Darwin's Theory of Evolution and Einstein's Theory of Relativity are two examples. Both were revolutionary in that they forced scientists to rethink certain basic assumptions they had previously taken for granted - the immutability of species in the case of Darwin. Such theories are rare, but they are integral to scientific advancement. Depending on the university, there is more or less flexibility on which subject matter exams one takes. However it is certainly likely that a student intending to major in biology would want to take the biology subject matter exam.
Biology is often one of the largest majors at universities.
Thus over a third of the exam has an evolutionary focus.