Anagenesis is the progressive evolution of species involving a change in gene frequency in an entire population rather than a cladogenetic branching event. 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 so that the ancestral population can be considered extinct. It is easy to see from the preceding definition how controversy can arise among taxonomists regarding when the differences are significant enough to warrant a new species classification. Anagenesis may also be referred to as phyletic evolution.
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