The Theories of Organic Evolution explains convincing the origin of life. It also explains how the wide variety of the plants and animals came into existence in the world. According to this theory, the world has been evolved and not been created.

Life originated spontaneously in the remote past from non-living inorganic substances. These substances reacted to organic compounds. The organic compounds developed into colloidal systems capable of stepwise improvements in order to give rise to a simple life.

Theories of Organic Evolution (Complete Notes)

Basic Theories of Organic Evolution


In the organic evolution, there are Different theories of evolution. But Five theories are accepted universally. They are

  • Theory of inheritance of acquired characters (Lamarckism)
  • Theory of natural selection (Darwinism)
  • Modern synthetic theory of evolution,
  • Weismanner’s germ plasma theory,
  • Hugo DeVries “mutation theory”

 

one

Theory of Inheritance of acquired characters (Lamarckism)


Lamarckism is the first theory of organic evolution, proposed by “Jean-Baptiste de Lamarck”(1744-1829), a French biologist. This is one of the theories of organic evolution Lamarckism.

Lamarck scientist’s famous book is “ philosophy zoologique” (1809).

He announced in 1801 a theory of organic evolution which has been known as “Theory of Inheritance of Acquired Characters“.

His evolutionary ideas are:

  • Internal forces of life tend to increase the size of the organism because of an “inner want” new structures appear.
  • The direct environmental effect over living organisms
  • Use of disuse
  • Inheritance of acquired characters

Example:

  • Giraffe due to the lake of surface vegetation gradually.
  • Aquatic birds –they had to go to water due to lack of food etc; some structures ‘web’ between the toes developed in the wings for fly gradually reduced.
  • Flatfishes (deep-sea fishes)
  • Whales lost their hind limbs
  • The Wanding birds (eg: -jacana) developed its long legs through generations of sustained stretching to keep the body above the water level.

Neo-LaMar kists:

The evolutionists who support the Lamarckian theory of inheritance of acquired characters come under the heading “neo-Lamarckian” they are:

  • Cope (1840-1897)
  • Giared (1846-1908)
  • Packard, Spencer & mc bride

 

two

The theory of natural selection (Darwinism)(in 1859)


Charles Darwin explain natural selection in his book “the origin of species” (full title “On the origin of species by means of Natural Selection” (or) “The preservation of favored races in the struggle for life”)

It includes the following elements:

  • The universal occurrence of variation
  • Overproduction (rapid multiplication ):
    • Every species, in the absence of environmental check, tends to increase in a geometrical manner.
    • A salmon produces 28000 eggs in a season.
    • Female rabbit gives birth to six young ones in one litter and produces 4 litters in a year (6 months old rabbit is capable of reproduction. due to environmental checks, it will be control
    • Elephants, in the absence of any checks, have 21 million descendants of the end of 800 years
  • The struggle for existence:
    • Intraspecific struggle between same species
    • Interspecific struggle between different species
    • Environmental struggle due to earthquakes, tidal waves, the bursting of volcanoes are all causes for killing large populations.
    • Survival of the fittest: in the struggle for existence, majority die out due to the fact those few survival have necessary advantageous variations. This idea of the survival of the fittest is the core theory of natural selection.
    • inheritance of useful variations: The organisms after getting fitted to the surroundings transmit their useful variations to the next generation, while non-useful variations are eliminated.
      • Neo-Darwinism: Neo-Darwinism’s a modified form of Darwinism.
      • Neo-Darwinist is:
        • H.huxely,
        • S.A.Geray,
        • Herbert Spencer,
        • Haeckel & D.S.Jordan,
        • Weismann (Germ-plasm theory)

 

three

Modern synthetic theory of Evolution


This theory is the result of the work of no. of scientists, namely T.Dobzhansky, R.A.Fisher, J.B.S.Haldan, Small Wright, Ernst Mayr & G.L.Stebbins. ‘Stebbins’ in his book “Process of organic evolution” discussed the synthetic theory. This is one of the proven Theories of Organic Evolution.

It includes the following factors:

  1. Mutation,
  2. variation (or) Recombination,
  3. Heredity,
  4. Natural selection,
  5. Isolation

(a). Mutation:

  • Due to gene mutation (or) point mutation, the chemistry of gene (DNA) alter & able to change its phenotypic effect.
  • Most of the mutant genes are recessive to normal gene and these are able to express phenotypically only in homozygous condition thus, gene mutation tends to produce variations in the offspring.
  • Mutations: What are Mutations and its types

(b). Variation (or) Recombination:

  • The production of gene combination containing the same individual, produce heterozygous individuals (meiosis)
  • Random mixing of chromosomes from two parents during sexual reproduction to produce a new individual.
  • The exchange between chromosomal pairs of particular alleles during meiosis, called “crossing over”, to produce new gene combinations.
  • Chromosomal mutations such as deletion, duplication, inversion, translocation, and polyploidy also result in variation.

(C). Heredity:

  • The transmission of variation from parent to offspring is an imp mechanism of evolution, favored in the struggle for existence, to benefit from the advantageous characteristics of their parents.

(D). Natural Selection:

  • Natural selection does not produce genetic change, but once it has occurred it acts to encourage some genes over others. Further natural selection creates new adaptive relations between population and environment by favoring some gene combinations, rejecting other and constantly modifying and molding the pool.

(E). Isolation:

  • Isolation of organism of a species into several populations (or) groups under psychic, physiological (or) geographical factors is supposed to be one of the most significant factors is supposed to be one of the most significant factors responsible for evolution.

 

four

Weismann’s Germ-plasm theory


August Weisman” (1834-1914) was a neo-Darwinian biologist who proposed the germplasm theory in his book “Das Keimplasma” The term Keimplasma means germplasm.

  • Theory based on two kinds of living bodies they are “Germplasm” (protoplasm of germ cells such as sperms and ova) and the “somatoplasm” (Protoplasm of somatic (or) body cells). The Germplasm is responsible for the development of reproductive ports and remains in the organism, the development of reproductive parts and remains in the organism, the development of reproductive parts and remains in the organism, sealed and secluded from the somatoplasm which develops out of the Germplasm.
  • Somatoplasm is a discontinuous entity and so any variation in it can’t be inherited. The somatoplasm doesn’t influence the Germplasm. So, Germplasm is immortal (perpetuated from generation to generation through meiosis) & Somatoplasm is mortal, it perishes with the death of an organism.
  • In 1904, Weizmann proposed a special particle the “idioplasm” (or) “determinant”, which is a distinct part of an organism is represented in the sex cells. Each determinant to be made up of still smaller units called “biophores”. These determinants are localized in the chromosomes of the nucleus.
  • In opposition of Lamarck’s inheritance of acquired characters, Weismann introduced in the idea of “parallel induction” According to this concept, the stimulus effects simultaneously the germ & somatoplasm.
  • Polymorphism, Weismann developed “hypothesis of germinal selection”. He said that. There are many no. of determiners struggling for expression within the Germplasm and finally, one set is selected out by the environment.
  • This is one of the Theories of Organic Evolution

Objections:

  • Lacks any experimental support,
  • It’s failure for explaining causes of asexual reproduction and regeneration.

Significance:

  • The idea of continuity and immortality of Germplasm
  • Understanding of the concept of particulate inheritance of Mendel.
  • It provides some clue about genes (determinants )
  • Proposes the division of Germplasm & Somatoplasm during cleavage of the zygote

 

five

Hugo de Vries “Mutation theory”


  • Hugo de Vries in his book “Die mutation the movie” (1901) proposed the mutation theory-
  • He observed on “Evening primrose”, “Oenothera Lamar Kiana”. He studied this plant in wild forms for many years continuously and observed certain spontaneous changes in some of these wild plants.

Mutation Theory:

Devries (1901), his theory states that evolution is a jerky process where new varieties and species are formed by a mutation that functions as a raw material of evolution.

Salient features are:

  • Mutation, sports (discontinuous variations) are the raw material of evolution.
  • The mutation appears all of a sudden.
  • Mutations do not revolve around the mean or normal character of these species.
  • The mutation can appear in no. of individuals of a species.
  • All mutations are inheritable.
  • Mutations in all conceivable directions.
  • Useful mutations are selected by nature, a lethal mutation from a single mutation.
  • Evolution is a jerky and discontinuous Process.

The importance of mutations in evolution:

  • De-Vries stressed on the randomness of variation, whereas Darwin considered the adaptiveness of the variation in evolution.
  • De Vries stated that new species arise by sudden jump, called mutation, whereas Darwin considered that new species arise by the cumulation of gradual fluctuating variations in a series of generations. •
  • H. Morgan observed mutations in Drosophila melanogaster and confirmed the mutation theory.

Objections:

  • It couldn’t explain the existence of a discontinuity
  • The device described certain numerical & structural changes in the chromosomes, but in “O.Gigas” mutant was found later to be due to polyploidy.
  • Provide raw material for other forces to act upon it.
  • It was not a satisfied explanation of the development of mimicry, the mutual dependence of flowers and pollinating insects.

 

Brush Back Your Basics with this MCQ:

Theories of Organic Evolution

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Reference: New Scientist