This document discusses several theories of evolution, including Lamarckism and Darwinism. Lamarckism, proposed by Jean-Baptiste Lamarck in 1809, suggested that acquired traits could be inherited and drive evolution. It proposed mechanisms like use and disuse of organs. Darwinism, proposed by Charles Darwin in 1859, introduced the theory of natural selection and survival of the fittest. It argued that overpopulation leads to struggle for existence, and natural selection favors individuals with traits best suited for their environment, driving evolution over long periods. Later theorists like Weismann introduced ideas like the distinction between somatic and germ-line cells.

1. THEORIES OF
EVOLUTION
RAAZ PRAKASH POKHREL

2. Organic Evolution
 It is slow, progressive, continuous
and irreversible change of
organisms from simple and
undeveloped form to complex and
developed form in long course of
time.

3. Lamarckism:-
The first theory of organic evolution
was postulated by a French biologists
Jean Baptiste Lamarck in 1809 A.D.,
which is popularly known as
Lamarckism.
 Lamarckism is also known as ‘ Theory
of inheritance of acquired characters'.

4. a) Tendency to
grow:
The internal forces of an organism
cause to increase in the size of parts of
an organism. The new structures
appear because of an inner want of the
organism. This increase in size is up to
a certain limit which is determined by
the life itself.

6. b) Environmental
Effects:
The change in environment brings
change in organ and habitat of
organism.

7.  The environment in which the
organisms live influences them
and this influence leads to change
in their habits. Then, the organs of
an animal became modified in
appropriate way in direct
response to a changing
environment. This results the
formation of new organs.

9. c) Use and disuse of
organs:
According to the Lamarckism, The
more continuously used organs
become more developed and
enlarged while unused ones
gradually weakens and finally
disappears.

11. d) Inheritance of
acquired characters:
According to this, Those characters
obtained by organisms during their life
time from the surrounding and the
changes in the surrounding are
transmitted to off springs. The process
continues and In a long course of
time, organisms become entire
different from their ancestors with new
characters acquired from the pre-
existing organisms.

12. Examples of Lamarckism:-
a) Formation of web toes
in water birds:
The present water birds and ancestors were
the land birds. Some of them could not find
sufficient food on the land and migrated into
water. They spread their toes while moving in
water. The skin extended between the toes
and formed webs for swimming.
This shows, formation of new organs or
environmental effects.

14. b) Formation of long
necked and legged giraffes:
The horse like short necked giraffes
were the ancestors of the present long
necked and legged giraffes. They used
to graze on ground. But during certain
course of time, there became the
scarce of the grass on which they feed.
Due to this, they had to depend on the
foliage of the trees.

16.  As the trees were high and big, they
continuously have to stretch their neck
and legs while eating. Due to
continuous use of necks and leg for
stretching, their neck and leg gradually
became long and strong. In a long
course of evolution the present giraffe
with long neck and legs are evolved.
This shows, Use and disuse of organs
and Inheritance of acquired characters.

19. c) Loss of limbs of
snakes:
The ancestors of the present limbless snakes with
limbs. In the course of time, those snakes had to
live in bushes and burrows. During which their
body became elongated as they had to pass
through them. For passing through the narrow
spaces their limbs were not used continuously
and finally, limbs were disappeared by continuous
disuse. Thus, present snakes are limbless unlike
other reptiles.
This shows the law, Use and disuse of organs.

21. Objection to Lamarckism:
 Our eyes are continuously used for
watching & as told by the law; use and
disuse of organs, it should be
developed. But the eyes become
weaker as we grow up older.
 The organs in the organisms which are
useless in the initial stages of life do
not vestige but develop later on in the
body and become functional.

23.  The characters acquired by the
organisms from surrounding are not
found to be heritable. Only those
characters affecting germ cells are
found to be heritable.
 Mutilations (body parts removal) are
not inherited from parents to offspring.
 Piercing of ears and nose is a tradition
but new babies are not born with such
holes.

24.  Germplasm Theory (Neo-Lamarckism):- This theory is
given by German biologists August Weishmann.
According to this theory, in living organisms, there are
two types of cells,Viz, Somatic cells and Germinal
cells.

25.  Germplasm Theory
 Weishmann proved that only the changes
in germinal cells are inherited from
parents to offspring but not the changes in
somatic cells. For this, Weishmann
 He cut the tail of mice for 20 generations
and measured the tails in each
generation. In all generation of the
mouse, he found the shape and size of
this tails similar or equal. This was
because there were changes in only the
somatic cells not in the germinal cells.

26. Darwinism
:

27. This is the most popular
theory of evolution given by a
British biologist Charles
Robert Darwin in 1859 AD. In
this theory, Darwin explained
about the origin of species by
natural selections. So, this
theory is also called, Theory
of Natural Selection.

28. Over population of off-
springs:

Nature has provided an enormous
power of reproductions or fertility,
which can lead to over population
of off-springs. This may cause
rapid increase in organism
number.

29.  Salmon: A salmon produces 28
million eggs in one season . If all
of them survive, a single species
will occupy all the sea after few
generations.

30.  An Oyster lays 60 millions eggs in a year. If all off
springs developed, the size of shells would be 8 times
the size of earth.

31.  Elephants are slowest breeders. A pair elephants can
produces 19 millions young ones at the end of 750
years.

32. Struggle for existence:

Due to increase in the number of organisms, there
arises the scarcity of food and space. Similarly,
organisms have to struggle for climate and breeding
place.

33.  Intra-species: This may occur between the members of
same species. For E.g. two dogs fighting for a meat-
piece.

34.  Inter-species: This occurs between the members of
two different species. E.g. cat chasing mouse and
mouse being escaped.

35.  Environmental Struggle:
The organisms have to struggle against the
environmental factors such as extreme heat or cold,
lightning, earthquake, volcanic eruption etc.

36. d) Natural Selection
(Survival of the fittest):

Due to overpopulation, the scarce of food, shelter and
breeding place takes place. In this course, organisms
struggle against different factors in environment. Those
who are best acclimatized in the environment will be
selected by the nature and other become extinct soon.
Herbert Spencer coined this term for the first time and
Darwin termed as natural selection.

37. Variation and Heredity:

According to this, variation are of two types; useful and
harmful variations. Harmful variation cause the
organisms unfit in the struggle for existence and cause
their elimination while useful variations make them
adapted in the environment.

38. Variation are of 2 types:-
 Continuous:
It is minute variation
between generations
to generations.
For E.g. variation between
two brothers etc.

39.  Discontinuous: It is the sudden variation which is very
different, called as mutation. For E.g. Occurrence of
five legs in a new calf.

40. Origin of new species
(Speciation):

In this course of struggle of existence and adaptation
in the environment, the successful and advantageous
variations are transmitted to the off-springs by the
organisms. The changing environment causes
variation in generations to generations over a long
period of time, and finally results in the origin of new
species.

41. Drawbacks Of Darwinism:-
 Darwin described about slow and small variations
during organic evolution but he didn`t describe about
usefulness of underdeveloped organs at initial stage.
 He didn`t mention abut use and disuse of organs and
also about vestigial organs.
 Darwin didn`t mention about mutation which is major
factor of organic evolution.

42.  Darwin described about survival of the fittest but not
about the arrival of the fittest.
 Darwin didn`t explain (differentiate) somatic and
germinal variation.
 Darwin did not describe about over specialization of
certain organs which may lead to the extinction of
organisms.
 The Pangenesis hypothesis of Darwin was kept in
dark.