What is the term for the evolutionary process where new species form?

The evolutionary process where new species form is called speciation. It happens when populations of a species become genetically different enough that they form separate evolutionary lineages and can no longer freely exchange genes.

Speciation often begins with a barrier to gene flow. Geographic isolation, such as the separation of populations on different islands, can allow mutations, natural selection, and genetic drift to accumulate independently. Over many generations, the populations may diverge in anatomy, behavior, habitat, or mating signals.

In allopatric speciation, physical separation drives divergence. In sympatric speciation, new species arise in the same geographic area, sometimes through changes in chromosome number, host preference, or mating behavior. Reproductive isolation is the key outcome: individuals may no longer mate, produce viable offspring, or produce fertile offspring together.

Speciation is different from mutation, which is a source of genetic variation, and from adaptation, which is the evolution of traits suited to an environment. Extinction is the disappearance of a lineage, not its division into new species. Darwin discussed how varieties might become distinct species in On the Origin of Species, published in 1859.