Final answer:
The typical rate of extinctions within a particular taxon under normal conditions is known as background extinction. This rate is drastically lower than what is currently observed due to human activities, which are causing extinction rates to approach those seen during historical mass extinction events.
Step-by-step explanation:
The typical tempo of extinctions for a taxonomic group under normal environmental conditions is referred to as background extinction. This is the standard rate of extinction in earth's history without the influence of catastrophic upheavals or human interference. It's estimated to be about one extinction per million species years (E/MSY).
In contrast, mass extinctions are rare events that have occurred five times in the fossil record, resulting in the loss of more than half of all species each time. These events are characterized by dramatically higher extinction rates than background extinction rates and are often due to extraordinary geological or climatic events.
Today, human activities have increased extinction rates to levels that are comparable to those of past mass extinctions. Current estimates suggest that extinction rates are now 500 to 1,000 times the background extinction rate, which could lead to a significant loss of biodiversity in the near future.