Anagenetic

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Anagenetic evolution refers to the evolution of an ancestral species to a descendant species without a split of lineage.

There are two ways in which evolution can produce a succession of species through time. The first is called anagenesis. This is where one species gradually evolves through such that after an extended period the population has changed sufficiently to be considered a new species. The second is cladogenesis. This is where a population splits and each part evolves separately along a different line from the other.

In the first case one species literally replaces another as the parent species evolves into the daughter species. In the second case the new and old species co-exist until one or both becomes extinct. The new species does not replace the old one, and sometimes the old species can outlive the new species.

It should be noted that nothing in these definitions assumes that the new species drives the old species to extinction.