Evolutionary Theories Georges Cuvier - PowerPoint Presentation

Slide1

Evolutionary Theories

Georges Cuvier

Charles Darwin

Jean-

Baptiste

Lamarck

Weismann

Slide2

The evolutionary theories

aims to explain the emergence of

complex living forms from

less complex ancestor.

for example:

Aristotle (384-322 B.C.) believed that there had been a gradual transition from the

imperfect to the perfect form,

Bacon (1561-1625 A.D.) proposed the effects of variations

upon the origin of new species

,

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Kent (1707-1788 A.D.) believed that

higher organisms

had developed from simpler forms

,Buffon

“who is considered as father of evolutionary concept” was the first to proposed the direct influence of the environment

and also suggested the concept of struggle for existence.

Slide4

So, number of naturalists proposed

various theories regarding the

E

volution of Life or

Organic Evolution.

Among these some important theories are as follows:

1.

Lamarck and Lamarckism or Theory of Inheritance of acquired characters.2.

Darwinism or Theory of Natural Selection.

3

.

Mutation

theory of De

Vries

.

4.

Neo-Darwinism

or Modern concept or

Synthetic theory

of evolution.

Slide5

1.

Lamarck and

Lamarkism or Theory of

Inheritance of acquired characters.

Jean Baptiste de

Lamark (1744-1829 AD) was the

first who gave extensive theory of organic evolution in the year 1809 when Charles Darwin was born.

He compared the contemporary species of his time with the fossil records by arranging them in the chronological series.

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He recognized different lines

of

descent or lineages of the present or modern species.

He observed that:

i. Species changes under changing external influences,

ii. There is a fundamental unity underlying the diversity of species

iii. The species progressively develop.

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Based on these observations he assumed or proposed that: (lamarck

propositions)

1. Living organisms as a whole or their parts tend to increase in size

continuously due to internal forces of life.2. New organs (characters/traits) are developed in order to meet the needs of the new want and are maintained (

acquired characters)

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3. The development of organs

and

their

use are proportional to the activities of

these organs

i.e. if an organ is used constantly, it will tend to become highly developed, whereas, the organ which is not in use will degenerate.

Thus the use and disuse

of organs result in variations.

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4. Every new character

that has been acquired in the life of an individual is

preserved and transmitted to the

next generation

i.e.

inheritance of acquired characters.Thus, Lamarck proposed that

variations among the organisms occurs because of

response of the organism to the needs of the environment.

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i.e. when the environment

of an organism

changes, its

needs also changes.

These changed needs lead to either

special use of organs or disuse

of organs.Organs that are continuously in use will increase more in size whereas organs which are not used continuously degenerate.

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Such acquired characters

or traits are

inherited to next generation.

He stress upon the fact that adaptation

is the main reason of evolution.Therefore, his theory is called as “

Theory of inheritance of acquired characters” or “Theory of use and disuse of organs

”.

Slide12

The explanation of the concept of lamarck

is known as “

Lamarckism

”. This concept can be explained as “

the changes in structure or function of any organ acquired during the life time of an individual in response to changes in the surrounding environment are inherited by its offspring and keep on adding up over a period of time”.

Such changes resulted in origin of new species.

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Examples or Evidences in favour of Lamarckism:

Lamarck cited several examples in support of his views:

1. Evolution of long neck of giraffe:

when the ancestor of giraffe faced inadequate supply of food i.e. grass or herbs, they started feeding on the leaves of trees. In the process of reaching the leaves of higher branches, its neck got stretched and the forelegs were raised.

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2.

Evolution of limbless condition of snakes:

ancestors of snakes were four limbed

but due to fear of mammals they started to creep on jungle floor and later on

adapted to burrowing to avoid predation.

During this adaptation they lost their limbs gradually

.3.

Evolution of aquatic birds by developing web

between toes.

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4. Evolution of horse and deer:

ancestors of horse lived in soft ground in jungles but when these jungles were replaced with grass lands,

these horse and deer were forced to graze on the ground that lead to

change molars and premolars, reduction

in digits and

increasing in length of legs.

Lamarck cited these examples to explain the use and disuse of organs.

Slide17

Criticism of Lamarckism:

Criticism of the 1st postulate:

Tendency to grow (increase in size)

According to Lamarck's principle of tendency to grow

‘every living organism has the tendency to increase its size continuously due to internal forces of life’.

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It seems to be true for the evolution of many lines or descent like

evolution of horse, elephant.

However, in plants, there are plenty of examples where the

decrease in size has been observed.

The ancestor of the present day angiosperm

was gigantic gymnosperm and fern.

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Criticism of the 2nd postulate:

Environmental effect and new needs

(acquired characters)

According to 2nd

proposition “New organs (characters/traits) are developed in order to meet the needs of the new want and are maintained (acquired characters

)” is also not accepted by scientific world because one can not sprout (germinate) wings if it wishes to fly.

Slide20

Criticism of the 3rd postulate:

Use and disuse of organs

According to 3rd proposition “if an organ is used constantly, it will tend to become highly developed, whereas, the organ which is not in use will degenerate” have some truth.

However, it is also not applicable in all i.e. the eyes of a voracious reader do not increase in size or power with age or continuous beating of heart.

Slide21

Criticism of the 4th postulate:

Inheritance of acquired characters

According to 4th proposition “

every new character that has been acquired in the life of an individual is preserved and transmitted to the next generation” was also discarded by the scientific community by conducting several experiments.

Among different scientists, Cuvier and Weismann

were the main critic.

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August Weismann 1890 conducted an experiment and observed that mutilation of tail of mice for 22 generations did not result in the birth of a tailless mouse.

On the basis of his experiment,

W

eismann totally discarded the concept of inheritance of acquired characters and put forward another theory known as “Theory of continuity of germ plasm

.

Slide23

According to this theory, the characters influencing the germ plasm are only inherited but the change in the somatic cells isn't.

Castle and Philip

also conducted a transplantation experiment to find out whether the environment brings any change or not.

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They surgically removed the ovary of black female guinea pig and transplanted it on the white one

.

This

white female guinea pig was mated with a white male guinea pig. As a result, all the offsprings were black.

Thus, they concluded that the environment doesn't affect the inheritance of acquired characters.

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The poring of the ear and the nose is a common tradition in the Nepalese culture since time immemorial. However, there is no trace of hole in new born babies.

The

wrestler’s powerful muscles are not transmitted to the offspring.

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Merits and Demerits of Lamarckism:

Merits of Lamarckism:

Lamarckian

theory was simple and provided a way

in which change in organism could come about.

It was the first completely widespread theory that was offered. It was the theory that tent itself to predictions and therefore to testing.

Slide27

Lamarckian theory proposed that the

environment of an organism

affected the usage of its various parts.

Most persons know that exercise results in the development of muscles.

In this way, Lamarck remains an important figure in the evolution of evolutionary biology.

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Demerits of Lamarckism:

There are so many demerits of Lamarckism which are as follows:

1. Lamarck suggests the tendency to

increase in size but evolution proceeded not only without any increase in size but rather through a reduction in size.

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2. The second Lamarckian principle that new organs result from new needs is quite false.

In other words

desire of the animal leads to

the formation of new structures not true

because it means that the man who mused “Birds can fly, so why can not I? should have grow wings and taken to the air.

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3. The other Lamarckian principle that the inheritance of characters acquired

during the lifetime of the individual found totally incorrect.

Many biologists who have devised many types of experiments which have refused it.

Slide31

Neo-Lamarckism

:

Neo-Lamarckism

is the first scientific modified version of Lamarckian principles.

A group of evolutionist, like Cope

, Giard,

Cannon,

Osborn, Packard,

Spencer, McBride

,

who were the main supporters of the Lamarck, try to modify the Lamarckian philosophy/doctrine of inheritance of acquired characters in order to make it acceptable.

Slide32

These Neo-Lamarckians

considered that

adaptations are universal which arises as a result of

casual relationship of structure

, function and

environment.

As the environment is ever changing, the changed environmental conditions alter habits of organism, hence, in response to new habits, organisms acquire the new structures in place of old structures due to their adaptations to changed environmental conditions.

Slide33

These variations finally become engrained in the heredity of the race.

The

criticism/objections raised by

Weismann as well as the theory of continuity of germplasm proposed by him against the theory of inheritance of acquired characters was also analyzed by these neo-lamarckian

.

Slide34

They also did not agree with Weismann’s theory of continuity of germplasm and says that in

some cases the characters are also transmitted through somatic cells

, e.g. regeneration in some animals like earthworms or vegetative propagation in plants.

This indicates that somatic cells also contain all the characters.

Slide35

They argue that the sex hormones influence the somatic cells

of an organism.

The

external conditions affecting the somatic cells stimulate the body cells to produce specific secretions

.

These secretions reach the sex cells through the blood. Such variations can be inherited by offspring.

Slide36

Guyer

and Smith

prove this view by conducting experiments by broking the lens of eye of male rabbit by means of a needle as a result the surrounding blood capillaries penetrates the damage area.

When this male rabbit was allowed to mate with a normal female, she gave birth to seven young ones, out of which 04 had defective eyes because the antibodies induce mutation in genes controlling eye development.

Slide37

Kammarer observed in

Proteus

anguinus,

which is a cave dweller the development of normal eyes and colour of the skin when exposed to daylight. This character is inherited to the next generation.

This indicates that acquired characters can be inherited, if the body-changes produce antibodies which in turn induce changes in the genes concerned with the character.

Slide38

Thus Neo-Lamarckism is a modern theory of evolution based on Lamarckism which retains the fundamental concept that acquired characters are inherited and emphasizing the influence of environmental factors on genetic changes.

Neo-Lamarckism proposes that:

(

i) Environment influences the organism and changes its heredity.

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(ii) At least some of the variations acquired by an individual can be transmitted to the next generation.

(iii) Internal vital force and appetency do not play any role in evolution.

(iv) Only those variations are transmitted to the next generation which affect germ cells or where somatic cells give rise to germ cells.