eating food that she was not supposed to. As it gets colder one day Miranda's mom Notices that it is snowing,but the snow is not normal, it is grayish from the volcanic ash being released into the atmosphere. Soon we find that Mrs. Nesbitt doesn't plan to live to much longer which she tells miranda. Very rough times lie ahead of the characters.
As it has been said the theme of survival of the fittest has come into play with the book. Earlier in the book we saw Miranda's mom giving more food to Matt than anyone else. survival of the fittest will most likely come into play even more with conditions getting worse and worse. Even today we see this role come into play in tough times. Survival of the fittest is a natural thing and has been observed for thousands of years.
In life As we knew it many things motivate the main character to survive,but sometimes motivation is not enough. For Miranda in the thought of her family is what keeps her going. Thinking of all the people she would have to leave behind would kill her so she keeps moving in. She may make a few mistakes, but she always keeps on going.