Extinction is the act or process of the dying-out of a species or evolutionary lineage. It is one of the most common of all ecological/evolutionary processes and represents an inevitable corollary of evolution by natural selection. Information on extinction comes from many sources, including laboratory experiments, field studies and the fossil record. The causes of short-term extinctions involving few species can, in principle, be well understood. The causes of long-term extinction events, such as those preserved in the fossil record, remain controversial. Although the contemporary biodiversity crisis is often referred to as a ‘sixth (mass) extinction’, levels of documented losses over the last 400 years have been modest by geological standards. Nevertheless, historical extinction rates are well above palaeontological background rates and are predicted to increase substantially over the next 200 years. Whether the number of species losses over this interval will reach geological magnitudes remains uncertain.

Key Concepts:

• Extinction is a common and perfectly natural process.
  
• The processes that lead to extinction operate at a variety of spatial, ecological and temporal scales.
  
• Extinctions can be studied in the laboratory, in the field (modern) and in the fossil record (ancient); however, the ability of researchers to determine extinction patterns accurate and evaluate causal models in detail decreased as the spatial and temporal scales along those spectrum increase and as the biology of the organisms in question grows less familiar.
  
• Extinction processes can also be studied via mathematical simulation models. In many (though not all) cases, there is good agreement between the predictions of these mathematical models and field observations.
  
• The fossil record of extinction events exhibits two compelling patterns: (1) a historical pattern of long periods of relatively modest (= background) extinction intensities over time, interspersed by short, nonperiodic intervals of elevated extinction intensity and (2) a secular decline in the magnitude of background extinction intensity from the mid-Palaeozoic to the Recent.
  
• Lower-level peaks in extinction intensity are probably due to the operation of single major or idiosyncratic combinations of moderate intensity process that foster long-term environmental change (e.g. sea-level change, continental glaciation, large igneous province volcanism and asteroid/comet impact).
  
• Higher-level peaks in extinction intensity (= mass extinctions) are probably due to idiosyncratic combinations of major process that foster long-term environmental change (e.g., sea-level change, continental glaciation, large igneous province volcanism and asteroid/comet impact).
  
• Over the last 400 years, extinctions have occurred at a rate that is well above background rates as inferred from the fossil record, but well below the range of even moderate geological extinction events. Models of maximum extinction intensities predicted over the next 200 years are matters for concern, but fall well below the intensities achieved by geological mass extinction events.