Evidence of evolution, speciation and the origin of life
Unit 7 of AP Biology, Natural Selection, carries one of the heaviest weightings on the exam, and a large share of its questions ask you to classify what is happening in a short scenario: which condition of Hardy–Weinberg equilibrium a population breaks, what kind of evidence a fossil or a reduced organ provides, which reproductive barrier keeps two species apart, whether speciation happened with or without geographic separation, and what a pattern in the fossil record says about the pace of evolution. This material trains that classification on invented scenarios rather than bare definitions.
You decide which Hardy–Weinberg condition is violated when crickets from a neighboring population cross a new highway corridor every year, and why the other conditions still hold. You read undisturbed rock layers and dated volcanic ash to give one fossil an age range and compare it with a fossil higher up. You identify the skin-covered, nonworking eyes of a cave fish as a vestigial structure and explain why that is evidence of common ancestry, and you separate convergent traits in unrelated organisms from shared ancestry.
Speciation questions ask you to tell behavioral isolation (different mating songs) from other prezygotic barriers, to recognize hybrid sterility and failed hybrid embryos as postzygotic barriers, to classify a tetraploid plant that can no longer breed with its diploid parents as sympatric speciation, and to recognize adaptive radiation when one ancestral bird species on an island chain gives rise to many species with specialized beaks. A fossil series of snail shells that stays the same for millions of years and then changes quickly illustrates punctuated equilibrium rather than gradualism.
The last questions connect genetic diversity to the risk of extinction, using a small island fox population facing a new disease, and cover the origin of life: the age of Earth, the earliest fossil evidence of life, the formation of organic molecules from inorganic precursors, and why RNA that both stores information and catalyzes reactions supports the RNA world hypothesis.
The material offers a quiz and a flashcard deck. The quiz explains why the correct classification fits the scenario and why each alternative does not. The flashcards review the fifteen key terms of these topics, from Hardy–Weinberg equilibrium and genetic drift to prezygotic and postzygotic barriers, polyploidy, punctuated equilibrium, adaptive radiation and the RNA world hypothesis.
- Identify which Hardy–Weinberg condition a scenario violates
- Interpret fossil, anatomical and vestigial-structure evidence of evolution
- Classify prezygotic and postzygotic reproductive barriers
- Distinguish sympatric from allopatric speciation, punctuated equilibrium from gradualism, and recognize adaptive radiation
- Explain the link between genetic diversity and extinction risk, and the evidence for models of the origin of life
Practice material written by Zestly, based on the College Board AP Biology course framework (Unit 7 topics 7.5, 7.6, 7.10, 7.11 and 7.12).
Sample question
In an invented cliff face, sedimentary rock layers lie undisturbed one on top of another. Fossil A is found in a layer near the bottom, and fossil B in a layer near the top. The volcanic ash layer immediately over fossil A is dated by radioactive decay to 52 million years ago, and the ash layer immediately under it to 55 million years ago. Which conclusion is best supported?
- Fossil A must be exactly 55 million years old, because it lies under the 52-million-year ash layer.
- The ages of the fossils cannot be compared, because only the layers around fossil A were dated.
- Fossil A is between about 52 and 55 million years old, and fossil B is younger than fossil A.
- Fossil B is older than fossil A, because the top layer was exposed to the air for longer.
See the answer
Fossil A is between about 52 and 55 million years old, and fossil B is younger than fossil A.
In undisturbed sedimentary rock, lower layers were deposited first, so fossil A is older than fossil B. The dated ash layers bracket fossil A between 52 and 55 million years old. Relative position in the rock allows fossil B to be compared even without a direct date.
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