Write down main points of theory of natural selection.

Theory Of Natural Selection

Main Points of Theory of Natural Selection

(Darwin’s Theory – Class 12 Biology Chapter 24 Evolution)

Charles Darwin (1859) in his book “On the Origin of Species” proposed the Theory of Natural Selection. It is the most widely accepted mechanism of evolution. The main points are:

1. Overproduction (Geometric Increase)

Every organism produces more offspring than can survive.
Example: A single pair of elephants can produce 19 million descendants in 750 years if all survived.

2. Struggle for Existence

Due to overproduction and limited resources (food, space, mates), there is a continuous struggle for existence.
Three types:
- Intraspecific struggle (same species)
- Interspecific struggle (different species)
- Environmental struggle (against climate, disasters)

3. Variation

All individuals of a population show heritable variations (differences in size, colour, shape, etc.).
Variations arise due to mutation, recombination and gene flow.
Some variations are advantageous, some neutral, some harmful.

4. Survival of the Fittest (Natural Selection)

Individuals with favourable variations are better adapted to the environment.
These individuals survive and reproduce more successfully → “Survival of the Fittest”.
Unfavourable variations are eliminated.

5. Origin of New Species (Speciation)

Accumulation of favourable variations over many generations leads to formation of new species.
Natural selection acts as the driving force of evolution.

Key Postulates (Summary Table – Draw in Exam)

Postulate	Explanation	Example
Overproduction	More offspring produced than can survive	Elephants, flies
Struggle for Existence	Competition for limited resources	Animals fighting for food/water
Variation	Differences among individuals	Different beak shapes in finches
Survival of the Fittest	Best adapted survive and reproduce	Long-necked giraffes
Speciation	New species formed by gradual change	Galápagos finches

Important Exam Lines:

“Natural selection is the differential survival and reproduction of individuals due to differences in phenotype.”
“It is a creative force that leads to origin of new species.”
“Darwin could not explain the source of variations (later explained by Neo-Darwinism).”

Density-Dependent and Density-Independent Factors: Mechanisms Influencing Population Dynamics
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