NEUTRAL THEORY OF MOLECULAR EVOLUTION PART B CSIR NET DEC 2017

The neutral theory of molecular evolution (proposed by MOTTO KIMURA) 1968 holds that at the molecular level most evolutionary changes and most of the variation within and between species is not caused by natural selection but by genetic drift of mutant alleles that are neutral.

SOME IMPORTANT POINTS -

• Except for advantageous mutations most alleles are under neutral condition.
• The rate of evolution for most genes will be equal to the neutral mutation rate .
• At the level of Dna sequences,genetic drift dominates evolution i,e genetic drift is the main force changing allele frequency .
• The rate of replacement in evolution resulting from the random genetic drift of effectively neutral mutations is equal to the mutation rate to such alleles, μ.
•  Explained the unexpectedly high rate of evolutionary change and very large amount of intraspecific variability at the molecular level that had been uncovered by new techniques in molecular biology. 
• A neutral mutation is one that does not affect an organism's ability to survive and reproduce. 
• The neutral theory allows for the possibility that most mutations are deleterious, but holds that because these are rapidly purged by natural selection, they do not make significant contributions to variation within and between species at the molecular level. 
• Mutations that are not deleterious are assumed to be mostly neutral rather than beneficial. 

CSIR NET  DEC 2017 PART B 

Which one of the following statements is NOT TRUE about the Neutral Theory as proposed by Motoo Kimura?

1.Except for advantageous mutations, most alleles are under neutral selection

2. The rate of evolution for most genes will be equal to the neutral mutation

rate

3. Advantageous mutations are exceedingly rare

4. At the level of DNA sequences, genetic drift dominates evolution

Answer-  3

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ECOTONE AND EDGE EFFECT

Ecotone-An ecotone is a zone of junction or a transitional area between two biomes [diverse ecosystems]. It is the boundry line where two communities meet and integrate.

 e.g. 1.The mangrove forests represent an ecotone between marine and terrestrial ecosystem

       2. grassland (between forest and desert), estuary (between fresh water and salt water) and river bank or marsh land (between dry and wet).

Characteristics of Ecotone

It may be narrow (between grassland and forest) or wide (between forest and desert). It may be local (zone between field and forest ) or regional (zone between forest and grassland ecosystems ).As it is a zone of transition, it has conditions intermediate to the adjacent ecosystems. It may also include a number of highly adaptable species that tend to colonize .Hence it is a zone of tension.Usually, the number and the population density of the species of an outgoing community decreases as we move away from community or ecosystem.

• Edge Effect –( Edge Species) It is the tendency for increased variety and density at community junctions .It was coined by Aldo Leopold(1933). OR Sometimes the number of species and the population density of some of the species in the ecotone is much greater than either community. This is called edge effect.
• The organisms which occur primarily or most abundantly in this zone are known as edge species.
• In the terrestrial ecosystems edge effect is especially applicable to birds. For example the density of birds is greater in the mixed habitat of the ecotone between the forest and the desert.

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TYPIFICATION TYPES -

The ICBN recognizes seven kinds of types (Article 9)

1.HOLOTYPE- A single specimen (single herbarium )or illustration used or designated by the author and the name is based as the nomenclatural type at the time of publication of the name of the taxon. (PART B -2017 DEC CSIR NET )

2.ISOTYPE -It is the duplicate of holotype specimen collected at the same time ,by the same person and from the same population.

3.LECTOTYPE -When no holotype was indicated or holotype is missing ,lost or damaged  then lectotype is the specimen used as a nomenclatural type which was taken from the orginal material.

4.SYNTYPE-Any one of two or more specimens that is listed in a species description where no holotype was designated. 

5.PARATYPE- These are not name bearing types. When the original description designated a holotype, there may still be additional specimens listed in the type series and those are termed paratypes.

6.NEOTYPE- Belonging to non original collection .A neotype is a specimen later selected to serve as the single type specimen when an original holotype has been lost or destroyed or where the original author never cited a specimen.

7.EPITYPE -Epitype can not be a part of original material. An additional, clarifying type(specimen or illustration) of a species or lower-order taxon , provided when the holotype  and paratypes from the original classification are demonstrably ambiguous or insufficient. 

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