Biology · Unit 7: Natural Selection · 14 min read · Updated 2026-05-10
Extinction — AP Biology
AP Biology · Unit 7: Natural Selection · 14 min read
1. Core Definitions of Extinction★★☆☆☆⏱ 3 min
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 two related terms: 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).
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. This topic contributes to the 13–20% of AP Biology exam score allocated to Unit 7: Natural Selection, and appears regularly on both multiple-choice and free-response sections.
2. Background vs. Mass Extinction★★★☆☆⏱ 4 min
Extinction is categorized into two main types based on rate, scale, and cause: background extinction and mass extinction.
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 extinctions are driven by global-scale disruptive events such as 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.
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★★★☆☆⏱ 4 min
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.
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)★★★★☆⏱ 3 min
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.
**Habitat loss and fragmentation**: splits large populations into small, isolated groups with high extinction risk
**Invasive species and disease**: outcompete or kill naive native species with no evolved defenses
**Overexploitation**: overhunting, overfishing, and poaching that reduces population sizes below viable levels
**Anthropogenic climate change**: 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.
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. AP Style Concept Check★★★☆☆⏱ 3 min
Common Pitfalls
Why: Students often see data about a species being lost from a region and incorrectly assume it is fully extinct.
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.
Why: Students memorize the 75% loss threshold but forget the short time window rule.
Why: Students focus only on the negative impact of extinction on biodiversity, not the evolutionary opportunity it creates.
Why: Both terms start with 'f', leading to quick mix-ups.