Extinction is the end of an organism or of a group of organisms (taxon), usually a species. The moment of extinction is generally considered to be the death of the last individual of the species, although the capacity to breed and recover may have been lost before this point. Because a species’ potential range may be very large, determining this moment is difficult, and is usually done retrospectively. This difficulty leads to phenomena such as Lazarus taxa, where a species presumed extinct abruptly “reappears” (typically in the fossil record) after a period of apparent absence.
Through evolution, new species arise through the process of speciation—where new variants arise that differ from existing populations—and existing species disappear through extinction. Speciation occurs when reproductive isolation between two groups becomes complete so that members of one group can no longer interbreed with members of another group even if they share habitat; this eventually results in populations that are morphologically and/or behaviorally distinct. Extinction then happens when there are no more individuals left alive who can reproduce and create offspring belonging to that particular lineage; it may also occur when an entire lineage goes extinct due to changes in environmental conditions or competition with other lineages. There are four main causes for natural extinction: habitat loss or destruction, overexploitation, disease/predation, and catastrophic events such as asteroid strikes.
Habitat loss or destruction is by far the most common cause of natural extinctions. It occurs when an environment suitable for sustaining a certain population can no longer do so because its resources have been depleted or it has been otherwise rendered uninhabitable (for example through pollution). Overexploitation occurs when a resource necessary for survival (such as food or water) is consumed at rates greater than its replenishment rate, leading to depletion and eventual extinction. Disease and predation typically work in tandem: predators hunt down sick individuals which weakens the prey population as a whole and makes them more susceptible to disease; meanwhile diseases reduce population numbers by killing off individuals directly. Catastrophic events like asteroid strikes are relatively rare but can lead to mass extinctions if they destroy enough habitats or kill enough individuals outright.
The history of life on Earth has been marked by numerous mass extinctions: times when global biodiversity took sharp dips due to some combination of these four main causes. The best known mass extinction event is probably the Cretaceous–Paleogene extinction event 66 million years ago which wiped out around three quarters of all plant and animal species on Earth including all non-avian dinosaurs; this event was likely caused by an asteroid strike resulting in widespread environmental devastation that made it impossible for many groups to survive. More recently, human activity has led to what some scientists refer to as the sixth mass extinction: starting around 1500 CE humans began colonizing new areas around the globe and bringing with them diseases that decimated local populations (as well as hunting animals into near-extinction for food & resources); at the same time clearance for agriculture & urbanization destroyed natural habitats on massive scales leading many plants & animals towards endangerment & possible extincti