Extinction — AP Biology Study Guide

For: AP Biology candidates sitting AP Biology.

Covers: Definitions of background and mass extinction, causes of extinction, the role of extinction in adaptive radiation, key connections between extinction and natural selection, and current human-driven extinction rates, aligned with official AP Biology CED requirements.

You should already know: Natural selection as the mechanism of evolution, Biological species concept for defining species, Ecological niches and basic biodiversity.

A note on the practice questions: All worked questions in the "Practice Questions" section below are original problems written by us in the AP Biology style for educational use. They are not reproductions of past College Board / Cambridge / IB papers and may differ in wording, numerical values, or context. Use them to practise the technique; cross-check with official mark schemes for grading conventions.

1. What Is Extinction?

Extinction is defined as the permanent global loss of all individuals of a species, such that no viable reproducing populations remain anywhere on Earth. This distinguishes full extinction from local extirpation (loss of a population from a single geographic region, with other populations surviving elsewhere) and functional extinction (a species with so few remaining individuals that it can no longer fulfill its ecological role, even if a small number of individuals are still alive). According to the AP Biology Course and Exam Description (CED), this topic is part of Unit 7: Natural Selection, which contributes 13–20% of the total AP Biology exam score. Extinction appears regularly on both the multiple-choice (MCQ) section, often as data analysis or concept application questions, and the free-response (FRQ) section, where it is commonly paired with topics like human impact, biodiversity, and macroevolution. Extinction is a naturally occurring process that is an expected outcome of natural selection: species unable to adapt to changing environmental conditions will be outcompeted or die off over time.

2. Background vs. Mass Extinction

Extinction is categorized into two main types based on rate, scale, and cause: background extinction and mass extinction. Background extinction is the steady, low rate of species loss that occurs under normal ecological conditions, driven by local factors like interspecific competition, small population genetic drift, minor regional climate change, or coextinction of species dependent on a lost host. The widely accepted baseline natural background extinction rate is approximately $1$ extinction per million species-years ($1$ E/MSY), meaning for every 1 million species living on Earth, we expect 1 extinction each year.

Mass extinction is a rare, catastrophic event where at least 75% of all species on Earth go extinct within a geologically short time window (fewer than 2.8 million years). Mass extinctions are driven by global-scale disruptive events: asteroid impacts, massive volcanic activity leading to rapid climate change, or rapid ocean acidification. There have been 5 major confirmed mass extinctions in Earth’s history, the most recent being the Cretaceous-Paleogene (K-Pg) extinction that eliminated non-avian dinosaurs 66 million years ago.

Worked Example

Paleontologists studying a 1-million-year interval of geologic time record 1500 extinctions among a sample of 2000 existing species. Is this event consistent with background extinction, or does it qualify as a mass extinction?

1. First, recall the two core criteria for mass extinction: ≥75% of all species lost in <2.8 million years. This time interval (1 million years) falls within the short time window requirement.
2. Calculate the percentage of species lost: $\frac{1500}{2000}\times 100=75\%$, which meets the percent loss threshold.
3. Compare the observed rate to expected background extinction: Expected background extinctions for 2000 species over 1 million years = $2000\times 1\text{ E/MSY}=2$ extinctions.
4. The observed 1500 extinctions are 750 times higher than the baseline background rate, so this event meets all criteria for mass extinction.

Exam tip: On AP Biology MCQ, you will often be given raw extinction counts and asked to classify the event; always check both the percentage of species lost and the time frame, not just the raw number of extinctions.

3. Evolutionary and Ecological Consequences of Extinction

Extinction is not only a loss of biodiversity; it is a core driver of macroevolutionary change. When species go extinct, they leave empty ecological niches (the specific role and resource set a species occupies in an ecosystem). Empty niches free up resources like food, habitat, and breeding sites for surviving species, reducing interspecific competition and creating new selective pressures that favor divergence. This process often leads to adaptive radiation, where a single surviving ancestral lineage rapidly diversifies into many new species, each adapted to a different empty niche.

A classic example is the adaptive radiation of mammals after the K-Pg extinction: non-avian dinosaurs occupied most large herbivore and carnivore niches for over 100 million years, keeping mammal populations small and restricted to few niches. After dinosaurs went extinct, mammals rapidly diversified into all the major mammalian groups we see today, including primates. Another key ecological consequence is coextinction: when a specialist species depends entirely on a host for survival, it will go extinct when the host goes extinct.

Worked Example

Hawaiian honeycreepers are a group of birds that evolved via adaptive radiation from a single ancestral colonizer species 5 million years ago, with each species evolving a unique beak shape to feed on specific native plant nectar or seeds. Over the past 200 years, 43 of the 53 known honeycreeper species have gone extinct due to invasive disease and habitat destruction. Explain how these extinctions will affect the evolution of the 10 remaining honeycreeper species.

1. Each extinct honeycreeper occupied a unique, now empty niche with unused resources from specific native plants.
2. Extinction of 43 species removes interspecific competition for these unused resources, so individual surviving honeycreepers with traits that allow them to use these resources will have higher survival and reproductive success.
3. Natural selection will favor trait divergence from the original niche of each surviving species, as individuals that use new resources leave more offspring.
4. Over generations, this divergent selection will lead to new adaptive radiation, with surviving lineages splitting into new species to fill the empty niches.

Exam tip: When asked to connect extinction to evolution on the FRQ, always explicitly link extinction to opening of niches, which then drives adaptive radiation; AP readers require this explicit causal connection, not just a vague reference to evolution.

4. The Current Sixth Mass Extinction (Human-Driven Extinction)

Scientific consensus confirms that Earth is currently experiencing a sixth mass extinction, entirely driven by anthropogenic (human) activity. Current extinction rates are estimated to be between 100 and 10,000 times higher than the natural background rate of 1 E/MSY, with most estimates falling around 1000 E/MSY. This rate is faster than almost all past mass extinction events.

The four main human drivers of elevated extinction rates are: (1) habitat loss and fragmentation, which splits large populations into small, isolated groups with high extinction risk; (2) invasive species and disease, which outcompete or kill naive native species; (3) overexploitation (overhunting, overfishing, poaching) that reduces population sizes below viable levels; (4) anthropogenic climate change, which alters temperature, precipitation, and ocean chemistry faster than most species can adapt. Species with small population sizes, long generation times, and specialized niches are disproportionately vulnerable to extinction, as they lack the genetic variation and generation turnover needed to adapt to rapid change.

Worked Example

Researchers compared extinction rates in fragmented Amazon rainforest (cut into small isolated patches by agriculture) vs. intact continuous rainforest. They found an extinction rate of 12 E/MSY in fragmented forest, compared to 0.5 E/MSY in intact forest. Explain this difference in the context of human-driven extinction.

1. Habitat fragmentation caused by clearing for agriculture reduces total available habitat and isolates populations into small, disconnected groups.
2. Small isolated populations have increased genetic drift and inbreeding depression, which increases the frequency of harmful deleterious alleles and reduces population fitness, raising extinction risk. Isolation also prevents recolonization by other populations after local extinction events.
3. Intact continuous forest has larger total population sizes, higher genetic diversity, and connectivity between populations, which supports gene flow and recolonization, reducing extinction risk.
4. The 24-fold higher extinction rate in fragmented forest is directly caused by human habitat fragmentation reducing population viability.

Exam tip: When asked to explain why a specific species is vulnerable to extinction, always link vulnerability to small population size, low genetic diversity, and long generation time; these are the key points AP exam graders look for.

5. Common Pitfalls (and how to avoid them)

• Wrong move: Confusing local extirpation with full global extinction when answering questions about the current mass extinction. Why: Students often see data about a species being lost from a region and incorrectly assume it is fully extinct. Correct move: Always check if the question specifies "global loss" vs. "local loss" before classifying the event as full extinction.
• Wrong move: Stating that extinction is only caused by human activity. Why: Modern curricula focus heavily on anthropogenic extinction, leading students to forget that extinction is a natural process that occurred for billions of years before humans evolved. Correct move: Always acknowledge that extinction is a natural outcome of natural selection, and human activity has only increased the rate far above natural baseline.
• Wrong move: Defining mass extinction by just a high number of extinctions, ignoring the time frame requirement. Why: Students memorize the 75% loss threshold but forget the short time window rule. Correct move: Always apply both criteria for mass extinction: ≥75% of all species lost within <2.8 million years.
• Wrong move: Forgetting to link extinction to adaptive radiation when asked for evolutionary consequences. Why: Students focus only on the negative impact of extinction on biodiversity, not the evolutionary opportunity it creates. Correct move: Whenever asked for evolutionary consequences of extinction, explicitly mention that extinction opens empty niches that drive adaptive radiation of surviving species.
• Wrong move: Confusing functional extinction with mass extinction on rushed MCQ questions. Why: Both terms start with "f", leading to quick mix-ups. Correct move: On test day, circle or underline the term in the question to confirm you are referencing the correct definition.

6. Practice Questions (AP Biology Style)

Question 1 (Multiple Choice)

Scientists studying coral reef biodiversity estimate that current extinction rates on tropical coral reefs are approximately 100 extinctions per million species-years. The natural background extinction rate for shallow-water marine species is approximately 0.1 extinctions per million species-years. Which of the following statements correctly compares the current rate to the natural baseline? A) The current extinction rate is 10 times higher than the natural background rate B) The current extinction rate is 100 times higher than the natural background rate C) The current extinction rate is 1000 times higher than the natural background rate D) The current extinction rate is 10,000 times higher than the natural background rate

Worked Solution: To find the multiplicative difference between the current rate and the baseline rate, divide the current rate by the background rate: $\frac{100}{0.1}=1000$. The question asks for how many times higher the current rate is, which calls for a multiplicative ratio, not an additive difference. This calculation matches the widely cited estimate that current extinction rates are 1000 times higher than natural baseline. The correct answer is C.

Question 2 (Free Response)

The five historic mass extinctions on Earth eliminated large numbers of species, but eventually led to long-term net increases in global biodiversity. (a) Define mass extinction and identify one natural cause of historic mass extinctions. (2 points) (b) Explain why mass extinction events often lead to long-term increases in global biodiversity. (2 points) (c) Explain why the current anthropogenic mass extinction is not expected to lead to a similar increase in biodiversity over the next 1000 years. (2 points)

Worked Solution: (a) A mass extinction is an event where at least 75% of all species on Earth go extinct within a geologically short time period (fewer than 2.8 million years). One natural cause of historic mass extinctions is a large asteroid impact (alternatively: massive volcanic activity causing rapid climate change). (b) Mass extinction eliminates most existing species, which leaves all of their ecological niches empty. Empty niches free up unused resources for surviving species, reducing competition and creating new selective pressures. Surviving lineages then undergo adaptive radiation, rapidly diversifying into new species to fill the empty niches, leading to a net increase in biodiversity over millions of years. (c) Speciation (the evolution of new species) takes thousands to millions of years to generate large numbers of new species, and the current extinction event is happening hundreds to thousands of times faster than past mass extinctions. Additionally, ongoing human activity continues to destroy habitat and alter climate, preventing surviving species from diversifying into empty niches, so no increase in biodiversity is expected over 1000 years.

Question 3 (Application / Real-World Style)

The black rhino is a large-bodied mammal with an estimated 5500 total individuals left in the wild, down from 500,000 in 1900. Black rhinos have a generation time of 25 years, require large individual territories, and are now split into small isolated populations in protected parks due to habitat fragmentation. Using what you know about extinction risk, explain why black rhinos are at extremely high risk of extinction in the next 100 years, even with full anti-poaching protection. Include reference to population genetics and natural selection in your answer.

Worked Solution:

1. Black rhinos have a very small total population size split into isolated groups, which increases genetic drift in each small population. Genetic drift raises the frequency of harmful deleterious alleles, causing inbreeding depression that reduces individual fitness and population growth.
2. Isolation of populations prevents gene flow between groups, so there is no new genetic variation introduced to counteract inbreeding depression.
3. Black rhinos have a long generation time of 25 years, so natural selection acts very slowly on the species. They cannot adapt fast enough to new threats like emerging diseases or rapid climate change, even if some genetic variation for adaptation exists.
4. Even with anti-poaching protection, these genetic and ecological factors make the black rhino extremely vulnerable to extinction. In context, this means active human intervention (like moving individuals between populations to increase gene flow) is required to prevent the loss of the species.

7. Quick Reference Cheatsheet

8. What's Next

Extinction is a core macroevolutionary process that sets the stage for the next topics in Unit 7: Natural Selection, specifically speciation and phylogenetic tree interpretation. Understanding extinction is required to correctly read phylogenetic trees that include extinct lineages, and to understand how speciation and extinction together shape global patterns of biodiversity. Without mastering the differences between background and mass extinction, and the evolutionary consequences of extinction, you will struggle to answer high-weight FRQ questions that connect macroevolution to human impact. Extinction also feeds directly into Unit 8: Ecology, where you will apply this understanding to study global biodiversity and conservation strategies.

• Speciation
• Phylogenetics
• Biodiversity
• Conservation Biology