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Anagenesis is a form of evolution in which a species as a whole changes significantly over time, so that the ancestral species is effectively extinct. This is what many people think of in reference to the evolutionary process, as entire generations of species develop and change over time to adapt to environmental needs or acquire new features. As these changes occur, the previous species simply ceases to exist, since the change occurs throughout entire generations and not simply isolated pockets of a species population. Anagenesis is in contrast to cladogenesis, which is a form of evolution in which part of a species changes to become a new species.
Also called phyletic evolution or change, anagenesis is the form of evolution that occurs when one entire species changes into another. As humanity has evolved from primates, for example, each species has changed and adapted to various needs to become a new species. Homo sapiens in existence now likely came from Homo erectus, but the process of evolution occurred for the entirety of the species, leaving Homo erectus to go extinct. This same process of anagenesis can be seen in other animals, in which ancient species have evolved into newer species and effectively gone extinct in the process.
The importance of anagenesis lies in the fact that it can be used to understand how species evolve in their entirety, and why some species end up going extinct. Such evolutionary changes occur in a variety of different ways, but they are not limited to pockets of a species population. When documented in a visual way, anagenesis usually takes the form of a single line with a slope that demonstrates the changes in a species over time. This is in contrast to cladogenesis, which is often depicted in graphical form as a line that branches out to produce new lines.
Cladogenesis, unlike anagenesis, is a process of evolution in which some part of a population changes and evolves, but not the entire species or generation. This leaves the original species more or less intact, allowing for both the older species and the new species to coexist. The test for evolution from one species into a truly new and unique species is typically whether or not the two species can mate with each other. In cladogenesis, the resulting species cannot still breed with the original species; anagenesis is not subject to this test, since the previous species has gone extinct during the period of evolution.