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Post Darwin theories are also termed as

Modern synthesis or synthetic theoriy


Discover of


Robert hook

Watson and Creek


Modern synthesis theory was put forward by:

R. A. Fisher, Dobzhansky, Seaswall Right and Ernst mayr


3 Premises of Modern synthetic theories are:

1. Evolution proceeds in a gradual manner with accumulation over small changes over a long period of time.

2. This change results from natural selection with differential reproductive success found on favorable traits.

3. explains changes not only within species but also higher level of processes such as origin of species producing great diversity of life in extent and also explains extinction.


Difference between Darwin and post-Darwin

D: identifies only competition, inheritance i.e. NS
PD: recognizes several forces of evolution that led to NS such as mutation, genetic drift and bottleneck effect.

PD: Characteristics are acquired as discrete entity known as genes and the variation within population is result of various Allele.
D: Doesn't explain it.

PD: Suggest accumulation/formulation of new species is usually due to gradual accumulation of small genetic change.
D: Didn't explain how.


Factors that produce and redistribute variation

1. Mutation
2. Genetic Drift
3. Gene Flow
4. Bottleneck effect
5. Recombination
6. Founder effect



Basic creative forces in evolution and only way to create variation

Alteration of existing genetic material.

It is mostly permanent and transmissible changes that occurs @ DNA or RNA or both

Themselves won't change in allied frequency but when supported by NS that leads to change in gene frequency in population.


Genetic Drift

Also known as Seawall right effect

Random Factor, occur by chance, mainly b'coz population is small and if any weak or rare trait, then that trait will drift away and disappear.

During Pleistocene period, people were disconnected and drift happened, so change occurred.

Resulting in decrease in size etc showing lack of resources.


Different kind of drift

Gene Flow
Bottleneck effect
Founder effect


Founder effect

Small band of founder leave their parental group and establish alone somewhere else.
as long as mates are chosen among themselves, they'll be having founder's gene, so any rare allele will become common traits

For example: Tay Sachs
Retinitis pigmentosa



1. Some population when passed through scarcity, it is K/A Bottleneck

2. There will be loss of genetic diversity and can be very detrimental

3. If bottleneck is removed, though population will be restored but several genes will be lost.

4. If we'll draw earlier and current gene on graph it'll give shape of bottleneck

5. Genetic drift can lead to bottleneck

6. Cheetah suffered catastrophic decrease for unknown reason and hence there was a loss of numerous allele.

7.Andaman Islander


Gene Flow

K/a migration

Exchange of gene between populations

Example: Britishers



No new gene but new genetic environment

Mixing and recombining the existing gene can lead to production of a new genotype.

Happen when chromosomes are dividing: During mitosis

Though there's new gene but it's k/a recombination as it doesn't change allele frequency but a new genetic environment is created and a change of gene function can invite NS to come and act upon.


Doll's rule

Not proved

Louis Dolls

Evolution is irreversible and irrevocable


Cope's rule

Not proved

Edward Cope

Studied fossil remains

Talked about:
1. un-Specialized animal survive.
Human are unspecialized

2. Organism has tendency to increase size in evolution:
Horse, camel herbivorous mammal evolved in size with time


Gause's rule

Not proved

competitive exclusion principle

2 species compete for exactly same resources and can't stably coexist

species with identical and ecological requirement can't occupy same ecological environment.