Anagenesis, also known as "phyletic change", is the evolution of species involving a change in gene frequency in an entire population rather than a branching event, as in cladogenesis. When enough mutations reach fixation in a population to significantly differentiate 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.
It is easy to see from the preceding definition how controversy can arise among taxonomists when the differences are significant enough to warrant a new species classification. Anagenesis may also be referred to as "gradual evolution".