Natural selection NEET


 Natural selection NEET
  • The novelty and brilliant insight of Darwin was this: he asserted that variations, which are heritable and which make resource utilisation better for few (adapted to habitat better) will enable only those to reproduce and leave more progeny.
  • Another interesting observation supporting evolution by natural selection comes from England.  
  • In a collection of moths made in 1850s, i.e., before industrialisation set in, it was observed that there were more white-winged moths (Biston bitularia) on trees than dark-winged or melanised moths. 
  • However, in the collection carried out from the same area, but after industrialisation, i.e., in 1920, there were more dark-winged moths in the same area, i.e., the proportion was reversed.
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  • The explanation put forth for this observation was that ‘predators will spot a moth against a contrasting background’. 
  • During postindustrialisation period, the tree trunks became dark due to industrial smoke and soots. Under this condition the white-winged moth did not survive due to predators, dark-winged or melanised moth survived. 
  • Before industrialisation set in, thick growth of almost white-coloured lichen covered the trees - in that background the white winged moth survived but the dark-coloured moth were picked out by predators. 
  • Do you know that lichens can be used as industrial pollution indicators? They will not grow in areas that are polluted. 
  • Hence, moths that were able to camouflage themselves, i.e., hide in the background, survived.
  • This understanding is supported by the fact that in areas where industrialisation did not occur e.g., in rural areas, the count of melanic moths was low. 
  • This showed that in a mixed population, those that can better-adapt, survive and increase in population size. Remember that no variant is completely wiped out. 
  • Branching descent (descent with modification) and natural selection are the two key concepts of Darwinian Theory of Evolution
  • Anagenesis : species formation without branching of the evolutionary line of descent.
Anthropogenic selection:
  • Excess use of herbicides, pesticides, etc., has only resulted in selection of resistant varieties in a much lesser time scale. (directional)
  • This is also true for microbes against which we employ antibiotics or drugs against eukaryotic organisms/cell. 
  • Hence, resistant organisms/cells are appearing in a time scale of months or years and not centuries. 
  • These are examples of evolution by anthropogenic action. 
  • This also tells us that evolution is not a directed process in the sense of determinism. 
  • It is a stochastic process based on chance events in nature and chance mutation in the organisms.

Types of natural selection: 

class 12 zoology chapter theories of evolution topic Natural ...
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ex:  
1. stabilizing : 
  • Human- height, birthweight, schizophrenia
  • Birds- Clutch Size − The number of eggs laid by a female bird (clutch size) is typically under stabilizing selection.
2. Directional: 
  • Industrial melanism 
3. Disruptive:
  • Polymorphism
  • Sexual dimorphism
  • Phenotypic plasticity
Speciation: formation of one or more new species from an existing species is called as speciation.
Types: Divergent speciation and convergent or transforming speciation

Divergent speciation:

  • Allopatric speciation: when a species split in to two or more geographically isolated populations and these population finally form a new species, It is called allopatric speciation.e.g., Darvin finches 
  • Sympatric speciation: in this type of speciation a sub population becomes reproductively isolated from its parent population. e.g., mainly in plants due to polyploidy. 
Transformation speciation: When an ancestor species changes into a new species. 
  • Phylatic speciation: Ancestor species changes into a new species by gradual changes in thousands of years. e.g., evolution of horse or human 
  • Quantum speciation: in this speciation sudden major changes appears in an ancestral species lead to formation of new species. no connecting links are available for this. it is caused by single step large mutation or saltation.


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