The continuous, low-level extinction of species that has occurred throughout the history of life on Earth is known as:

Background extinction

Explanation:

The background extinction refers to the diminishing of the species at a fairly slow and constant rate. This extinction takes place because of the various environmental factors namely the changing climatic conditions, spreading of diseases, and destruction of habitat. It plays an important role in the natural process of evolution. This has been taking place continuously throughout the geological time and the species are undergoing continuous evolution over a specific period of time.