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Transformation of a particular lineage of organisms to a different state which can be justified as a new species from its ancestral species is known as anagenesis. It is the constant evolution of a species that continues to exist as an interbreeding population. It is the mechanism in which one species shelves onto another due to evolutionary changes within a lineage. The newly formed species completely overshadow the ancestral species causing them to be extinct. It does not always indicate the formation of new species. It takes thousands and millions of years for a new species to evolve.
Read Also: Homo Erectus
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Key Terms: Anagenesis, Cladogenesis, Evolution, Phyletic evolution, Punctuated Equilibrium
What is Anagenesis?
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Anagenesis (ana-new, genos-race) is the collective changes that transform one species into a different species with different characteristics. It is also called phyletic evolution or progressive evolution. During Anagenesis a species slowly accumulates changes until it reaches points where it’s no longer the same species, which is very rare.
It is very less common evolution as it is non-branching evolution in which there will be extinction of parental species as shown in figure. Species A transforms into B, B into C and so on. Species A goes extinct when it became species B. In this transformation the number of species will remain same.
Cladogenesis
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Cladogenesis occurs when there is branching or splitting, leading to two or more lineages and resulting separate species. Cladogenesis (Klados-branch) is the separation of a gene pool into two or more gene pools, said pools then eventually produce one or more new species. Cladogenesis is the only evolutionary change that can promote biological diversity by increasing the number of species.
In Cladogenesis, one species rapidly transforms into another so it is the more frequent evolutionary change. Branching Evolution and punctuated Equilibrium is the other name for Cladogenesis.
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| Biomass | Carbon cycle | Types of Natural Selection |
| Adaptive Radiation | Plant Breeding | Green Revolution |
Anagenesis & Cladogenesis
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Cladogenesis contrasts anagenesis, as Cladogenesis refers to the phenomenon of evolutionary splitting of a parent species into two distinct species forming a clad.
This may occur naturally when few organisms end up in often distant new areas or when environmental changes cause several extinctions, to find out whether a species is formed through Anagenesis or Cladogenesis several scientists and researchers examine fossil, molecular evidence from the DNA of different living species, etc.
Although both are different from each there are some similarities as both are part of evolution. Here are some of similarities between Anagenesis and Cladogenesis,
- Both occur in response to the change in environment
- Both leads to speciation
- Both are types of evolutionary mechanism
- Both can be categorized based on the speed of occurrence of each process.
Characteristics of Anagenesis
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- It increases the resistance to the changes in the environment
- It divides the labor into the body parts
- By this process, arthropods and crustaceans show a high tendency towards speciation
- To improve the functions the complexity and rationalization of organs is increased
- The central nervous system complexity is increased
- It degenerates the unnecessary organs into parasites by increasing parasitic nature
Cause of Anagenesis
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Anagenesis occurs when changes accumulate in a population to the point where the ancestral species is no longer found in the population causing it to effectively go extinct.
As per the theory, in an anagenetic evolution, during the event of speciation, the original population tends to quickly increase and hence achieve genetic variations eventually through recombination of genetic material or mutation provided a stable environment. Other contributors that have some influence on genetic information and phenotype are genetic drift and selection process which can cause a species to be different from previous forms.
By observing evolutionary lineage, there are two mechanisms that are needed to be considered. One is when there is a change in the genetic information through the interaction of genes between species over a period of time. The second process is speciation which is closely related to cladogenesis.
Anagenetic Evolution
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The evolution process can take place by Anagenesis. In anagenetic evolution, changes occur within a lineage or by cladogenesis in which lineage splits into two or more separate lines. In this new species are formed after many long years.
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.
Anagenesis Example
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This is a slow and gradual process which can take thousands or millions of years. These are two examples of Anagenesis,
- Evolution of horse began from a small mammal Eohippus, meaning “dawn horse”. It looked more like a deer than a horse. A mammal of the family Equidae, occurred over a geologic time scale of 50 million years, transforming the small, forest-dwelling Eohippus into the modern horse Equus.
Much of this evolution took place in North America, where horses originated but became extinct about 10,000, years ago.
- Another example of Anagenesis is the peppered moth. The collections of moths were made in England in the year 1850 before industrialization. It was observed that there were more white-winged moths than black-winged moths, But after industrialization nearly around 1920 it was noticed that there were more black-winged moths then white-winged moths.
After observations, it was concluded that predators spot a month against a contrasting background. Due to industrialization trees, trunks become black due to smoke and soot. So, white-winged moths didn’t survived.
But before industrialization set in, the white-winged moths survived and dark-winged moths were picked out by predators. This shows that in a mixed population the one who can better adapt can survive and increase in population size.
Read Further: Classification of Animal Kingdom
Difference Between Anagenesis and Cladogenesis
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| Parameters | Anagenesis | Cladogenesis |
|---|---|---|
| Definition | Anagenesis is an evolutionary change of a single lineage in which one taxon is replaced by another without branching. | Cladogenesis is an evolutionary change of a particular species in which new species are branched off from a common ancestral species. |
| Branching | No branching of the lineage occurs. | Branching of the lineage occurs. |
| Type of evolution | Evolution with a lineage. | Evolution which results in the splitting of a lineage. |
| Alternative name | Phyletic evolution or progressive evolution | Branching evolution |
| Gene pool | One gene pool is converted into another gene pool. | A single gene pool is split into several gene pools. |
| Biological diversity | Does not promote biological diversity. | Promotes biological diversity since it increases the number of species. |
| Transition of species | It is a slow transition of one species to another. | The species are rapidly separated into two or more species. |
| Modes | Tachytely, horotely and Bradytely. | Tachyschizia, Horoschizia and Bradyschizia. |
| Example | The evolution of horse, giraffes. | Darwin’s finches |
Check Further: Industrial Melanism
Things to Remember
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- Evolution can take place by anagenesis, in which changes occur within a lineage, or by cladogenesis, in which a lineage splits into two or more separate lines.
- 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.
- Genetic drift, Mutation and natural selection leads to speciation.
- Evolutionary changes occurring in one species are not shared with other species. Over time, species diverge more and more from one another as a consequence of anagenetic evolution.
- As per the punctuated equilibrium (Cladogenesis) hypothesis, anagenesis can be rare and the pace at which evolution occurs is accelerated immediately after the division that might lead to cladogenesis. Speciation can be affected by factors such as geographical isolations and hybridization.
- Anagenesis can be viewed as the processes of sexual and natural selection, and genetic drift's effect on an evolving species over time.
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Sample Questions
Ques. Name the phenomenon by which rapid speciation takes place? (1 Mark)
Ans. Genetic drift is a phenomenon by which rapid speciation takes place.
Ques. What is the cause of speciation according to Hugo De Vries? (1 Mark)
Ans. Mutation is the cause of speciation according to Hugo De Vries.
Ques. What is meant by Gene pool? (1 Mark)
Ans. Gene pool refers to the total of different kinds of genes pooled by all the members of a population.
Ques. What is Cladogenesis? (1 Mark)
Ans. Cladogenesis is the splitting of parent species into two different species forming a clade.
Ques. What is speciation? List any two events that lead to speciation. (2 Mark)
Ans. Speciation refers to the origin of new species or phenomena of development of new species from pre-existing ones. The two events which lead to speciation are – Genetic drift, mutation, and natural selection.
Ques. What is phyletic evolution? (2 Mark)
Ans. Phyletic evolution is a model of evolution which states the speciation is a slow, uniform and gradual process. This type of evolution leads to transformation of species into a completely new one. It can be drawn as a line. Species A becomes Species B, which becomes species C.
Ques. What is genetic drift? (2 Mark)
Ans. Genetic drift is a mechanism of evolution. It refers to random fluctuations in the frequencies of alleles from generation to generation due to chance events. Genetic drift can cause traits to be dominant or disappear from a population. The effects of genetic drift most probably occur in small, isolated populations.
Ques. Differentiate anagenesis from cladogenesis giving one example of each. (2 Mark)
Ans.
| Anagenesis | Cladogenesis |
|---|---|
| In anagenesis, evolution takes place within a single lineage, and the parent species becomes extinct. | In cladogenesis, an existing species diverges into two different species, and the parent species exists along with the newly formed species. |
| Example : Evolution of horse, Giraffe | Example : Darwin’s finches |
Ques. By taking industrial mechanisms as an example, explain the concept of natural selection by evolution? (3 Marks)
Ans. Natural selection is a mechanism through which populations of living creatures adapt to changes to survive. It claims that because of the survival of the fittest, species evolve rapidly. Minimal differences are preserved and transmitted, giving rise to new forms over time. A collection of moths was discovered in the year 1850. White-winged moths outnumbered dark-winged moths in this collection. The population of dark-wringed moths rose with industrialization.
The explanation for this is because, throughout the post-industrial period, tree trunks became darkened by industrial smoke, preventing the white-winged moth from surviving. However, it is also known that before industrialization, trees were coated with thick growths of off-white-colour lichens, allowing white-winged moths to thrive while dark colour moths were easily identified by predators. As a result, it may be argued that nature chooses the best species for the job.
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