Glossary

**Extinction** is the complete and permanent disappearance of a biological species, occurring when no living individuals of that species remain anywhere on Earth. Species become extinct due to a range of environmental and evolutionary factors, including habitat fragmentation, climate change, natural disasters, overexploitation, interspecific competition, genetic inbreeding, and declining reproductive success. An estimated 99% of all species that have ever existed are now extinct. Under normal conditions, species disappear at a low, continuous rate of roughly one to five species per year across the entire fossil record, a process termed **background extinction**. Periodically, however, extinction rates spike dramatically during **mass extinction** events, in which a substantial proportion of Earth's biodiversity — typically 75% or more of species — is lost within a geologically brief interval. These catastrophic events, driven by asteroid impacts, large-scale volcanism, rapid climate shifts, and other global-scale perturbations, fundamentally restructure ecosystems and open ecological niches for surviving lineages, thereby shaping the trajectory of evolution.