Newborn babies can range anywhere from 5.5-10 lbs at birth - with 7.5 lbs being the average birth weight. Some babies are even smaller if born prematurely. Babies that are too small tend to have weaker immune systems and are more susceptible to infectious diseases. Babies that are too big can be challenging to deliver through the birth canal.

a. What type of selection is this an example of?

b. What is driving this type of selection?