What is the process through which one species separates into two or more species called?

Speciation is the process through which one species diverges into two or more distinct species, resulting in increased biodiversity. This typically occurs through a variety of mechanisms, such as geographic isolation, where physical barriers prevent populations from interbreeding, or reproductive isolation, where differences in mating behaviors or times prevent interbreeding even when populations are in proximity.

As populations accumulate genetic changes over time due to natural selection, genetic drift, or mutation, they may become sufficiently differentiated that they can no longer interbreed, leading to the formation of new species. Speciation is a fundamental concept in evolutionary biology, as it helps to explain how new species arise and how life's diversity has developed over millions of years.

While extinction, evolution, and adaptation are important biological concepts, they refer to different phenomena. Extinction involves the loss of a species, evolution describes the overall change in species over time, and adaptation relates to the process of becoming better suited to an environment. Speciation specifically focuses on the formation of new species, making it the most accurate term for the process described in the question.