Extinction
Extinction - the permanent loss of a species - is a normal part of evolutionary history, sometimes occurring in mass events that reshape biodiversity.
Background extinction vs. mass extinction
Extinction is the natural end point for the overwhelming majority of species that have ever existed - estimates suggest over 99% of all species that ever lived are now extinct. Background extinction refers to the low, relatively steady rate of species loss that occurs continuously over geological time due to ordinary processes: competition, gradual environmental change, or simply being outcompeted by better-adapted lineages.
Mass extinctions are much rarer, dramatic events in which a large fraction of species disappear within a geologically short window of time, usually due to a sudden, severe environmental disruption that outpaces most species' ability to adapt or migrate. The fossil record documents five major mass extinctions, the largest being the end-Permian extinction (~252 million years ago), which eliminated an estimated 96% of marine species, and the most recent being the end-Cretaceous (K-Pg) extinction (~66 million years ago), which eliminated roughly 75% of species, including all non-avian dinosaurs, likely triggered by a massive asteroid impact combined with intense volcanic activity.
What makes species vulnerable
- Small population size and low genetic diversity reduce a population's capacity to adapt to new pressures and make it vulnerable to drift and inbreeding depression.
- Habitat specialization: species that depend on a narrow, specific habitat or resource are more vulnerable to environmental change than generalists.
- Slow reproductive rate (long generation time, few offspring) limits how quickly a population can rebound from losses or evolve in response to new pressures.
- Restricted geographic range, especially on islands, increases vulnerability to localized disturbances (invasive species, habitat destruction, disease).
Extinction opens the door for adaptive radiation
Mass extinctions, while catastrophic in the short term, have historically been followed by episodes of adaptive radiation, in which surviving lineages rapidly diversify to exploit newly vacated ecological niches with reduced competition. The extinction of non-avian dinosaurs at the end of the Cretaceous is widely credited with opening ecological opportunities that allowed mammals - previously small and ecologically constrained - to rapidly diversify into the wide range of forms and niches they occupy today.
This pattern illustrates an important evolutionary principle: extinction is not simply a loss but also a major driver of subsequent biodiversity, reshuffling which lineages have access to which resources and habitats.
Key terms
4
- Extinction
- The permanent disappearance of a species from Earth.
- Mass extinction
- A relatively short period during which a large percentage of Earth's species go extinct, usually due to a major environmental disruption.
- Background extinction rate
- The relatively constant, low rate of species extinction occurring outside of mass extinction events.
- Adaptive radiation
- The rapid diversification of a lineage into many new species, often filling ecological niches left vacant after a mass extinction or opened by a new adaptation.
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