That obviously became harder to do as breakthroughs in genetic research and cellular organization, and momentous events like the cold war, the civil rights movement, Watergate and the war on terror, began to elbow their way onto the lists. College professors could pick and choose what to cover in their introductory survey classes. But because the A.P. test can touch on almost anything, high school juniors and seniors must now absorb more material than most college freshmen.

So perhaps it is no surprise that while the number of students taking the A.P. biology test has more than doubled since 1997, the mean score has dropped to 2.63, from 3.18. On the exam last May, slightly under half of the test-takers scored at least a 3, which equates to a C in a college course. And while 19 percent of students earned 5’s, almost twice that many got 1’s, which could be a failing grade in college.

A committee of the National Research Council, a part of the National Academy of Sciences, called attention to these problems in 2002. It criticized A.P. science courses for cramming in too much material and failing to let students design their own lab experiments. It also said the courses had failed to keep pace with research on how people learn: instead of listening to lectures, “more real learning takes place if students spend more time going into greater depth on fewer topics, allowing them to experience problem solving, controversies and the subtleties of scholarly investigation.”

A few top universities have become more choosey about giving credit. In 2007, the Massachusetts Institute of Technology, for instance, stopped giving credit for A.P. biology, and developed its own placement exam. Stuart Schmill, M.I.T.’s dean of admissions, says the biology department found that even some of the students who scored 5’s did not have the problem-solving skills needed for higher-level courses. The University of Texas has also tightened its rules for biology placement, giving credit for 5’s only, though many large universities still accept 4’s or 3’s.

Several elite private high schools have also dropped A.P. courses. In defiance, the public school district in Scarsdale, N.Y., created its own in-depth courses called Advanced Topics. (For college credit, students still have to do well on the A.P. or another placement exam.)

The College Board took the criticisms to heart, and has been working with hundreds of college professors and high school teachers to develop the new approach.

For biology, the change means paring down the entire field to four big ideas. The first is a simple statement that evolution “drives the diversity and unity of life.” The others emphasize the systematic nature of all living things: that they use energy and molecular building blocks to grow; respond to information essential to life processes; and interact in complex ways. Under each of these thoughts, a 61-page course framework lays out the most crucial knowledge students need to absorb.