Extinction is the irreversible disappearance of all signs of life pertaining to a particular group of organisms. This group can be any of the accepted categories of taxonomic nomenclature, including kingdom, phylum, class, order, family, genus, species, and even subspecies. Most often, extinction is used in reference to the species level organism. The organism may have gone extinct at any time within the past 600 million years of the fossil record or may be in the process of going extinct today. Furthermore, this phenomenon does not pertain only to animals but is also applicable to plants, fungi, protists, bacteria, and archaebacteria.

Many situations can theoretically lead to the extinction of an organism. Determining the cause of extinction is very difficult, especially when information from the fossil record must be used to reconstruct a sequence of events. In many cases, a direct explanation is not forthcoming, and many hypotheses must...