Submitted By

SHRUTI KUMARI
SEMESTER 2ND P.G. ZOOLOGY
Year 2019-2021
CC 8 Biosystematics and Evolution
PURNEA UNIVERSITY, PURNEA.

DARWINISM
Charles Robert Darwin
Charles Robert Darwin (12 February 1809 – 19 April 1882) who was an
English naturalist geologist and biologist.
Best known for his contributions to the science of evolution.
His proposition that all species of life have descended over time from
common ancestors is now widely accepted, and considered a
foundational concept in science.
At first he was admitted to school of medicine at Edinburgh but soon
left it and joined Christ college of Cambridge from where he
graduated in 1831.
1831 he got the opportunity to go on a voyage as a naturalist on ship
named HMS beagle with Dr John Stevens Menslow.
During the journey (27th December 1831 to 2ⁿᵈ October 1836) the
vessel explore South America, Atlantic Islands, New Zealand,
Australia, Tasmania, Galapagos Island and many other regions and
collected numerous biological living and fossil specimens and
recorded various informations.
Darwin read an essay of Thomas Malthus in 1838 on the principles of
population
Keeping views of Malthus, Darwin formulated an idea that there must be
some things which control population.
He guessed this might be done by nature through the process of selecting
suitable one and eliminating others and does conceived his theory of natural
selection in 1838
While writing his theory in 1858 he received an essay from Alfred Russel
Wallace (young English naturalist exploring Malaya Archipelago) the
described him the same idea as his.
Darwin and Alfred adapted ideas of Malthus and put forth all these ideas in
the journal of Proceedings of Lineann society the same year.
In 1859 Darwin published his theory of natural selection in detail in his book
’The Origin of Species’.
Darwinism
The theory of evolution by means of natural selection proposed by Darwin
is known as Darwinism.
This is based mainly on three observations and two conclusions derived
from these observations. These are as follows :
1. Observation 1st; over production or enormous fertility –
Each living organism is in possession of power of reproduction in so much
large number, sometimes too enough than required to maintain its
generation.
Also they multiply in geometric ratio.
For example, an oyster may produce over 60 to 80 million eggs per year.
2. Observation 2ⁿᵈ ; Population –
Organism multiply in a geometric ratio while the food supply increases in
an arithmetic ratio, yet its number remains constant for a prolonged
period.
This is due to intense competition between the organisms to ensure your
living to obtain maximum amount of food and shelter.
3. Conclusion 1ˢᵗ; struggle for existence –
Based on above observations Darwin suggested that competition for food,
space and mates among individuals such as check this was called struggle for
existence, which is categorised into three categories. They are
A. Intraspecific struggle- struggle among individual of same species. It is the
most severe check because need of food, mate or space are similar.
it can be seen in between Nations, can occur in form of cannibalism, most
obviously seen in case of plants.
B. Interspecific struggle- struggle between the individuals of different species
and occurs mostly due to similar food habit as well as one may serve as the
food of other, that is struggle between a Aggressor and a Prey.
C. Struggle with environment- the struggle of organism against natural
calamities such as drought, extreme cold or extreme heat,
earthquakes,volcanic eruptions, lightning, etc. is known as a struggle with
environment for extra specific struggle.
4. Observation 3ʳᵈ ; Variation amongst organism-
Differences that exist among organisms are called variations.
Variations may be harmful, neutral or useful.
Variations that are passed on from generation to generation are called
heritable variations and these form the raw material for evolution. Small
heritable useful variations passes to the next generation and increases in extent
in successive generation
These variations arise due to changes in the genes or the chromosomes.
5. Conclusion 2nd ; Natural Selection –
During the struggle for existence, the individuals that exhibit variations
beneficial in facing the environment will survive, while those that cannot face
the hardship will be eliminated.
Those organisms best able to survive and reproduce will leave more offspring
than those unsuccessful individuals. This is referred to as Survival of the fittest(by
Herbert Spencer) or Natural Selection (by Darwin)
According to Darwin, the
giraffe exhibited variations in
the length of the neck and
legs.
When the grass on the ground
became scarce, giraffes with
long necks and legs had an
advantage over those with
shorter neck and legs, as they
could feed on the tall trees.
So these forms survived and
reproduced and became
abundant.
Over a period of time, giraffes
with short necks starved and
became extinct.
Origin of Species
As a result of struggle for existence,
variability and inheritance, individuals that
are better adapted, survived and became
abundant.
Slowly over a period of time, this group,
which was remarkably different from the
original population, becomes established as
a new species.
This group is also subject to the same forces
of change as their ancestors were and this
process continues to give rise to new
species.
Members of this group may possess
variations that may be beneficial to them
in another environment.
As a result, two or more species may arise
from a single ancestral species.
Over many generations, unequal
reproduction among individuals with
different genetic traits changes the overall
genetic composition of the population. This
is evolution by natural selection.
This mechanism can cause a population to
change so much, that it becomes a new
species. This is known as speciation.
 Criticism of Darwinism:
1. To explain the mechanism of inheritance of characters, Darwin
proposed the theory of pangenesis to explain this phenomenon. He
said that every cell or organ produces minute hereditary particles
called pangenes or gemmules. These were carried through the
blood and deposited in the gametes. This theory was not accepted.
2. It is important to valuation but does not explain the cause and
mechanism of appearance of variation.
3. Unable to make distinction between heritable and non heritable
variations.
4. Suggest new species arises by gradual accumulation of heritable
useful variation but does not provide any reason for absence of
transitional form in most cases
5. Mutation has not been given any importance.
6. Explain only survival of fittest but does not explain the arrival of fittest.
7. It does not explain elimination of the unfit.