In evolutionary biology, change within a single lineage without branching into separate species is called anagenesis.
Anagenesis describes evolutionary transformation along one continuous lineage. Over generations, inherited changes accumulate in a population until the later form differs substantially from its ancestor. The lineage itself persists, rather than splitting into two or more independently evolving branches.
This contrasts with cladogenesis, in which one lineage divides into separate lineages. Cladogenesis commonly increases biodiversity because a single ancestral species can produce multiple descendant species. Anagenesis changes one lineage but does not directly increase the number of species.
The terms are often used in discussions of fossil sequences, where gradual morphological change may appear as a succession of forms. However, fossil records are incomplete, so deciding whether a sequence represents anagenesis or hidden branching can be difficult. Anagenesis is also called phyletic evolution.