Class 10 | Science 2 | Chapter 1 | Easy notes on Heredity and evolution | Darwin's theory of natural selection and Lamarckism

Easy notes on Chapter 1. Heredity and Evolution | 

Darwin's theory of natural selection | Lamarckism

Darwin's theory of natural selection :

i) According to Darwin, all the organisms reproduce prolifically.

ii) All the organisms compete with each other in a life-threatening manner.

iii) In this competition, only those organisms sustain which show the  modifications essential for winning the competition.

iv) Nature selects only those organisms which are fit to live and the rest perish.

v) Sustaining and selected organisms can perform reproduction and thereby  

give rise to the new species with their own specific characters. 

Darwin’s theory of natural

selection was widely accepted but it has some objections. Some of the main objections are -

1. Natural selection is not the only factor responsible for evolution.

2. Darwin did not mention any explanation about useful and useless modifications.

3. There is no explanation about slow changes and abrupt changes.

Lamarckism :

Jean-Baptiste Lamarck proposed that morphological  changes occurring in living organisms are responsible for evolution.

The reason behind those morphological changes is activities or laziness of that organism.

He called this concept as principle of ‘use or disuse of organs’. 

For examples, 

i) Neck of giraffe : The neck of giraffe has become too long due to browsing on leaves of tall plants by extending their neck for several generations.

ii) Shoulders of the ironsmith have become very strong due to frequent hammering movements.

iii) Wings of birds like ostrich  and emu have become  weak  due to no use. 

iv) Legs of the birds like swan and duck have become useful for swimming due to living in water

v) Snakes have lost their legs by modifications in their body for burrowing habit.

All the above examples are types of ‘acquired characters’ and are transferred from one to another generation. This is called as theory of inheritance of acquired characters or Lamarckism.

Speciation :

Formation of new species of plants and animals is the effect of evolution.

Species is the group of organisms that can produce fertile individuals through natural reproduction.

Genetic variation is responsible for formation of new species from earlier one.

Geographical and reproductive changes are also responsible for speciation.