मराठी

Basic Postulates of Darwinism

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Estimated time: 14 minutes

CISCE: Class 12

1. Enormous Fertility in Living Beings

All organisms have an amazing fertility and an innate desire to multiply in a geometric ratio. However, because space and food are limited, the actual number of individuals in a species remains roughly constant under stable conditions.

Examples of High Productivity:

Organism	Reproductive Rate	Theoretical Impact (if all survived)
Elephant	6 young in a 90-year lifespan	19 million descendants from one pair in 750 years.
Oyster	6 million eggs a year	Shells would equal the size of the Earth in one year.
Green fly	High multiplication	One summer's descendants would weigh down China.

CISCE: Class 12

2. Struggle for Existence

Because more individuals are produced than can survive, there is an everlasting competition at all levels of life.

Type of Struggle	Between	Reason for Struggle
Intraspecific	Members of the same species	Exactly similar needs (food, shelter, light, mates).
Interspecific	Members of different species	Predator-prey relationship for food.
Extraspecific	Organisms and the environment	Natural calamities (drought, cold, floods, earthquakes).

CISCE: Class 12

3. Variations Under Nature

No two individuals of a species are exactly alike. Evolution relies entirely on these variations, provided they can be inherited.

Type of Variation	Description	Inherited?
Meristic	Change in number of recurring body parts (e.g., 6 digits).	Yes
Substantive	Change in physical shape, size, or colour.	Yes
Continuous	Small, fluctuating changes on either side of an average.	Yes
Discontinuous	Sudden, conspicuous new characters ("sports" or mutations).	Yes
Somatogenic	Acquired characters developed due to the environment.	No
Blastogenic	Genetic changes occur in the germ cells.	Yes

CISCE: Class 12

4. Natural Selection or Survival of the Fittest

The Mechanism: Due to the struggle for existence, only individuals with useful heritable variations best fitted to the conditions survive.
Survival of the Fittest: A term coined by Herbert Spencer, meaning nature automatically weeds out less perfect forms.
Giraffe Example: In a population with variable neck lengths, short-necked giraffes starved and died out. Giraffes with longer necks could reach tall trees, got enough food, survived, and left more offspring.

CISCE: Class 12

5. Origin of Species

Because the environment is constantly changing, surviving individuals continuously develop new adaptations. Over long periods of geological time, descendants become completely distinct from their ancestors, resulting in the origin of new species.

CISCE: Class 12

Wallace’s Chart: Summary of Natural Selection

Facts Observable in Nature	Wallace's Inferences
Enormous fertility + Constant population size	Struggle for existence.
Struggle for existence + Heritable variations	Natural selection (Survival of the fittest).
Survival of the fittest + Changing environment	Origin of new species.

Maharashtra State Board: Class 12
CISCE: Class 12

Key Points: Basic postulates of Darwinism

Enormous fertility: Organisms produce far more offspring than can survive, while resources like food and space are limited.
Struggle for existence: Due to overproduction, organisms face a constant struggle - within the same species, between different species, and with the environment.
Variations: Individuals of a species show heritable variations, and only inherited (germinal) variations are important for evolution.
Natural selection (Survival of the fittest): Individuals with favourable variations survive, reproduce, and leave more offspring, while less fit ones are eliminated.
Origin of species: Accumulation of favourable variations over long periods leads to adaptations and the formation of new species.