Evolution

EVOLUTION:
In Biology, the term evolution refers to a process of development of an entity in the course of
time through gradual sequence of changes from simple to complex form.

THE CONCEPTS OF EVOLUTION: There are two schools of thoughts. One of the
schools of thoughts believe in Divine Creation while the other in origin through simple to
complex forms.
Theory of special creation: The theory of special creation believes that living entities are
Created by God. They were created either at the same time or at some intervals. However, the
species do not have any inter-relationship with each other from the origin point of view.
Theory of Evolution: The theory of evolution believes that organisms are evolved through
gradual process of changes from simple to complex forms during the course of time. Thus
plants and animals have developed in continuous, orderly way, under the guidance of natural
laws.

EVOLUTION FROM PROKARYOTES TO EUKARYOTES

i) Membrane invagination Theory: It suggests that the cell membrane of prokaryotic cells
invaginated to enclose the genetic material. This accounts for the development of true nucleus
while the other portions enclosed other necessary materials to transform into different
organelles.
ii)Endosymbiotic Theory: It was suggested by Lynn Margulis. According to this hypothesis,
the eukaryotic cell might have evolved when a large anaerobic amoeboid prokaryote ingested
some aerobic bacteria (mitochondria) and rather than digesting started living with it in
endosymbiotic relationship. Likewise, some of such eukaryotic cell, endocytosed autotrophic
photosynthetic bacteria and transformed into ancestral autotrophic plant like organism. The
endosymbiotic theory seems more powerful in dealing with the evolution of eukaryotes since
both mitochondria and chloroplast have following similar features like prokaryotes;
i. Circular DNA
ii. Ribosomes
iii. Metabolism
iv. Binary fission way of reproduction.

EVIDENCES IN FAVOR OF EVOLUTION:

Following are some evidences in favor of organic evolution.
1. Evidences from fossil record:
From the fossil records it has been concluded that evolution has taken place from simple to
complex in a gradual manner. The example of such evidence is Archaeopteryx. This bird had
lived 150million years ago. The fossils of this bird had following features.
Reptilian Characters of Archaeopteryx:
(a) The body axis is more or less lizard-like,
(b) The jaws are provided with teeth.
(c) The wings have claws.

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Avian Characters of Archaeopteryx:
(a) Presence of feathers on the body.
(b) The two jaws are modified into a beak.
(c) The fore limbs are modified into wings.

2. Evidence From Homologous Organs:

The organs which have the same fundamental structure but different in functions are called
homologous organs. These organs follow the same basic plan of organization during their
development. But in the adult condition, these organs are modified to perform different
functions as an adaptation to different environments. Homology indicates common ancestry.
Examples: The fore-limbs of man, cheetah, whale and bat have the same basic structural plan
But the fore-limbs of these animals have different functions. In man they are used for
grasping, in cheetah for running, in whale for swimming and in bat for flying.
3. Evidence from vestigial organs:
The organs which are present in reduced form and do not perform any function in the body
but correspond to the fully developed functional organs of related animals are called vestigial
organs. They are believed to be the organs which were complete and functional in their
ancestors.
Example:
(a) Coccyx is a tail bone in man which is well developed in other vertebrates.
(b) Vermiform appendix in man is small finger like projection which has no function but in
grazing animals it helps in cellulose digestion by microorganisms.
4. Evidence from Embryology:
Embryological study shows similarities between different animals groups through which
embryo convert into Adult. Example: Vertebrates history shows that fishes, amphibians,
reptiles, birds and mammals including man had a common ancestor due to the similarity
between their embryological development pattern.
5. Evidence from Molecular Biology:
The blood serum contains certain specific proteins which gives evidence in favour of
evolution. Example: (a) serum blood test have shown the relationship between man and
monkey. (b) the blood protein of carnivores like cats, dogs also closely related. (c) herbivores
like cow, goat, sheep also have protein resemblance in their serum.

LAMARCKISM:
The theory of evolution was put forth by French biologist Lamarck has come to be known as
Lamarckism. This theory is also known as Inheritance of Acquired characteristics.
This theory has three salient features:
Effect of Environment: According to Lamarck, when the living organisms live in a
particular environment for a long period of time, it has a great influence on it. If a living
organism migrate from one place to another place in a different environment, their needs and
habitat are gradually changed. These changes may be the case of use and disuse of the organs.
Use and Disuse of Parts: According to Lamarck, continuous use of a part results in it being
well-developed and disuse of a part over a long period of time will result in its degeneration.
For example,
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 Giraffes were forced to extend their necks and stretch their legs to reach higher vegetation
over a period of time. This resulted in every generation having a little longer neck and
legs than the previous one.
 Webbed feet in aquatic birds are thought to have developed due to constant spreading of
toes and the stretching of the skin between.
 The snakes are limbless, it Is because that their ancestors were lizard like, when mammals
appeared on earth, these snakes were compelled to live in narrow cave, so they were
crawling in this way they did not use their limbs which were disappear.

Inheritance of Acquired Characters: According to Lamarck, the characters that an

organism acquired due to a change in their environment such as long neck, webbed feet, etc.
were passed on to the next generation. In this way, evolution from simpler to complex forms
took place. However, this theory was not widely accepted as it is known that acquired
characters are only phenotypic changes and not genotypic. Thus, while the cases of giraffe,
aquatic birds and snakes do show that evolution has occurred, Lamarckism does not provide a
satisfactory answer to the mystery of evolution.
Drawback of Lamarckism: Environmental changes will affect only the somatic cells and
did not affect the germ cells or the gametes. Only the changes that affect the germ cells will
be inherited by successive generations.

DARWINISM: (THEORY OF NATURAL SELECTION)

The theory of natural selection was put forth by Charles Darwin in his book 'On the Origin of
Species by Means of Natural Selection'.
According to Darwin, nature has its own ways of selecting the best from the available species
for continuation of life. The Main points of Darwin’s Theory are
 Over production
 Struggle for existence
 Variation and heredity
 Survival of the fittest
 Origin of new species.

Over Production:
Living organism is fertile. They have very fast process of reproduction. They increase their
number rapidly. It helps to adjust their existence in that environment. Some examples of over
production are as follows.
Cod fish produce 5 to 7 million eggs in a season.
A common fly lays eggs, six times in every summer, and these are about 120n number. A
starfish lays one million eggs in one year.
Struggle For Existence:
If all the seeds of the plants germinates and grows up into new adult plants, in a short period
of time they would cover a very large area. This will lead to struggle for nutrition in that area.
Those plants will compete with each other for their existence. This struggle may of three
types.

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Evolution
Intraspecific struggle: Struggle between same kinds of species.
Interspecific struggle: Struggle between Different kinds of species.
Environmental struggle: this is the struggle against the severe climatic conditions, such as
light, temp, rain etc.
Variations and Heredity:
All organism show variation in their size, shape colour, physiology and habit. Those
individuals who show suitable variations will continue to exist and reproduce, while those
which show unfavorable variation will be eliminated.
Survival Of The Fittest:
In the competition or struggle for existence, the individual who shows favourable variations
will survive and they will be able to reproduce. But those living organisms which shows
unfavorable variations, will be unable to survive and therefore will die.
For example, if an animal give birth to 5 offsprings, those who have the suitable changes
according to the environment, will survive, because they have the ability to face those harsh
conditions in which others could not survive.
Origin of New Species:
Variations of inherited genes are responsible for the development of new characters in their
offsprings. The better genes will be able to transfer into the next generation, which are more
suitable to that environment.

Hardy-Weinberg Theorem: “The frequencies of dominant and recessive alleles in a

population will remain constant in a population in the absence of external force”.
(p + q)2 = 1
p2 + 2pq + q2 = 1

p = the frequency of the dominant allele (represented here by A)

q = the frequency of the recessive allele (represented here by a)
The three terms of this binomial expansion indicate the frequencies of the three
genotypes:
p2 = frequency of AA (homozygous dominant)
2pq = frequency of Aa (heterozygous)
q2 = frequency of aa (homozygous recessive)
Let us examine a hypothetical population in which allele „A‟ has a frequency of 0.19
and allele „a‟ has a frequency of 0.81.
p2 + 2pq + q2 = 1
(0.19) 2 + 2(0.19) (0.81) + (0.81) 2 = 1
0.0361 + 0.3078 + 0.6561 = 1
0.0361= frequency of AA (homozygous dominant)
0.3078= frequency of Aa (heterozygous)
0.6561= frequency of aa (homozygous recessive)

Factors Affecting Hardy-Weinberg Theorem

i) Mutations: It introduces new genes into population
ii) Selection: Either due to natural or artificial selection, some species may be given greater
opportunity to reproduce and increase the number of their offspring which can affect the
frequencies of alleles.

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iii) Non-random mating: The process of mating should be non-random in a given
population and the population size should be large enough.
iv) Gene flow: It refers to the exchange of genes between different populations of the same
species. It may be either due to migration of individuals from population to another of the
same species or to the transfer of gametes.
GENETIC DRIFT (NEUTRAL SELECTION):
It refers to the changes in allelic frequency in a population from generation to generation. It
occurs when allele frequencies grow higher or lower by chance and typically takes place in
small populations. There are two types of genetic drift
i) Bottleneck effect: In this kind of genetic drift, the size of the population is decreased due
to some natural catastrophes like volcano eruption, earthquake, flood, fire, etc. As a
consequence, a number of individuals would be eliminated leaving behind few live
individuals to reproduce.
ii) Founder effect: In this kind of genetic drift, a new small population separated from the
larger One due to some geographical or physical barriers starts reproducing within itself. As a
consequence, the allelic frequencies will be different from their original stock.
SPECIATION AND ITS MECHANISM:
It is the biological process of formation of new species of living organisms. It occurs when a
group of individuals within a population develop distinct characteristics and becomes
reproductively isolated from the rest. There are different ways for speciation process.
Sympatric speciation: In this case, one of the populations of a species occupying the same
geographical area becomes distinctly different features so that it is unable to mate with its
original stock. There could be different reasons for sympatric speciation such as polyploidy,
habitat differentiation and sexual selection. It is more commonly observed among plants than
animals.
Allopatric speciation: In this kind of speciation, the populations become geographically
separated from each other so that they become reproductively isolated from the rest of their
populations. As a consequence, the gene flow stops among them and depending upon their
environmental factors during the course of time, they do genetically differ from them. It is
one of the very common ways of speciation.
Peripatric speciation: When small groups of individuals break off from the larger group and
form a new species, this is called peripatric speciation. Like allopatric speciation, although
there is a geographical isolations exists in this kind of speciation, it differs in way that the
separated group is much smaller than the original one.
Parapatric speciation: In this speciation, the populations are not geographically separated
from each other, but they enter a quite different habitat within the same area of the parent
species. In such case, the populations may interbreed but develop distinct features and habits.
The reproductive isolation in this case is behavioral rather than geographical. For instance, it
is observed in plants living on boundaries between distinct climates may flower at different
times in response to their environments. Thus, they cannot interbreed with the parental types.