Anagenesis, also known as "phyletic transformation", is when the new morphospecies is a result of rapid evolution in the ancestral form without speciation taking place, such that there are no remaining other populations of the ancestor species and the species can be considered extinct. The ancestor species is therefore superseded by the new species it morphs into. Anagenesis is in contrast to the branching speciation known as cladogenesis. Anagenesis rejects animals related by convergent evolution and unlike the study of behavioral homologies permits the inclusion of animals related by parallel evolution (Yarczower & Hazlett 1977).
However, the Punctuated Equilibria Hypothesis was proposed, which explained the gradual direction of phyletic change. Under this hypothesis, phyletic transformation is almost meaningless, because if it is correct, lineages do not change during its lifetime, they remain static until speciation occurs so that anagenesis is not possible (Gutman & Altava 2008).
Recent evolutionary studies are looking at anagenesis and cladogeneis for possible answers in developing the hominin phylogenetic tree to understand morphological diversity and the origins of "Australopithecus anamensis", and this case could possibly show anagenesis in the fossil record (Kimbel et al. 2006).
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 series of such species is collectively known as an evolutionary lineage. The various species along an evolutionary lineage are chronospecies. If the ancestral population of a chronospecies does not go extinct, then this is cladogenesis, and the ancestral population represent a paraphyletic species or paraspecies, being an evolutionary grade. This situation is quite common in species with widespread populations.
It is easy to see from this 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.
- The University of California, Berkeley resource on understanding evolution defines a lineage as "A continuous line of descent; a series of organisms, populations, cells, or genes connected by ancestor/descendent relationships." Understanding Evolution, Glossary of Terms
- The Oxford English Dictionary defines biological lineage as "a sequence of species each of which is considered to have evolved from its predecessor."OED definition of lineage
- Ereshefsky, M. (2001), "Philosophy of Biological Classification", Encyclopedia of Life Sciences, Wiley-Blackwell, doi:10.1038/npg.els.0003447, ISBN 0470016175
- Leonid Grinin & Andrey Korotayev. Social Macroevolution: Growth of the World System Integrity and a System of Phase Transitions. World Futures, Volume 65, Issue 7 October 2009 , pages 477 - 506; Aromorphoses in Biological аnd Social Evolution: Some General Rules for Biological and Social Forms of Macroevolution in Social Evolution & History (Vol. 8 No. 2, September 2009: 6-50).
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