Anagenesis, also known as "phyletic change", is the evolution of species involving an entire population rather than a branching event, as in cladogenesis. When enough mutations have occurred and become stable in a population so that it is significantly differentiated from an ancestral population, a new species name may be assigned. A key point is that the entire population is different from the ancestral population such that the ancestral population can be considered extinct. A series of such species is collectively known as an evolutionary lineage.

It is easy to see from the preceding definition how controversy can arise among taxonomists as to when the differences are significant enough to warrant a new species classification. Anagenesis may also be referred to as "gradual evolution".

The philosopher of science Marc Ereshefsky argues that paraphyletic taxa are the result of anagenesis. The lineage leading to birds has diverged significantly from lizards and crocodiles, allowing evolutionary taxonomists to classify birds separately from lizards and crocodiles, which are grouped as reptiles.

Regarding social evolution, it has been suggested that social anagenesis/aromorphosis be viewed as universal or widely diffused social innovation that raises social systems' complexity, adaptability, integrity, and interconnectedness.