Monday, July 27, 2020

Evolution - Notes | Class 12 | Part 3: Theories of Evolution



Lamarckism (Theory of Inheritance of Acquired characters)

It is proposed by Lamarck. It states that evolution of life forms occurred by the inheritance of acquired characters.

Acquired characters are developed by use & disuse of organs.

o Evolution by use of organs: E.g. Long neck of giraffe is due to continuous elongation to forage leaves on trees. This acquired character was inherited to succeeding generations.

o Evolution by disuse: E.g. Disappearance of limbs in snakes.

This theory was eliminated out because it is proved that the characters are inherited only through genes.

Darwinism (Theory of Natural selection)

-    Proposed by Charles Darwin.

-    It was based on observations during a sea voyage in a sail ship called H.M.S. Beagle.

-    Alfred Wallace (a naturalist worked in Malay Archepelago) had also come to similar conclusions.

-    Work of Thomas Malthus on populations influenced Darwin.

Darwinism is based on 2 key concepts:

o Branching descent: It explains that all organisms are modified descendants of previous life forms.

o Natural selection: Consider a bacterial colony A growing on a given medium. If the medium composition is changed, only a part of the population can survive under new condition. This variant population (B) outgrows the others and appears as new species, i.e. B is better than A under new condition. Thus, nature selects for fitness.

Natural selection is based on the following facts:

·     Heritable minor variations: It is either beneficial or harmful to the organisms.

·     Overproduction: Population size grows exponentially due to maximum reproduction (E.g. bacterial population).

·     Limited natural resources: Resources are not increased in accordance with the population size.

·     Struggle for existence: It is the competition among organisms for resources so that population size is limited.

·     Survival of the fittest: In struggle for existence, organisms with beneficial variations can utilize resources better. Hence, they survive and reproduce. This is called Survival of the fittest. It leads to a change in population characteristics and new forms appear.

Darwin ignored about origin of variation and mechanism of evolution or speciation.


-    Hugo de Vries proposed Mutation Theory of evolution.

-    He conducted experiments on Oenothera lamarckiana

(evening primrose) and believed that evolution takes place through mutation and not by minor variation.

-    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).

-    Mutation is the origin of variation for evolution.


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