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Anagenesis is the gradual
evolution Evolution is change in the heritable characteristics of biological populations over successive generations. These characteristics are the expressions of genes, which are passed on from parent to offspring during reproduction. Variation ...
of a
species In biology, a species is the basic unit of classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individuals of the appropriate s ...
that continues to exist as an interbreeding population. This contrasts with
cladogenesis Cladogenesis is an evolutionary splitting of a parent species into two distinct species, forming a clade. This event usually occurs when a few organisms end up in new, often distant areas or when environmental changes cause several extinctions, ...
, which occurs when there is branching or splitting, leading to two or more lineages and resulting in separate species. Anagenesis does not always lead to the formation of a new species from an ancestral species. When speciation does occur as different lineages branch off and cease to interbreed, a core group may continue to be defined as the original species. The evolution of this group, without
extinction Extinction is the termination of a kind of organism or of a group of kinds (taxon), usually a species. The moment of extinction is generally considered to be the death of the last individual of the species, although the capacity to breed and ...
or
species selection A unit of selection is a biological entity within the hierarchy of biological organization (for example, an entity such as: a self-replicating molecule, a gene, a cell, an organism, a group, or a species) that is subject to natural selection. Th ...
, is anagenesis.


Hypotheses

One hypothesis is that during the speciation event in anagenetic evolution, the original populations will increase quickly, and then rack up genetic variation over long periods of time by
mutation In biology, a mutation is an alteration in the nucleic acid sequence of the genome of an organism, virus, or extrachromosomal DNA. Viral genomes contain either DNA or RNA. Mutations result from errors during DNA replication, DNA or viral repl ...
and recombination in a stable environment. Other factors such as
selection Selection may refer to: Science * Selection (biology), also called natural selection, selection in evolution ** Sex selection, in genetics ** Mate selection, in mating ** Sexual selection in humans, in human sexuality ** Human mating strateg ...
or
genetic drift Genetic drift, also known as allelic drift or the Wright effect, is the change in the frequency of an existing gene variant (allele) in a population due to random chance. Genetic drift may cause gene variants to disappear completely and there ...
will have such a significant effect on
genetic material Nucleic acids are biopolymers, macromolecules, essential to all known forms of life. They are composed of nucleotides, which are the monomers made of three components: a 5-carbon sugar, a phosphate group and a nitrogenous base. The two main clas ...
and physical traits that a species can be acknowledged as being different from the previous.


Development

An alternative definition offered for anagenesis involves progeny relationships between designated taxa with one or more denominated taxa in line with a branch from the evolutionary tree. Taxa must be within the species or genus and will help identify possible ancestors. When looking at evolutionary descent, there are two mechanisms at play. The first process is when genetic information changes. This means that over time there is enough of a difference in their
genome In the fields of molecular biology and genetics, a genome is all the genetic information of an organism. It consists of nucleotide sequences of DNA (or RNA in RNA viruses). The nuclear genome includes protein-coding genes and non-coding g ...
s, and in the way that species' genes interact with each other during the developmental stage, that anagenesis can thereby be viewed as the processes of sexual and natural selection, and genetic drift's effect on an evolving species over time. The second process, speciation, is closely associated with cladogenesis. Speciation includes the actual separation of lineages, into two or more new species, from one specified species of origin. Cladogenesis can be seen as a similar hypothesis to anagenesis, with the addition of speciation to its mechanisms. Diversity on a species-level is able to be achieved through anagenesis. Anagenesis suggests that
evolution Evolution is change in the heritable characteristics of biological populations over successive generations. These characteristics are the expressions of genes, which are passed on from parent to offspring during reproduction. Variation ...
ary changes can occur in a species over time to a sufficient degree that later organisms may be considered a different species, especially in the absence of fossils documenting the gradual transition from one to another. This is in contrast to cladogenesis—or speciation in a sense—in which a population is split into two or more
reproductively isolated The mechanisms of reproductive isolation are a collection of evolutionary mechanisms, behaviors and physiological processes critical for speciation. They prevent members of different species from producing offspring, or ensure that any offsprin ...
groups and these groups accumulate sufficient differences to become distinct species. The punctuated equilibria hypothesis suggests that anagenesis is rare and that the rate of evolution is most rapid immediately after a split which will lead to cladogenesis, but does not completely rule out anagenesis. Distinguishing between anagenesis and cladogenesis is particularly relevant in the fossil record, where limited fossil preservation in time and space makes it difficult to distinguish between anagenesis, cladogenesis where one species replaces the other, or simple migration patterns. Recent evolutionary studies are looking at anagenesis and cladogenesis for possible answers in developing the hominin phylogenetic tree to understand morphological diversity and the origins of ''
Australopithecus anamensis ''Australopithecus anamensis'' is a hominin species that lived approximately between 4.2 and 3.8 million years ago and is the oldest known ''Australopithecus'' species, living during the Plio-Pleistocene era. Nearly one hundred fossil specimens ...
'', and this case could possibly show anagenesis in the fossil record. When enough
mutation In biology, a mutation is an alteration in the nucleic acid sequence of the genome of an organism, virus, or extrachromosomal DNA. Viral genomes contain either DNA or RNA. Mutations result from errors during DNA replication, DNA or viral repl ...
s 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 A chronospecies is a species derived from a sequential development pattern that involves continual and uniform changes from an extinct ancestral form on an evolutionary scale. The sequence of alterations eventually produces a population that is p ...
. If the ancestral population of a chronospecies does not go extinct, then this is
cladogenesis Cladogenesis is an evolutionary splitting of a parent species into two distinct species, forming a clade. This event usually occurs when a few organisms end up in new, often distant areas or when environmental changes cause several extinctions, ...
, and the ancestral population represents a paraphyletic species or
paraspecies A paraspecies (a paraphyletic species) is a species, living or fossil, that gave rise to one or more daughter species without itself becoming extinct. Geographically widespread species that have given rise to one or more daughter species as perip ...
, being an
evolutionary grade A grade is a taxon united by a level of morphological or physiological complexity. The term was coined by British biologist Julian Huxley, to contrast with clade, a strictly phylogenetic unit. Definition An evolutionary grade is a group of sp ...
. This situation is quite common in species with widespread populations.


In humans

The modern human origins debate caused researchers to look further for answers. Researchers were curious to know if present day humans originated from Africa, or if they somehow, through anagenesis, were able to evolve from a single archaic species that lived in Afro-Eurasia.
Milford H. Wolpoff Milford Howell Wolpoff is a paleoanthropologist and professor of anthropology at the University of Michigan and its museum of Anthropology. He is the leading proponent of the multiregional evolution hypothesis that explains the evolution of ''Ho ...
is paleoanthropologist whose work done when studying human fossil records explored anagenesis as a hypothesis for
hominin The Hominini form a taxonomic tribe of the subfamily Homininae ("hominines"). Hominini includes the extant genera ''Homo'' (humans) and '' Pan'' (chimpanzees and bonobos) and in standard usage excludes the genus ''Gorilla'' (gorillas). The ...
evolution. When looking at anagenesis in hominids, M. H. Wolpoff describes in terms of the ‘single-species hypothesis,’ which is characterized by thinking of the impact that culture has on a species as an adaptive system, and as an explanation to what conditions humans tend live in based on the environmental conditions, or ecological niche. When judging the effect culture has as this adaptive system, scientists must first look the modern Homo sapiens. Wolpoff contended that the ecological niche of past, extinct hominidae is distinct within the line of origin. Examining early
Pliocene The Pliocene ( ; also Pleiocene) is the epoch in the geologic time scale that extends from 5.333 million to 2.58Miocene The Miocene ( ) is the first epoch (geology), geological epoch of the Neogene Period and extends from about (Ma). The Miocene was named by Scottish geologist Charles Lyell; the name comes from the Greek words (', "less") and (', "new") and mea ...
s findings helps to determine the corresponding importance of anagenesis vs. cladogenesis during the period of morphological differences. These findings propose that branches of the human and chimpanzee once diverged from each other. The hominin fossils go as far as 5 to 7 million years ago (Mya). Diversity on a species-level is able to be achieved through anagenesis. With collected data, only one or two early hominin were found to be relatively close to the
Plio-Pleistocene The Plio-Pleistocene is an informally described geological pseudo-period, which begins about 5 million years ago (Mya) and, drawing forward, combines the time ranges of the formally defined Pliocene and Pleistocene epochs—marking from about 5&nb ...
range. Once more research was done, specifically with the fossils of ''A. anamensis'' and ''A. afarensis'', researchers were able to justify that these two hominin species were linked ancestrally. However, looking at data collected by William H. Kimbel and other researchers, they viewed the history of early hominin fossils and concluded that actual macroevolution change via anagenesis was scarce.


Phylogeny

DEM (or Dynamic Evolutionary Map) is a different way to track ancestors and relationships between organisms. The pattern of branching in phylogenetic trees and how far the branch grows after a species lineage has split and evolved, correlates with anagenesis and cladogenesis. However, in DEM dots depict the movement of these different species. Anagenesis is viewed by observing the dot movement across the DEM, whereas cladogenesis is viewed by observing the separation and movement of the dots across the map.


Criticism

Controversy arises among
taxonomists In biology, taxonomy () is the scientific study of naming, defining ( circumscribing) and classifying groups of biological organisms based on shared characteristics. Organisms are grouped into taxa (singular: taxon) and these groups are given ...
as to when the differences are significant enough to warrant a new species classification: Anagenesis may also be referred to as gradual evolution. The distinction of speciation and lineage evolution as anagenesis or cladogenesis can be controversial, and some academics question the necessity of the terms altogether. The philosopher of science
Marc Ereshefsky Marc Ereshefsky is a professor of philosophy at the University of Calgary, specializing in the philosophy of science and the philosophy of biology. His research focuses on issues on the intersection of philosophy and biology. Ereshefsky is speci ...
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.


Applications

Regarding
social evolution {{unreferenced, date=February 2015 ''Social Evolution'' is the title of an essay by Benjamin Kidd, which became available as a book published by Macmillan and co London in 1894. In it, Kidd discusses the basis for society as an evolving phenomenon ...
, 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.


See also

* Multigenomic organism


References


External links


Diagram contrasting Anagenesis and Cladogenesis from the University of Newfoundland
{{Authority control Evolutionary biology concepts Evolutionary biology terminology Rate of evolution Speciation