Evolution | Plus 2 Zoology| Exam Capsule Notes (Web and PDF)

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EVOLUTION: CHAPTER AT A GLANCE

THEORIES OF ORIGIN OF LIFE
  1. Spontaneous generation: Life came out of decaying & rotting matter like straw, mud etc. Louis Pasteur disproved this theory.
  2. Biogenesis: Life originates from pre-existing life.
  3. Panspermia: Units of life spores were transferred to planets including earth.
  4. Special creation: Living things were created by a supernatural power.
  5. Chemical evolution: By Oparin & Haldane. Life was originated from inorganic & organic molecules.
Miller experiment to prove Chemical evolution. 

As a result, some amino acids are formed. 



EVIDENCES FOR EVOLUTION

1. Paleontological evidences: 

    Study of extinct animals and geological period.

2. Morphological & Anatomical evidences

    a. Homologous organs: 
  • The organs having fundamentally similar structure and origin but different functions. This phenomenon is called Homology. 
  • E.g. 1. Human hand, Whale’s flippers, Bat’s wing & Cheetah’s foot. 2. Thorns of Bougainvillea and tendrils of Cucurbita. 
  • Origin of homologous organs is due to Divergent evolution (related species become less similar in different environmental condition). 
    b. Analogous organs: 
  • The organs having similar function but different structure & origin. This phenomenon is called Analogy. 
  • E.g. Wings of insects & wings of birds, Sweet potato & Potato, Eye of the octopus & of mammals.
  • Origin of analogous organs is due to Convergent evolution (unrelated species become more similar in similar environmental condition). 
3. Adaptive radiation: 
  • It is the evolution of different species in a geographical area. 
  • E.g. Darwin’s finches, Australian marsupials (Marsupial radiation), Placental mammals in Australia.

4. Biochemical evidences:
  • Organisms show similarities in proteins, genes, biomolecules & metabolism. It indicates common ancestry.
5. Embryological evidences:
  • Ernst Haeckel observed that all vertebrate embryos have some common features that are absent in adult.
  • Karl Ernst von Baer rejected this proposal.
6. Natural selection:

It is the process in which the organisms with better, favourable & heritable variation are survived and reproduced. E.g.
  • In England, before industrialization (1850s): More white-winged moths than dark winged (melanised) moths.
          After industrialization (1920): More dark-winged and less white winged.
          
Reason:
Before industrialization: Due to covering of white lichens on the trees, white winged moths survived but dark winged moths were picked out by predators.
After industrialization: No lichens. Tree trunks became dark due to smoke and soot. So, predators identified white winged moths easily. Dark winged moths survived due to dark background.
  • Development of resistant varieties in organisms against herbicides, pesticides, antibiotics or drugs etc. These are the examples for natural selection by anthropogenic action (evolution due to human activities).

THEORIES OF BIOLOGICAL EVOLUTION

Theory of Inheritance of Acquired characters: 
  • Proposed by Lamarck. 
  • It states that evolution occurred by the inheritance of acquired characters. 
  • E.g. Long neck of giraffe.
Theory of Natural selection:
  • Proposed by Charles Darwin.
  • Key concepts: Branching descent & Natural selection.
  • Natural selection is based on these facts: Heritable minor variations, Overproduction, Limited natural resources, Struggle for existence & Survival of the fittest.

MECHANISM OF EVOLUTION

Hugo de Vries conducted experiments on evening primrose and proposed that evolution takes place through mutation.

Mutation is the origin of variation for evolution.

Darwinian variation is minor, slow and directional. It results in gradual evolution.

Mutational variation is sudden, random & directionless. Here, speciation is by saltation (single step, large mutation).

HARDY-WEINBERG PRINCIPLE

It states that allele frequencies in a population are stable and is constant from generation to generation in the absence of disturbing factors.

E.g. in a diploid, p & q are the frequencies of alleles A & a respectively.

Frequency of AA = p2 
Frequency of aa = q2 
Frequency of Aa = 2pq

Hence p2 + 2pq + q2 = 1

Factors affecting Hardy-Weinberg equilibrium:

    Gene migration, Genetic drift, Mutation, Genetic recombination & Natural selection.

Gene migration: Gene flow from one population to another.

Genetic drift: Gene flow by chance. Original drifted population becomes founders (founder effect).

Natural selection: It is 3 types.
  • Stabilizing selection: Here, more individuals acquire mean character value and variation is reduced.
  • Directional selection: Individuals of one extreme are more favoured.
  • Disruptive selection: Individuals of both extremes are more favoured.


ORIGIN AND EVOLUTION OF MAN

Sequence of Human evolution:

Dryopithecus → Ramapithecus → Australopithecus → Homo habilis → Homo erectus → Homo neanderthalensis → Homo sapiens 

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