Anagenesis

Darwin explained evolution as “descent with modification” whereby organisms evolve and accumulate changes from their ancestors which help them adapt and survive. Since Darwin, there have been many speculations and attempts to understand exactly how the origin of species occurs- has evolution been a result of one species being transformed into another, or is it due to branching off from an ancestral species? Although both events lead to evolution, the difference between them lies in whether they are occur in the same lineage (anagenesis) or split into different lineages (cladogenesis). Moreover, is evolution a result of gradual changes over time or did it occur in rapid bursts intermitted by long periods of no apparent changes? This article attempts to address these questions in detail.

What is Anagenesis?

Anagenesis (aka phyletic transformation or gradualism) is a form of progressive evolutionary event, whereby gradual changes are introduced within a population. This form of evolution gives rise to a new species after appreciable changes have been accommodated within the ancestral population. This new species has characteristics entirely different from the “old” or ancestral species. In anagenesis, the changes within the population are due to mutations and recombination.

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Causes of Anagenesis

Anagenesis occurs due to changes in the gene pool or gene frequency of the population. These changes can be associated with genetic drift, mutations, recombinations and sometimes natural selection. These changes lead to morphological variations as well. Ultimately, the amount of changes in the population accumulate at appreciable levels, enough for them to be classified as a separate species from the original ancestral population.

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Characteristics of Anagenesis

Anagenesis is the transformation of or “replacement” of one species by another species which has originated from the same lineage. The process of anagenesis helps in establishing newer and more improved characteristics and abilities in the descendant group, which eventually accumulate appreciably enough to be designated as a new species. The characteristics of anagenesis are −

• It is the evolution within a lineage.

• It is a progressive evolution, gradual evolutionary changes occur over a long period of time.

• The new species has entirely different characteristics from the ancestral species.

• There is no increase in the number of species. One species is simply transformed into another.

• The ancestral species eventually may experience extinction, owing to the low numbers that will remain as new species are formed.

Example of Anagenesis

The evolution of the modern day horse (Equus spp.) is thought to be via anagenesis, evolving from a much smaller, five-toed mammal called the Hyracotherium. Another example is the increased size of cranium in humans.

What is Cladogenesis?

Cladogenesis is an evolutionary event that results in lineage splitting, as a result of which, two or more descendant (sister) species are formed from the ancestral population. Cladogenesis results in the process of speciation, which is the evolution of new species from a single ancestral species.

Cladogenetic events are a result of geographical barriers that caused division by preventing one group from mating with the other, eventually leading to accumulation of several different evolutionary changes that culminated in speciation. A prime example is the phenomenon of adaptive radiation. Cladogenesis is synonymous with the theory of punctuated equilibrium.

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Anagenesis vs Cladogenesis

Both events are forms of population differentiation. The difference lies in their pace and whether they occur in a directional or branching manner.

Anagenesis	Cladogenesis
Involves changes in the traits of lineage	Evolution of two or more new species from an ancestral lineage
Evolution within same lineage	Branching off and separation into different lineages
Also known as phyletic evolution	Also known as branching evolution
May or may not result in formation of a new species	New species resulted
One gene pool changes into another	Evolution of new gene pools
High genetic diversity within the population due to mutations and recombinations	Lesser genetic variation within individual species
Geographical isolation not required	A result of geographical isolation
Number of species remains the same	Number of species increases due to splitting from ancestral lineage
Changes are restricted only to descendants of a particular species or the same lineage No effect on biological diversity	Responsible for diversification of the biological world, increased number of species, each with a unique set of characters
Progressive evolution, gradual changes occur within a population	Jerky evolution, speciation occurs between periods of stasis

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Punctuated Equilibrium

The concept of punctuated equilibrium was introduced by Niles Eldridge and Stephen Tay Gould in 1972. Thai theory advocates that evolution is not a gradual process but is a result of long periods of stasis (i.e., periods where there is no appreciable change in the species, even for a million of years), interspersed by rapid bursts of instantaneous changes that culminate in speciation. In such cases therefore, the transitional changes in traits of a population are omitted and not fossilised, as is apparent from the “missing” fossil records.

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What is Phyletic evolution?

Phyletic evolution or phyletic gradualism refers to the evolutionary changes within a single population or a lineage. According to this theory introduced by Darwin, evolution occurs at a constant rate and new species are the result of gradual changes in the ancestral population. Phyletic evolution involves the accumulation of sufficient changes within a lineage, such that it can be differentiated into and recognised as a species separated from its ancestor. Essentially, phyletic evolution is simply another name for anagenesis.

What is Genetic Drift?

Genetic drift refers to the random changes in allele frequencies, from one generation to another, that occur within a population. Genetic drift is an event that is based on sampling error or chance. Quantitatively, the gene frequency changes in a population due to genetic drift are inversely proportional to the size of the population. The effects of genetic drift, although not significant from one generation to the next, lead to major changes in the overall characteristics of a population after many generations.

Conclusion

• Anagenesis is the transformation of one species into another, within the same lineage and is a linear event and may or may not lead to evolution.

• Anagenesis is related with phyletic evolution which is the gradual and constant evolutionary changes that occur within a single population.

• Cladogenesis is the branching off of two or more new species from an ancestral species and leads to speciation.

• Cladogenesis is extended into the Punctuated equilibrium model that proposes the evolution of species as a rapid burst of changes interrupted by periods of stasis.

• Genetic drift is the random change in allele frequencies due to sampling error and its effects are most obvious in small populations.

FAQs

Q1. Does the parental species get extinct during the formation of a new species in cladogenesis?

Ans. No, although it can be a possibility. In cladogenesis, the new species simply branch off from the parent species. The parent species and the new species can coexist at the same time.

Q2. Is speciation a result of anagenesis or cladogenesis?

Ans. According to the majority of the views, speciation is a result of cladogenesis.

Q3. What is the bottleneck effect and how is it related to genetic drift?

Ans. Evolutionary bottlenecks are events (habitat destruction, disasters, etc.) whereby a population experiences a drastic reduction in its size. The remaining population faces stronger effects of genetic drift and the less commonly occuring alleles may be lost. The gene pool of the small population is altered greatly, and can lead to evolution of new species.

Q4. How is genetic drift different from natural selection?

Ans. The main difference between these two is that while genetic drift is a random, chance-related event and has no direction, natural selection favours the spread of those alleles whose phenotypic traits favour survival. Natural selection is a non-random, carefully guided process.

Q5. Is speciation a result of anagenesis or cladogenesis?

Ans. According to the majority of the views, speciation is a result of cladogenesis.