15 Main Theories of Biological Evolution of Man (with Statistics)
Read this essay to learn about the 15 main Theories of Biological Evolution of Man !
Read this essay to learn about the 15 main Theories of Biological Evolution of Man !
1. Theory of Eternity:
This is an orthodox theory. It believes that some organisms were
there from the very beginning of the Universe. Those organisms still
exist and will be continued in future in addition to some new forms.
According to this theory, the original forms are eternal, and they have
been preserved automatically. But this view is not at all popular; it is
held by a few people only.
2. Theory of Divine Creation:
A Spanish Monk, Father Sudrez (1548 – 1617) proposed this theory. It
was based on the Biblical book of Genesis. According to Genesis, of Old
Testament of Bible, the world was created by the supernatural power
(God) in six natural days.
The theory specifies that all creations, including plants,
animals and man on earth were created during those six days. Since all
species were made individually by god, the theory does not accept the
idea of origin of new species from ancestral forms. Life is considered
as a vital spirit according to this theory.
The Hebrew and the Christian Church authorities had supported
this view for many Centuries. To them, god created Adam and Eve, the two
companions of opposite sex about 6,000 years ago, from whom the human
beings have descended.
Archbishop Ussher (1581 – 1656) pointed out 4004 BC as the exact
year for the creation of man. Each and every followers of this theory
believed that all creations of god are arranged in a chain where human
is posited at the top.
3. Theory of Spontaneous Origin:
The theory contends that life had originated repeatedly from
inanimate materials or non-living things in a spontaneous manner. The
concept was held by early Greek philosophers like Thales (624 – 547BC),
Empedocles (485 – 425BC), Democritus (460 – 370BC), Aristotle (384 –
322BC) and others.
Aristotle thought that fireflies originated from morning dew and
mice from the moist soil spontaneously. All succeeding Greek
philosophers and many scientists shared Aristotle’s view till the middle
of the seventeenth Century. Louis Pasteur partially accepted this
theory.
4. Theory of Catachysm or Catastrophism:
French geologist Georges Cuvier (1769 – 1832) proposed this theory.
His observation was based on the fossil remains of varied organisms.
According to him, the earth had to face severe natural calamities at
different times for which many animal species have been destroyed. But
each time when the earth settled after a great Catastrophe, relatively
higher forms of animals appeared to replace the situation.
Cuvier did not believe in continuous evolution. To him the
species never evolved by modification and re-modification; a series of
Catastrophes were responsible behind changes where previous sets of
living creatures get replaced by new creatures of complex structure.
As per his scheme, corals, molluscs and crustaceous appeared in
the first phase. Then came the first plants being followed by the fish
and reptiles. The birds and mammals appeared thereafter and in the last
phase man emerged about five to six thousand years ago.
5. Theory of Uniformitarianism:
This theory was presented by Charles Lyell (1797 – 1837) in his work
‘Principles of Geology’. Being a geologist, he could not accept the
concept of an unchanging earth. By studying the rocks and geological
processes, he came to the conclusion that, at the beginning, some forces
were in operation to shape and reshape the earth. Animal forms
gradually evolved along with this change. Fossils were the main support
for his evidence. This theory on one hand discarded the “theory of
Catastrophism” and on the other hand nullified the “theory of divine
Creation”.
6. Theory of Cosmic Origin of life:
This theory advocated that the first life seed had been transported
through the cosmic particles from other planet. Richter (1865) developed
this theory and he was supported by Thomson, Helmholtz (1884), Von
Tieghem (1891) and others.
According to them the meteorites that travelled through the
earth’s atmosphere, contained embryos and spores in them; those
gradually grew and evolved into different types of organisms. But the
concept lacked evidences and interplanetary exchange of viable spores
and embryos could hardly be possible in the light of current
understandings.
7. Theory of Cynogen:
German scientist Fluger proposed this theory. According to him, the
cynogen, a complex chemical compound was developed by sudden reaction
between the atmospheric nitrogen and carbon. This cynogen later gave
rise to first protein substance, which ultimately created life through
various types of chemical synthesis.
8. Theory of Chemo-synthesis:
This theory also recognized a complex type of chemical synthesis. It
pointed out different kinds of materials, which in varied natural
environment produced a large number of actions and interactions. As a
consequence, life developed in a peculiar set up following a complicated
situation.
9. Theory of Virus
Some scientists believed that virus was initially responsible for the
emergence of life. The viruses hold a transitional stage between living
and non-living. By nature a virus is non-living, but when it reaches to
the body cell of the living host, it behaves as living. Therefore, it
was thought that such a creature might possess a role in the emergence
of life.
10. Theory of Organic evolution
According to this theory, origin of life must have taken place in
this world. First living existence was very minute and in the form of
unicellular structure. As the time passed on, most of the unicellular
forms were transformed to multicellular forms under the various
environmental oscillations. Gradually and gradually simple form of
animals was converted to very complex type of animals.
As a matter of fact, the geo-environment of the earth underwent a
process of continuous change and influenced the animal forms. Complex
forms of animals evolved out of the simple forms in a slow and steady
way. This process of change has been designated as organic evolution.
The conception of organic evolution maintains its conformity with
ancient Hindu religious thought. B.M. Das (1961) wanted to prove this
with the example often incarnations of Lord Krishna (Dasha avatar).
He mentioned that the first incarnation was a fish (Matsya
avatar). He justified his remark by comparing it with the western belief
where the life was thought to be originated in water. The second
incarnation according to Das was a turtle (Kurma avatar), an amphibian.
The next incarnation was a wild pig (Baraha avatar) which represents
land-living animals. The fourth incarnation was a mixed form with half
man and half animal (Nrisingha avatar). This idea complies with
anthropological outlook.
All of the anthropologists now agree that the stage before true
man was a combination of man and ape. However, the fifth one was a
short-statured incarnation (Baman avatar). It indicates the fact that
early men were short stature.
In this way Prof Das described not only the biological evolution,
but the cultural revolution too. He also mentioned that Parasurama was
defeated by Rama, as Rama possessed bow and arrow, a superior weapon
than the axe. The stage corresponded to the food-gathering stage of
prehistory and it was followed by a food-producing stage as depicted in
the story of Lord Krishna who used to look after the cattle in his
childhood and his elder brother Balaram carried a plough most of the
time.
In the Christian era, before Darwin, several scientists and
philosophers expressed their views regarding the evolution. In this
context, Carl Linnaeus (1707 – 1778) made a classic work “Systema
Natural” where he described a system of classification involving the
plants and animals, known as taxonomy.
He placed man in the order Primate along with apes and monkeys, but
he did not suggest any common ancestry for them. Further, he believed
that each species was created specially and separately; their position
remains unchangeable. In this way, the proposition of Linnaeus was a
combination of the Old belief and the new thought.
Men boddo (1714-1790) by observing the origin of species traced
the evolution of man from the monkeys. Bonnet (1720 – 1793) also worked
on the process of evolution and proposed a ‘scale of beings’. His
proposition went on an ascending order from the mineral to man. Many
more scientists worked with the origin of man. Among them, the
contributions of Erasmus, Darwin (1731- 1802), Karl von Baer
(1792-1876), Schopenauer (1788 -1860) and Charles Lyell (1797 – 1875)
seem to be indispensable for proper understanding of the facts of
evolution. Imanuel Kant (1724 – 1804) proposed that the man be descended
from the monkey.
According to a group of scholars, the expression of Goethe (1749 –
1832) was so meaningful in respect of evolution that he may be regarded
as a predecessor of Charles Darwin. Again, another scientist, Malthus
(1766 -1834) kept valuable contribution towards formulating the theory
of natural selection. It is justified to trace the history of
evolutionary thought from the beginning of nineteenth Century. The first
systematic attempt was made by Jean Baptiste Lamarck (1744 – 1829), a
French biologist who was an eminent pre-Darwian student of evolution.
His theory was published in 1802 in which he proposed the
‘inheritance of acquired characters’ during the lifetime of the
individual. Following Lamarck’s proposition, Charles Darwin and Alfred
Russell Wallace jointly proposed the theory of the ‘Origin of Species’
by Natural Selection.
Charles Darwin’s evolutionary theory had its base on the
accumulation of small fluctuating variations. He had realized that
heredity was an essential factor in the study of evolution, though he
did not put much importance to it. August Weismann realized the
importance of heredity better than Darwin did.
He emphasized on the ‘continuity- of the germ plasm’ and tried to
project the transmission of inherited qualities from generation to
generation by the germ cells. Hugo de Vries, one of the re-discoverers
of Mendel’s laws of heredity, announced mutation theory of evolution in
1901. He considered mutation (i.e. sudden hereditary changes) as a
factor behind evolution.
Natural selection found very little or no place in his mutation
theory. But, later the geneticists, biometricians, and palaeontologists
revived the faith in natural selection. Of these, the most important
development took place in the field of genetics; the natural selection
was started to be restudied and reinterpreted by the geneticists.
Mention may be made of Theodore Dobzhansky and R.B. Goldschmidt, who
laid the foundation for the Neo-Darwinian theory.
The genetic theory of Natural Selection is therefore referred as
Neo-Darwinism. R.S. Fisher, J.B.S. Haldane and Sewall Wright made
valuable contribution to the statistical analysis of population and
secured own position among the principal proponents of Neo-Darwinism.
However, the important theories have been discussed in the following
pages.
11. Theory of Lamarck (Lamarckism):
He was the first scholar to recognize the distinction between
invertebrates and vertebrates. But Lamarck’s name is usually associated
with the ‘theory of inheritance of acquired characters’. Of his several
writings, mention must be made about three publications relating to the
theory of evolution: Recherches Sur L ‘Organization des Corps Vivant
(1802), Philosophic Zoologique (1809) and Historie Naturelle des Animaux
sans Vertebrates (1815 – 1822).
Lamarck expressed the fact that the acquired characters could
be inherited. His theory, known as Lamarckism was based on two laws:
i. The law of use and disuse of organs, and
ii. The inheritance of acquired characters.
According to Lamarck, a living body is always influenced by the
environmental factors and ultimately this phenomenon initiates an
adaptation of organism to its surroundings. As per necessity, some parts
of the body may be used more and more.
Therefore, those parts tend to show more development or changes
in course of time. On the contrary, other parts of the body, which may
not be required much, will be weak or demolished due to constant disuse.
This change in body structure is reflected in future generations. This
means, the characters that are acquired by the use or disuse of
different organs can be transmitted to the succeeding generations.
To support his theory Lamarck presented several examples. The
most remarkable one is associated with the long neck and high front legs
of giraffes. He stated that this animal originally possessed short neck
and small front legs.
As an herbivorous animal, the forerunners of modern giraffe were
acquainted with grass and the leaves of dwarf trees. But following a
sudden scarcity of these plants, giraffes had to stretch out their necks
to reach the leaves of the tall trees. This stretching affected the
muscles and bones of the neck, which started to be modified with time.
Not only had the neck become longer the front legs also increased in
size. This phenomenon is nothing but an adaptation to the environment,
in the way to survival.
The modified traits were continued in subsequent generations and
eventually all the giraffes got very long necks and well-built long
front legs. In another example, he mentioned that the ducks are unable
to fly because their wings became weak when they stopped flying.
Again, the birds that started to live in an aquatic environment
gradually acquired webbed feet through the conquest of survival. Lamarck
also cited other examples like limblessness in snake, blindness of
moles and certain cave-dwelling forms, aquatic plants with dimorphic
leaves (having submerged and aerial leaves), etc. All these changes were
held to be cumulative from generation to generation, and also
hereditary.
Lamarck’s theory had met criticisms from several angles. Although
some of his views were admitted by a few scholars, most of the scholars
did not accept his theory. The German scientist August Weismann
ridiculed the essence of Lamarckism (inheritance or acquired characters)
by his experiments, which involved cutting of tails of mice for over
twenty generations.
All tailless mice in all generations (even in the last
generation) produced their offspring’s with tails. Therefore he reached
to the conclusion that the environmental factors might have an influence
on the body cells, but it is not enough to profess a change of
reproductive cells.
Characters of an organism would not be inherited unless the
change could occur in the reproductive cells. However, the proposition
of Weismann is popularly known as ‘Germ-Plasm theory’ as contrary to the
theory of Lamarck. According to Weismann the body of an animal is
composed of two parts viz. Germplasm (germ cells) and somatoplasm (body
cells); only those characters which are located in the germplasm will be
inherited by the offspring.
The evidence against Lamarckism was also criticized by others, on
the ground that cutting of tail is rather mutilation, in which the
animal did not participate actively so some specific cases were required
where organisms can actively participate in the activity. In this
respect, McDougall (1938) conducted a series of experiments on learning,
using white rats. He designed a water tank having two exits, one
lighted and the other dark.
The lighted exit received electric shock, while the dark exit did
not have any arrangement to receive the electric shock. The white rats
were dropped into such an experimental tank, and then trained to escape
through the dark exit. A number of trials were required for the rat to
learn the way to escape from the dark exit. These trials constituted a
measure of the speed of learning.
The trained rats were bred, and their offspring’s were taught the
same problem. In this manner, he subjected the rats for
experimentation, for forty-five generations. McDougall observed that the
number of errors made in learning, the problem decreased progressively
from generation after generation. On the basis of this experiment, he
concluded that an acquired character (learning or training) is
inherited.
Unfortunately, McDougall’s experiments met with severe criticism,
mainly because the repetition of similar experiments in other
laboratories had failed to produce similar results. They could not
control the genetic constitution of the experimental rats.Limitation of
various other experiments probably initiated the scholars for seeking
evidence in favour of Lamarck. A new school of thought in the name of
Neo-Lamarckism soon appeared in the scene, which tried to modify the
principles of Lamarck in order to make it acceptable to the students of
evolution.
The foremost position was occupied by Giard (1846 – 1908) of
France and Cope (1840 – 1897) of America. However, Neo- Lamarckism was
based on the idea of adaptation, integrated with direct and casual
relationship between structure-function and environment. The difference
between the Lamarckism and Neo- Lamarckism was that, Lamarck believed in
direct action of the environment, which, he thought was responsible to
achieve final perfection of the individual. But Neo-Lamarckism omitted
the very idea.
The Neo-Lamarckians argued that a considerable period of time was
required for getting the effect of external factors. They also pointed
out that if the external factors failed to influence the reproductive
cells of the parents, their offspring’s would never inherit any of the
modifications.
Rapid progress of science in twentieth Century favoured the
growth of ‘genetics’, which supported none of the theories – Lamarckism
and Neo-Lamarckism. Still Lamarck deserves appreciation as his
proposition helped to open new avenues of thought in the science of
evolution.
12. Theory of Darwin (Darwinism):
In 1825, Darwin was sent to Edinburgh to study medicine, but soon
he discontinued the course. After this his father wanted him to be
prepared for the post of a clergyman, in the Church of England. So
Darwin was sent to Cambridge. While studying at Cambridge, he gained
friendship with some distinguished men of science, such as, the botanist
Dr. Henslow and the geologist Sedgwick. Dr. Henslow’s friendship
entirely changed the course of Darwin’s life; he nominated Darwin in the
position of a young naturalist for the voyage on H.M.S. Beagle (a ship,
in which Charles Darwm sailed around the world).
The voyage on the Beagle started on 27th. Dec. 1831 and Darwin
visited many Islands in Atlantic ocean, some of the islands in the
Pacific ocean including Galapagos islands, many places on the coasts of
South America and finally returned after five years on 2nd. Oct. 1936.
While on the Beagle, Darwin took notes on the flora, fauna, and the
geology of the places visited; and also made extensive collections of
living and fossil minerals. All these constituted the basis for his
future publications.
Darwin’s first publication, Journal of Researches (1839) met with
immediate success. In October 1838 he accidentally came across Robert
Malthus’ essay on population. This essay provided a clue for which
Darwin was able to think of the ‘struggle for existence’ among the
animals and plant kingdom.
In this respect, he started to collect the data from 1842. The
famous geologist of that period, Sir Charles Lyell suggested him to
write about the origin of species. In 1858, when Darwin was halfway in
his writing, he received a manuscript entitled, “On the tendency of
varieties to Depart Indefinitely from the Original type” from Alfred
Russell Wallace (1823 – 1913).
Wallace requested Darwin to read his essay and to make comments
on it. Darwin found that the essay was complete in all respects and
contained the essence of his theory of natural selection. Being
generous, he decided to withhold his half-completed work, in favour of
Wallace. So he wrote to Lyell with a recommendation to publish Wallace’s
paper at once.
But Lyell, being aware of Darwin’s strenuous effort since 1842,
urged Darwin to write a short abstract of his theory. He wished that
Wallace’s paper would be published simultaneously with Darwin’s
abstract. Reluctance of Darwin could not stand against the insistence of
Lyell.
Thus, in 1859, Wallace’s paper and an abstract of Darwin’s
manuscript together appeared in the Journal of the Proceedings of the
Linnean society. To start with, Darwin intended to complete his work in
four volumes but subsequently he condensed the work into a single
volume, entitled ‘Origin of Species’ which was published in November
1859.
The work of Darwin was submitted fifty years after Lamarck and
his theory is commonly known as Darwinism. But, the credit went to both
the scholars – Darwin and Wallace; the first systematic as well as
comprehensive approach in the perspective of evolutionary development
was made by them.
Darwin’s theory of evolution is based upon four main, rather
easily understandable postulates, which may be summarized as follows:
1. Prodigality of Nature:
All species have a tendency to produce more and more offspring’s
in order to increase the number of population. For example, a salmon
produces 28,000,000 eggs in a single season; a single spawning of an
Oyster may yield as many as 114,000,000 eggs; a common roundworm
(Ascaris lumbricoides) lays about 70,000,000 eggs in a day.
Darwin has even cited examples from slow breeding animals.
Elephants appear to be one of the slowest breeders, having a life span
of about hundred years. The active breeding age continues from thirty to
ninety years, during which a single female may produce six young ones.
Taking this estimation into consideration, Darwin calculated that
a single pair of elephants, at this rate of reproduction (provided all
the descendants survived and reproduced at the same rate) would produce
19,000,000 elephants after 750 years.
All these examples furnish instances of tremendous reproductive
potential among all species of organisms. The basic reason behind this
huge production is to ensure the survival. Because, in reality we find
that, in spite of the rapid reproductive potential, the size of a given
species, in a given area, remains relatively constant.
2. Struggle for Existence:
Above observation led to the conclusion that all the progeny
produced by any generation do not complete their life cycle, many of
them die during juvenile stages. Darwin therefore proposed his of
Struggle for Existence’; the struggle is often generated for the want of
enough resource All individuals cannot survive under struggle.
According to Darwin, the Struggle for existence may be of
different types. It may be a Struggle to overcome adverse environmental
conditions (like cold or drought), or to obtain food from a limited
source of supply. It may be a fight for occupying a living pace, or even
to escape from the enemies. However, any of these said situations,
evidently leads the members of a group towards competition, in order to
meet their requirements.
Thus the nature of struggle may be of three types according to the situations:
(i) Intra-specific struggle:
When the members of a same species struggle among themselves, the
situation is considered as intra-specific struggle. Such a struggle is
usually centered round the consumption.
(ii) Inter-specific struggle:
The individuals from different species also may go on fighting
for survival. An individual from one species may hunt another individual
of other species as food. For example, tiger hunt goat and deer; cat
hunt rat; lizard hunt cockroach and different small insects; and so on.
According to Darwin in the animal kingdom, a species often stand as prey
to other species, which clearly indicates a struggle for existence.
Such happenings have been referred as Inter-specific struggle.
(iii) Environmental struggle:
The environmental struggle is different from the inter-specific
or intra-specific struggle. Here individuals irrespective of their
species-identity struggle against the environmental hazards like
earthquake, flood, drought etc. Those who have greater potentiality for
resistance, only they survive.
Darwin believed that the struggle is a continuous phenomenon in
the way to survival It is severe among the members of the same species
(intra-specific competition), as they depend on identical requirements
of life. The inter-specific competition is though very common, but its
frequency is lesser than the intra-specific competition.
3. Organic variation:
Darwin observed that variation is a universal phenomenon. Except
the identical twins no two organisms are exactly alike. Even the two
leaves of a plant or two peas in a pod often show easily recognizable
differences. Therefore individuals of a single species must vary from
each other.
At times, an entire population may exhibit a definite pattern of
variation for which it is distinguished from the rest of the species.
Such a population showing definite pattern of variation is often
referred to as subspecies. Darwin considered these subspecies as
incipient species, and he believed that in course of time, these
subspecies would be subjected to further variation to give rise a new
species.
Although natural variations are neither advantageous nor
disadvantageous to the species concerned but some variations are
considered as favourable and others are unfavourable. In fact, the
variations in terms of physiological, structural and behavioural traits
play very important role for adaptation in the environment. The new
variants are produced continuously but when those variants cannot cope
up the environment, it is termed as un-favourable variation.
Organisms with un-favourable variation easily get defeated in the
struggle for survival and in course of time they become eliminated from
the world. On the other hand, the new variants that are capable to
adopt the pressure of the environment survive long. The new traits of
advantageous characteristics pass on to the future generation.
Darwin recognized two main types of variation in nature, viz.
Continuous variation and discontinuous variation. By the term continuous
variation he wanted to mean small fluctuations of evolutionary
significance. It was held as a force for attaining perfection being
selected by nature For example, the long neck of giraffe was evolved out
of continuous evolution.
Contrary to this discontinuous variation is mostly large and rare
in occurrence. However, they appear suddenly and do not show any graded
series. Such discontinuous variations have been regarded as ‘sports’ by
Darwin; to which, Hugo de Vries has given the name ‘mutation’, at a
later period. In the eye of Darwini discontinuous variation had no
evolutionary importance.
Darwin draws the example of Dinosaurs. The enormous size and
giant stature of Dinosaurs were the result of discontinuous variation.
He found negative mode of natural selection behind the extinction of
those Dinosaurs.
4. Natural Selection:
Natural selection is the final outcome of Darwin’s evolutionary
thought. Individuals differ from each other because of organic
variation, which evidently means that some individuals are better
adapted to survive under the existing environmental conditions than
others.
In the struggle for existence, the better-adapted individuals
possess a better chance of survival than those who are less adapted. The
less adapted individuals therefore get eliminated before reaching
maturity and thus a large number of individuals die in the struggle for
existence.
However, the traits having greater survival value are preserved
in the individuals and transmitted to the offspring’s, who are supposed
to be the progenitors of the next generation. Darwin called this
principle, by which preservation of useful variation is brought about,
as natural selection. The same principle (natural selection) has been
designated by Herbert Spencer as ‘survival of the fittest’. In the words
of Darwin “the expression often used by Spencer, of the survival of the
fittest, is more accurate, and is sometimes equally convenient”.
The theme of Darwin’s theory may finally be summed up in the
following words: The organisms always struggle to maintain their
existence as nature decides the survival of the fittest one. Adaptive
traits preserved through natural selection gradually change the
characteristics of species and thus evolution occurs.
The theory of the origin of species by natural selection, though
is regarded as a major advancement in evolutionary thought, it lacked
the knowledge of heredity, which was essential for the understanding of
evolutionary studies. It was really unfortunate that Darwin never came
across Mendel’s work, who by then invented the basic principles of
heredity. Had Darwin come across Mendel and his work, he would not have
to write in the last edition of his ‘origin of species’ that “the
fundamental principles of heredity are still unknown”.
The human ancestry was discussed by Darwin in his book, ‘The
Descent of Man’ which was published in 1871. He said that life ascended
from simplest form of minute organisms to the complex forms through
different stages of evolution where man is found at the summit.
But, at the time of Darwin very few fossil evidences were
discovered; those were insufficient to establish the proposition. This
was the first weakness of Darwinism. The second weakness was hidden in
the process itself. Darwin wanted to explain heredity by the ‘theory of
Pangenesis’, which declared that all parts of the body produce minute
particles called pangenes that ultimately get deposited to the sex-cells
being carried by blood.
Those particles are further carried to the next generation when
fertilization takes place and same kinds of organ, cell, tissue etc. are
reproduced. However, the theory of Pangenesis, like the Lamarck’s
principle, accounts for the inheritance of acquired characters. But it
too was universally discarded for the lack of evidence. The flaws of
Darwin were rectified later, after the development of the science of
genetics and the rectified theory was known as Neo-Darwinism.
13. Mutation Theory of Hugo De Vries:
The publication of de Vries’ work raised much controversy among
the adherents of Darwinism and the mutationists. The early geneticists
extended their wide support to de Vries’ theory, mainly because the
variations, which they noted in their experiments, conformed to de
Vries’ observations but hardly with Darwin’s concept.
Even eminent geneticists like William Bateson, Thomas Hunt Morgan
and others were attracted by this mutation theory Mutation theory
distinguished heritable variations from environmental variations, which
Darwin failed to understand in his ‘Natural Selection’. As a
consequence, in the early years of twentieth Century Darwin’s natural
selection was totally rejected in explaining the process of evolution.
14. Theory of Gregor Mendel:
De Vries’ hypothesis on mutation highlighted chromosomal changes,
rather than the changes in the gene themselves. So his mutation theory
is considered as out modded on the ground that it did not indicate true
mutation.
The mutations as understood today are concerned with genes, the
discrete units of heredity, which occupy particular loci on the
chromosomes. It tells that each gene controls a specific developmental
process and responsible for the appearance of specific traits in an
organism.
Mendel used the term ‘factor’, when he described his ‘Law of
Inheritance’. But in 1900 the term was replaced by the new term ‘gene’
and a new science gradually developed with the name ‘Genetics’ Now It IS
known that a gene represents a specific segment of the DNA molecule.
The product of a gene action in many cases, is a protein; and the
developmental process in a given organism depends on specific kind of
proteins produced under the instruction of a particular set of genes. A
mutation in a gene often causes corresponding changes in the protein
concerned. If mutation occurs in the gem cells of an organism, the
change will be inherited by its offspring.
Therefore, only those mutations that cause changes in the
reproductive cells of the organism are of evolutionary significance But
the structural changes of chromosomes cannot be undermined because they
often bring considerable effects in the evolution as found in many
plants and a few animals like Drosophila, crepis etc.
Although the knowledge of genetics brought a revolution in the
field of evolution Mendel’s Law of Inheritance’ is fundamental in
identifying the nature of the offspring’s. It explained the basic
process of heredity.
15. Synthetic Theory of Evolution (Neo-Darwinism):
Darwinism in its original form failed to explain satisfactorily the
mechanism of evolution and the origin of new species. The inherent
drawbacks in the Darwinian ideas were the lack of clarity as to the
sources of variation and the nature of heredity.
In the middle of twentieth Century, Scientists had come to a
consensus to employ all sorts of knowledge – genetic, ecological,
geographical morphological, palaeontological etc. in order to understand
the actual mechanism of evolution. Due importance was given to both
mutation and natural selection, among other forces of evolution This led
o the emergence of a synthetic theory of evolution, which we also call
as Genetical Theory of evolution, or ‘Biological theory of Evolution’.
Some authors namely David J. Merrell (author of ‘Evolution and
Genetics’) and Edward O Dodson (author of ‘Evolution: Process and
Product’) have called this new theory as Neo-Darwinism. But, George
Gaylord Simpson and his followers strongly warned against equating the
synthetic theory of evolution with ‘Neo-Darwinism’. Simpson argued that
the synthetic theory had no Darwin. It was not only different from
Darwin’s; it had drawn its material from a variety of non-Darwinian
sources.
After the development of the science of genetics, it has been
known mat a population snares a common gene pool. Accordingly, the
evolution denotes a change of gene -frequency in the gene pool of a
population over certain span of time.
The synthetic theory of evolution does not discard all previous
propositions, rather considers them as partially important. Therefore,
we find amalgamation of various concepts viz. Natural selection,
Mendelian principles, Mutation, population genetics in this theory of
evolution. But it is interesting to note that modem genetics does not
acknowledge to mutation theory in its original form, as proposed by de
Vries. Because that original theory had out- rightly rejected the basic
concept, ‘natural selection’ as delivered by Darwin and advocated
‘mutation’ as the sole force of evolution. However, at present evolution
appears to be a complex process involving several complex forces.
This page was last modified on November 19, 2014.
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