EVOLUTION

 

MECHANISM OF EVOLUTION:

• In the first decade of twentieth century, Hugo deVries based on his work on evening primrose brought fourth the idea of mutations.
• Mutation is the large difference arising suddenly in a population.

How deVries theory of mutation differs from Darwin’s theory of natural selection?

• It is the mutation which causes evolution and not the minor variations that Darwin talked about.
• Mutations are random and directionless while Darwinian variations are small and directional.
• Evolution for Darwin was gradual while deVries believed mutation caused speciation and hence called it saltation (single step large mutation).

HARDY – WEINBERG PRINCIPLE:

• In a given population one can find out the frequency of occurrence of alleles of a gene on a locus.
• This frequency is supposed to remain fixed and even remain the same through generations.
• Hardy-Weinberg principle stated it using algebraic equations.
• The principle states that allele frequencies in a population are stable and is constant from generation to generation.
• The gene pool (total genes and their alleles in a population) remains a constant. This is called genetic equilibrium:
• Sum total of all the allelic frequencies is 1.
• (p + q)² = p² + 2pq + q² = 1.
• When frequency measured, differs from expected values, the difference (direction) indicates the extent of evolutionary change.
• Disturbance in genetic equilibrium, or i.e. change of frequency of alleles in a population would then be interpreted as resulting in evolution.


• Five factors are known to affect Hardy-Weinberg equilibrium:
  • Gene migration or gene flow.
  • Genetic drift.
  • Mutation.
  • Genetic recombination.
  • Natural selection.
  
  
• Gene migration: When migrations of a section of population to another place occur, gene frequencies change in the original as well as in the new population. New genes /alleles are added to the new population and these are lost from the old population.

• Gene flow: Gene migration occurs many time is termed as gene flow.
• Genetic drift: change in gene frequency takes place by chance.
• Founder effect: sometimes the change in allelic frequency is so different in the new sample of population that they became a different species. The original drifted population becomes founder species and the effect is called founder effect.

Operation of natural selection on different trait:

• Natural selection can lead to :
  • Stabilization: in which more individuals acquire mean character value.
  • Directional changes i.e. more individuals acquire value other than the mean character value.
  • Disruption: more individuals acquire peripheral character value at both ends of the distribution curve.

A BRIEF ACCOUNT OF EVOLUTION:

• About 2000 million years ago (mya) the first cellular forms of life appeared on earth.
• Some cellular form had the ability to release O₂.
• Slowly single cell organisms became multi-cellular life forms.
• By the time of 500 mya invertebrates were formed and active.
• Jawless fish probably evolved around 350 mya.
• Sea weeds and few plants existed probably around 320 mya.
• Coelacanth a lobe finned fish discovered in South Africa in 1938 evolved into first amphibians that lived on both land and water. These were ancestors of modern day frogs and salamanders.
• The amphibian evolved into reptiles.
• Reptiles’ lays eggs which don not dry up in sun unlike those of amphibians.
• Giant ferns (pteridophytes) were present but they fell to form coal deposits slowly.
• Some of the reptiles went back into water to evolve into fish like reptiles probably 200 mya ( Ichthyosaurs)
• The land reptiles were the dinosaurs.
• The biggest dinosaurs are Tyrannosaurus rex was about 20 feet in height and had huge fearsome dagger like teeth.
• About 65 mya the dinosaurs suddenly disappeared from the earth.
• Some of them evolved into birds.
• The first mammals were like shrews. Their fossils were small sized.
• Mammals were viviparous and protected their unborn young inside the mother’s body.
• Due to continental drift, pouched mammals of Australia survived because of lack of competition from any other mammals.

 ORIGIN AND EVOLUTION OF MAN:

• About 15 mya primates called Dryopithecus and Ramapithecus were existing.
• They were hairy and walked like gorillas and chimpanzees.
• Ramapithecus was more man like while Dryopithecus was more ape-like.
• Few fossils of man-like bones have been discovered in Ethiopia and Tanzania.


• Two mya Australopithecines probably lived in East African grasslands.
  • They hunted with stone weapons.
  • Essentially ate fruit.
  
  
• The first human-like being the hominid and was called Homo habilis.
  • Brain capacity wee between 650 – 800 c.
  • They did not eat meat.
  
  
• Fossils discovered in Java in 1891 revealed the next stage i.e. Homo erectus about 1.5 mya.
  • Had large brain around 900 cc.
  • Probably ate meat.


• Neanderthal man:
  • Brain size 1400 cc
  • Lived in east and central Asia between 1, 00,000-40,000 years back.
  • They used hides to protect their body.
  • Buried their dead.
  
  
• Homo sapiens:
  • Arose in Africa and moved across continents and developed distinct races.
  • During ice age between 75,000-10,000 years ago modern Homo sapiens arose.
  • Pre historic cave art developed about 18,000 years ago.
  • Agriculture came around 10,000 years back and human settlement started.

 

CBSE Biology (Chapter Wise) Class XII ( By Mr. Hare Krushna Giri )
Email Id : [email protected]