Theories explaining Origin of life

seven major theories are proposed to explain the origin of life on earth. these theories are as
follows:
Theory of special creations: the theory of special creation is proposed that life on earth is created
by a supernatural power, the god. according to the christian belief, god has created the universe,
planet, animal, plant and human in six natural days. similar beliefs are also been proposed by other
religion as well.
These points are as follows:
a) All living organisms were created same day [no difference in their appearance].
b) they were created in the present form [no evolution].
c) their bodies and organs are fully developed to meet the requirement to run the life [no
adaptation]
objections to the theory of special creation: • It was purely based on religious belief. • There was no
experimental evidences to support the assumptions. • The age of different fossils proves that living
organism appear on earth in different time frame.
Theory of eternity of life: This theory assumes that life had no beginning or end. it believes that life
has ever been in existence and it will continue to be so ever.
It further believe that there is no question of origin of life as it has no beginning or end. the theory is
also known as steady state theory.
The main objection against the proposed theory that it could not be able to explain evidences,
support that initially earth forms and then life appeared on it. where life exist before the formation of
earth?

Theory of catastrophism: This is the extension of the theory of special creation. This theory
assumes that life is originated by the creation and it is followed by catastrophe due to geographical
disturbances. Each catastrophe destroyed the life completely whereas each creation forms life
different from the previous one. Hence, each round of catastrophe/creation is responsible for
evolvement of different types of organisms on earth. No scientific experiment to support the
hypothesis and mostly based on imaginary concepts.
Theory of cosmozoic: This theory was put forward by Richter and strongly supported by
Arrhenius. The theory assumes that life was present in the form of resistant spores and appeared on
earth from other planet. Since the condition of earth was supporting the life, these spores grew and
evolved into different organisms. This theory was also known as “theory of panspermia or spore
theory”.

The theory initially got the support from the fact that fossils of microorganism were found in
meteorites. But no mechanism is known about the transfer of spores from other planet or whether
these spores could survive the journey in space. The absence of life forms on any planet except
earth and no details about the spores, its origin and mechanism of crossing interplanetary space and
reaching earth. No scientific experiment were given to support the theory. As a result, the
hypothesis didn’t receive much attention.
one meteorite that fell in 2000 in Canada contained tiny organic
structures dubbed "organic globules." NASA scientists think this
type of meteorite might have fallen to Earth often during the
planet's early history, seeding it with organic compounds
Theory of spontaneous generations: The theory of spontaneous generation or abiogenesis
assumes that non-living material in a spontaneous manner give rise to life.
There are several observations supporting this theory:
• Hair of horse tail dipped in the water gives rise to horsehair worm, gordius.
• Fly larvae/maggots developed on a rotten meat.
• In ancient egypt, it was believed that the mud of the nile could give rise to frogs, toads,
snakes, mice and even crocodiles when warmed by the sun
- van helmont (1577-1644) claimed that he can produce mice from the dirty shirt and
handful of wheat grains kept in dark cupboard in 3 weeks.
Evidences against the theory of spontaneous generation: Theory of spontaneous generation
was criticized by lazzaro spallanzani, francisco redi and louis pasteur. These great
scientists performed well designed scientific experiments to disprove the theory of
spontaneous generations.
Redi’s experiment: Francisco redi did conclusive and well designed experiment to disapprove the
theory of spontaneous generation. He placed meat and fishes in 3 separate jars. jar no. 1 was left
open, no. 2 was covered with gauze and the third one was covered with paper. the meat/fishes
decayed in all three jars and attracted flies. In jar no. 1, flies entered and layed eggs which eventually
gave birth new larvae. whereas in jar no. 2, flies couldn’t be able to enter and no larve was found
inside the jar. but flies laid eggs on gauze that produced larvae. this has conclusively proves that
organisms arise from the pre-existed organism rather than non-living matter.
Spallanzani’s experiment: In this experiment, spallanzani has prepared
animal or vegetable broth and boiled them for several hours and then
either remained open or sealed immediately. These sealed broth
remained free from microorganism growth. He concluded that high
temperature boiling had killed all microorganisms and in the absence of
microorganism life could not appear. But, the broth left open or exposing
of sealed broth shows growth of microorganisms.
Louis Pasteur Experiment: In another conclusive experiment, Pasteur had
designed experiment in a flask with “S” shaped curve tube (Figure below).
He took hay infusion in the flask and boiled for several minutes. After
cooling, the steam condensed into the lower part of tube and act as barrier to
stop the entry of microorganisms. No life appeared in the flask for several
months. Analysis of condensed water indicates appearance of microorganism
in the neck of the tube. Breaking of “S” tube allowed the growth of
microorganisms in the flask.
Hydrothermal vent/ Deep sea vent theory:
• The deep-sea vent theory suggests that organic molecules and life may have begun at deep sea or
submarine hydrothermal vents, spewing key hydrogen-rich molecules. Hydrothermal vents form at
locations where seawater meets magma.
• Hydrothermal vents are the result of seawater percolating down through fissures in the ocean crust.
The cold seawater is circulated through hot volcanic rocks and heated by hot magma and reemerges
to form the vents. Seawater in hydrothermal vents may reach temperatures of over 700° Fahrenheit
(300-400 °Celsius).
• The hot solution emerging into cold seawater precipitate minerals deposits that are rich in iron,
copper, zinc and other metals. These minerals solidify as they cool, forming chimney-like structures.
• Their rocky nooks could then have concentrated these molecules together and provided mineral
catalysts for critical reactions. Even now, these vents, rich in chemical and thermal energy, sustain
vibrant ecosystems devoid of energy from the sun.
• Extremophilies now known to exist around the vents; these ecosystems are totally independent of
energy from the Sun, depending not on photosynthesis but rather on chemosynthesis by sulfur-fixing
bacteria.
Black smokers” are chimneys formed from deposits of iron sulfide, which is black.
“White smokers” are chimneys formed from deposits of barium, calcium, and
silicon, which are white.
Modern theory:
The modern theory is also known as “chemical theory/ chemical evolution” or theory of
primary abiogenesis or theory of biochemical evolution.
The idea of chemical theory was put forward by two scientist, Alexander Oparin and J.B.S
Haldane (1929).
In the modern theory, the hypothesis of abiogenesis was proposed with a condition that
the non-living materials can give rise to life in the condition of primitive earth. The condition
of the primitive earth is different from the present conditions which do not permit
abiogenesis.
“High energy discharges (UV/Lightening) in the reducing environment of primordial earth
would have favoured the synthesis of simple organic molecules from existing geological
molecules on earth surface.”
Assumptions:

1.Spontaneous generation of life under the present environment is not

possible.
2. Earth’s atmosphere, ~1 billion years is very different from the current
conditions.
3. Primitive earth’s atmosphere was reducing in nature (because of
presence of nitrogen, ammonia, methane, carbon dioxide, and it
actually did not have any atmospheric oxygen.)
4. Under these conditions, the chemical molecules (inorganic
molecules) react with each other through a series of reactions to form
organic substances and other complex biomolecules. The solar energy
and UV radiation provided the energy for the chemical reactions.
 Theory explains:

1. Life originated from simple inorganic molecules.

2. Inorganic molecules transformed into organic molecules.
3. Organic molecules are later transformed into colloidal substances.
4. Colloidal substances acquired life (capacity of propagation/reproduction)
5. The colloidal substances with life behaved like present day prokaryotic
cells.
According to the chemical theory of origin of life, a series of chemical synthesis give rise to life. As per this
hypothesis proposed, origin of life have four major steps:
1. Formation of Inorganic molecules
(Seventy percent of our body weight is made up of water.
ninety percent of a dry weight of any living being essentially consists of these major elements
– the most abundant being the carbon, hydrogen, oxygen, nitrogen, phosphorous and sulphur.
In addition to these, some trace elements like iron, copper, zinc; but in terms of bulk quantity,
the bulkiest, or the one which is available in most quantity is the carbon.)
The high temperature of primitive earth didn’t allow the condensation of atoms to form
inorganic molecules. As temperature of earth goes down, condensation of different atoms give
rise to simpler molecules. The elements most abundant on the primitive earth are hydrogen,
nitrogen & carbon, sulphur, oxygen, etc. The reaction of these molecules give rise to the
different gases, such as hydrogen, nitrogen, ammonia, methane, carbon dioxide and water
vapour. The energy for these reactions was provided by sunlight, lighting or volcanic
eruptions.
2. Spontaneous formation of monomeric organic compounds:
-The simple molecules interact with each to form simple monomeric organic compounds.
When temperature of earth gradually cooled down then the highly reactive free
radicals CH and CH2 condensed to form variety of saturated and unsaturated hydrocarbon.
-The reactions between the inorganic molecules to give simple organic molecules occurs
in reducing environment inside ocean. These simple elements were washed from
atmosphere to ocean through rain wherein polymerization and condensation reactions
formed simple organic compounds such as sugars, amino acids, fatty acids, glycerol and
nucleotides.
-water show condensation, reduction and polymerization reactions
-The inorganic molecules were condensed in the form of rain as temperature of earth lower
down. Hence, the mixture of inorganic compounds and simple organic compounds that
were present in the primordial ocean of the primitive earth is called prebiotic or Primordial
soup.
3. Spontaneous formation of complex organic compounds:
The small, simpler organic compounds react to form complex organic compounds.
The complex organic molecules were formed in oceans as facilitated by presence of
high temperature and presence of monomers.
The simple amino acids reacts to form polypeptides, sugar reacts to form large sugar
molecules, fatty acid and glycerol combined together to give fat.
Heat of the sun is utilized for providing energy for these reactions.
 4. Spontaneous formation of molecular aggregates:
 Large organic molecules came together to form large colloidal aggregates called
as coacervates (to assemble together). conglomeration of organic molecules and
other polymers with positive and negative charges.
 A layer of water molecules forms around the protein molecules present in
coacervates. The membrane present around the molecules protect the molecule
and bring high local concentration to enhance the chemical reactions. The colloidal
aggregates absorb protein and other molecules from the ocean. This results in
growth in coacervate as well as internal complexity. As coacervate divides into
multiple small ones.
 These coacervates are the initial species present in the ocean to start the
formation of primary cells. This process is accomplished in two steps:

4.a.)Formation of eobionts or protocells: The coacervates has the ability to take up

new molecules to replace the degraded molecules and maintain the size. Thus,
coacervates has the basic property of living system but it doesn’t have complex
molecules such as enzyme etc.
The process of acquiring new molecules was not regulated. Later, nucleic acid is
entrapped within the coacervates and process of division became precise and controlled.
This form of coacervates with nucleic acid is known as eobionts or protocells/protobionts.

4.b.) Formation of first cells: Protein molecules and appearance of enzymes has
enhanced the synthesis of several of biomolecules in eobionts. RNA and DNA developed
and these molecules has taken over the protein synthesis. Interaction of lipid and protein
allowed the formation of biomembrane which has provided selectivity in the primitive
cell for intake or exlusion of material. It allowed the appearance of membrane bound
protocell and that has eventually given first cell on earth. The mutation in DNA and
selection of fast growing cell give rise to the appearance of first primordial cell. The first
cellular form on earth appeared ~3.8 billion years ago.
Experimental verification of chemical origin of life -
Experiment
Stanley Miller and Harold Urey.
Twenty-four years later, in 1953, Stanley Miller and Harold Urey, actually demonstrated chemical
theory to be true, by conducting experiment in mimicking primitive earth environment.
Experiment:
1. Miller and Urey designed an closed system or apparatus to mimic conditions thought at that time
to have existed on the early Earth.
2. The apparatus, A flask for half filled boiling water simulated the primeval sea, The water was
heated so that some vaporized and moved into a second (spark chamber), higher flask containing
the “atmosphere”—a mixture of gases, containing methane, ammonia, hydrogen and water
vapours in the ratio 1:2:2 without air (oxygen). The spark chamber had two tungsten electrodes.
Side tube from spark chamber connected to a condenser and U-shaped trap to collect sample .
3. The electric discharge was created by using tungsten electrodes in a atmosphere chamber. This
mimics the lighting in the primitive atmosphere.
• They passed the mixture through a condenser.
• They circulated the mixture within the closed glass system for more than 2 weeks continuous and
then analysed the chemical composition of the liquid inside the apparatus
Observation:
The analysis of mixture indicates the presence of amino acids such as glycine, alanine, aspartic acid,
nitrogen base adenine and simple sugar as ribose.
Chemistry of experiment:
In addition, he found hydrogen cynide (HCN), formaldehyde (HCHO) and other active intermediate
compounds such as acetylene etc.
• Formation of HCN, HCHO
CO2 → CO + [O] (atomic oxygen)
CH4 + 2[O] → CH2O + H2O
CO + NH3 → HCN + H2O
• Formation of Glycine: The formaldehyde, ammonia, and HCN then react to form glycine.
CH2O + HCN + NH3 → NH2-CH2-CN + H2O
NH2-CH2-CN + 2H2O → NH3 + NH2-CH2-COOH
Conclusions/ significance:
-All these complex organic compounds including amino acids, were synthesized non-
enzymatically. This proved that complex organic compounds could be synthesized in reducing
atmosphere without the help of enzymes and thus supported the theory of chemical evolution.
-The experiment significantly proved that origin of life on the planet Earth had occurred because of the
chemical conditions or through Abiogenesis.

Drawbacks:
Debates : whether brown soup/ primordial soup can support life. The substances are highly toxic.
(gaseouc CO, cynadies).
-unsure of exact composition of earths original atmosphere. Urey-Miller may have miscalaculated the
exact % of each substances. (there were evidences of major volacanic erruptions 4 billion years ago,
which would have added co2, nitrogen, hydrogen sulfur, sulfur dioxide into the atmosphere.
-No actual organism were formed in this experiment.
Theories of Evolution : Darwin’s Theory of Evolution
The theory of natural selection was proposed by Charles Darwin and Alfred Russel Wallace. Charles
Darwin travelled for 5yrs expedition around the world on the ship H.M.S Beagle. During this journey, he
made observation of several animals and plants. He keenly observed the similarities among organisms
and draw evolutionary relationship. Charles Darwin to proposed the theory of natural selection in his
book “Origin of Species”.

Darwin defined evolution as "descent with modification," the idea that species change over time, give rise
to new species, and share a common ancestor.

Darwin's theory of evolution, also called Darwinism, considers evolution as “evolution as such”, common
descent, gradualism, multiplication of species and natural selection.
• Evolution as such: new species arise naturally rather than created by god.
• Common descent: common decent involves the formation of new species from an ancestral
population.
• Gradualism: evolution proceeds chiefly by the accumulation of gradual changes.
• New species form by speciation/ multiplication, in which an ancestral population splits into two or
more genetically distinct descendant populations. Speciation involves reproductive isolation of groups
within the original population and accumulation of genetic differences between the two groups.

• Natural selection is the process through which populations of living organisms adapt and change.
Individuals in a population are naturally variable, meaning that they are all different in some ways.
This variation means that some individuals have traits better suited to the environment than others.
Because resources are limited in nature, organisms with heritable traits that favor survival and
reproduction will tend to leave more offspring than their peers, causing the traits to increase in
frequency over generations.
Natural selection causes populations to become adapted, or increasingly well-suited, to their
environments over time. Natural selection depends on the environment and requires existing heritable
variation in a group.
Finches that he observed on the Galapagos islands,
Finches, small sparrow-like black birds with strong, short
beaks. finches varied on different islands, but they were
closely related to one another.

These finches evolved from a common ancestor to have

different beaks well-suited for different types of food they
feed on. Usually, long and pointed beaks are more fitted
for seed and cactus feeders. Even some short-
beaked finches tear up the base of the cactus and feed on
its pulp. Most finches that feed on seeds on the ground
(ground finches) have short and stout beaks. Slender
and sharp beaks are common in insect-eating finches.
This adaptation helps them occupy and survive in
different niches. Most of them evolved from regular seed-
eating finches to insectivorous and plant-eating finches.
The process by which they successfully evolved and
radiated to different habitats is termed as adaptive
radiation.
The theory of natural selection is based on following points (how it works):

1. Rapid Multiplication: Every organism has enormous ability to reproduce to continuance of

the species. All animals and plant tend to multiply in geometrical progression. For example, an
organism will be double in Ist year, four times by 2nd yr and 8 times in a third year and so on.
Ex : Elephant has an average life span of 90 years and during the whole life span he can produce
only six offsprings. If all the offsprings survived, single elephant pair would produce
19,000,000 elephants in 750 years.

2. Limited natural resources: Inspite the enormous capacity of an organism to reproduce, the
number of individual species remain constant. It is due to increase in population in animal or
plant requires more space and food. Ultimately the food to plant or animal is provided by the
CO2 from air, water and mineral from soil. The amount of these basic material is limited in
universe. Hence, it does not allow the population of organism beyond the limit and an
equilibrium is reached.
3) Struggle for existence: Due to shortage of food, water and space, there is severe
competition among the off spring for existence. Every individual has few basic
requirements, such as food, space, water, mate to reproduce and protection from enemies. In
order to achieve basic needs, organisms compete with each other and it is known as struggle
for existence.
The struggle for an individual can be of 3 types:
-Intraspecific Struggle: The competition of the individuals of the same species. For
example, fight between two dogs for a piece of meat.
-Interspecific Struggle: The competition of the individuals of the different species. For
example, tiger attacks on deer for food.
-Environmental Struggle: Every individual struggle against the change in environment such
as temperature, humidity, level of water, rain, climate etc.
4. Variation: Each and every individual varies in several aspects to other individual. Even the
offsprings produced by parents also differ in many aspects. The two individuals can be different
from each other in their behavior, color, size, strength etc.

5. Natural Selection: Due to variation among different individual, they struggle towards their
existence with different potentials. These variation in an individual may allow him to survive and
complete its life cycle. Wheras, if the variations are unfavorable, the individual will struggle
against every odds and as a result it may not be able to complete the life cycle. Another factor is
ability to adopt into the changed environment. Both Darwin and Wallace, recognized the
environment as the principal factor for natural selection.
For example, fast running deer has better chance to escape from the tiger compared to the slow
runner.
6. Inheritence of useful variations: The individual survived due to unique variations, they transfer
the useful variations to the next generations (offsprings) and allow the individual to multiply.
Darwin believed that any variation which can help the individual to survive and help to favouable
for struggle will be inherited. He considerd the variation may be acquired or inheritable.

7. Formation of new Species/survival of the fittest: As a result of struggle and natural selection,
only the individual fits to the environmental conditions will survive and complete its life-cycle. As
a result, the number of these individual will increase over the course of time compared to the less
favorable organism. In addition, the variation favoring will be inherieted to the next generation
whereas unfavorable variation will be discarded. Due to continous selection, a new organism will
appear which will be different from their ancesteral form.
For example, in the beginning there were two different types of giraffe present on earth; (1) Deer
like short height and (2) Long neck and forearms. Until the grass was available on land, both of
these species were surviving and be able to complete their life cycle. With change in climate and
reduction of grass, there might be a struggle for food. The giraffe with long neck and forearms can
still be able to eat leaves on the tress but the deer like giraffe could not be able to reach there and
died.
Sexual selection
-Mutual attraction between the sexes is an important factor in reproduction. The males and females of
many animal species are similar in size and shape except for the sexual organs and secondary sexual
characteristics.
-There are, however, species in which the sexes exhibit striking dimorphism (or physical difference).
-Particularly in birds and mammals, the males are often larger and stronger, more brightly coloured, or
endowed with conspicuous ornamentation. the long plumage of male peacocks (Pavo cristatus) and birds
of paradise (Paradisaea) and the enormous antlers of aged male deer (Odocoileus) These traits,
-However, make animals more visible to predators and are also cumbersome loads. Darwin knew that
natural selection could not be expected to favour the evolution of disadvantageous traits, and he was able
to offer a solution to this problem.
-He proposed that such traits arise by “sexual selection,” which “depends not on a struggle for existence
in relation to other beings or to external conditions but on a struggle between the individuals of one sex,
generally the males, for the possession of the other sex.”
-The concept of sexual selection as a special form of natural selection. This concept was put forth by
Charles Darwin in The Descent of Man (1871).
Define:
-Organisms more proficient in securing mates have higher fitness. There are two general
circumstances leading to sexual selection. One is the preference shown by one sex (often the
females) for individuals of the other sex that exhibit certain traits. The other is increased strength
(usually among the males) that yields greater success in securing mates.

-Thus theory postulates that the evolution of certain physical traits—such as pronounced
coloration, increased size, or striking adornments—in animals may grant the possessors of these
traits greater success in obtaining mates. From the perspective of natural selection, such increases
in mating opportunities outweighs the risks associated with the animal’s increased visibility in
its environment.
Artificial Selection:

--From ancient times, man is selecting good breed animals and plant for their
use. In addition, they are performing cross breeding of these species to
develop newer breed with desirable characters.
--Selective breeding is the process by which humans use animal breeding
and plant breeding to selectively develop particular phenotypic traits by
choosing which typically animal or plant males and females will sexually
reproduce and have offspring together.
--Artificial selection works the same way as natural selection, except that
with natural selection it is nature, not human interference, that makes these
decisions.
--Natural selection is a slow process but much more complex compared to
the artificial selection procedure.
Darwin was also inspired greatly by the evolution that he saw in the traits of pigeons, not due to natural
selection but rather artificial selection. Breeding pigeons was a popular hobby in England in Darwin ’s
time. By selecting which pigeons were allowed to mate, people had a profound effect on their appearance,
such as the shape and size of their beaks and the color of their feathers.

Examples : Dog breeding is another prime example of artificial selection. The use of artificial selection
has allowed humans to drastically alter the appearance of dogs. For centuries, dogs have been bred for
various desired characteristics, leading to the creation of a wide range of dogs, from the tiny Chihuahua to
the massive Great Dane.

Artificial selection has long been used in agriculture to produce animals and crops with desirable traits.
The meats sold today are the result of the selective breeding of chickens, cattle, sheep, and pigs. Many
fruits and vegetables have been improved or even created through artificial selection. For example,
broccoli, cauliflower, and cabbage were all derived from the wild mustard plant through selective
breeding. Artificial selection appeals to humans since it is faster than natural selection and allows humans
to mold organisms to their needs.
Mendelian inheritance : also called Mendelism
The principles of heredity formulated by Austrian-born botanist, teacher and priest, Gregor Johann Mendel in
1865. Gregor Mendel is often referred to as the Father of Genetics.

In 1900, Hugo De vries, Karl correns, Eric Tschermack rediscovered medels hypothesis.

Mendel studied the inheritance of seven different traits in garden peas, each trait being controlled by a different
gene.

Allele: an allele is a variant form of a gene. An allele is one of two or more versions of a gene. An individual
inherits two alleles for each gene, one from each parent. If the two alleles are the same, the individual is
homozygous for that gene or if different, he is heterozygous. Alleles determine the phylogenetic traits.

Mendel’s research led him to formulate three principles of inheritance: (1) the alleles of a gene are either
dominant or recessive, (2) different alleles of a gene segregate from each other during the formation of gametes,
and (3) the alleles of different genes assort independently.
Medel’s Three principles are
The Principle/law of dominance
The Principle/law of segregation
The Principle/law of independent assortment

The principle of dominance: This principle is a statement about genetic function.

Some alleles evidently control the phenotype even when they are present in a single copy. In a
heterozygote, one allele may conceal the presence of another. The allele that exerts greater influence
than the other on the same inherited character is called dominant allele while the other is known as
recessive allele.

Mendel developed the concept of dominance from his experiments with plants, based on the
supposition that each plant carried two trait units, one of which dominated the other.
 For example:
Monohybrid Cross: Intercross between two organisms, pure for any one pair
characteristics.

If a pea plant with the alleles T and t (T = tallness, t = shortness) is equal in height to a
TT individual, the T allele (and the trait of tallness) is completely dominant. If the Tt
individual is shorter than the TT but still taller than the tt individual, T is partially or
incompletely dominant—i.e., it has a greater influence than t but does not completely
mask the presence of t, which is recessive.
InComplete dominance or partial Dominance:
An allele is dominant if it has the same phenotypic effect in heterozygotes as in homozygotes—that
is, the genotypes Aa and AA are phenotypically indistinguishable. Sometimes, however, a
heterozygote has a phenotype different from that of either of its associated homozygotes.

For example: Flower color in the snapdragon, Antirrhinum majus, is an example. White and red
varieties are homozygous for different alleles of a color determining gene; when crossed, they
produce heterozygotes that have pink flowers. The allele for red color (W) is therefore said to be
incompletely, or partially, dominant over the allele for white color (w).
Co-dominance
A variation on incomplete
dominance is codominance, in
which both alleles for the same
characteristic are simultaneously
expressed in the heterozygote..

In a self-cross between
heterozygotes expressing a
codominant trait, the three possible
offspring genotypes are
phenotypically distinct.
The law of segregation: This principle is a statement about genetic transmission.

Explains the process by which alleles are transmitted. In a heterozygote, two different alleles
segregate from each other during the formation of gametes i.e. eggs and sperms.

According to the law of segregation, only one of the two gene copies present in an organism is
distributed to each gamete (egg or sperm cell) that it makes, and the allocation of the gene copies is
random. As a result, each gamete contains one allele of each gene. This idea is called the principle
of segregation.

When an egg and a sperm join in fertilization, they form a new organism, whose genotype consists
of the alleles contained in the gametes

An allele is transmitted faithfully to the next generation, even if it was present with a different
allele in a heterozygote. The biological basis for this phenomenon is the pairing and subsequent
separation of homologous chromosomes during meiosis.
For example: As the parental strains, the tall and dwarf pea plants form the P generation of the
experiment. Their hybrid progeny are referred to as the first filial generation, or F1, from a Latin
word meaning “son” or “daughter.” Because each parent contributes equally to its offspring, the
genotype of the F1 plants must be Dd; that is, they are heterozygous for the alleles of the gene
that controls plant height. Their phenotype, however, is the same as that of the DD parental
strain because D is dominant over d.

These F1 plants produce two kinds of gametes, D and d, in equal proportions. Neither allele is
changed by having coexisted with the other in a heterozygous genotype; rather, they separate, or
segregate, from each other during gamete formation. This process of allele segregation is
perhaps the most important discovery that Mendel made. Upon self-fertilization, the two kinds
of gametes produced by heterozygotes can unite in all possible ways. Thus, they produce four
kinds of zygotes (we write the contribution of the egg first): DD, Dd, dD, and dd. However,
because of dominance, three of these genotypes have the same phenotype. Thus, in the next
generation, called the F2, the plants are either tall or dwarf, in a ratio of 3:1.
Mendel took this analysis one step further. The F2 plants were self-fertilized to produce an F3.
All the dwarf F2 plants produced only dwarf offspring, demonstrating that they were
homozygous for the d allele, but the tall F2 plants comprised two categories. Approximately
one-third of them produced only tall offspring whereas the other two-thirds produced a
mixture of tall and dwarf offspring. Mendel concluded that the third that were true-breeding
were DD homozygotes and that the two-thirds that were segregating were Dd heterozygotes.
These proportions, 1/3 and 2/3, were exactly what his analysis predicted because, among the
tall F2 plants, the DD and Dd genotypes occur in a ratio of 1:2.

Why do the two phenotypes appear in a 3 : 1 ratio in the F2 generation?

The chance that two independent events occur together—like a sperm from an Rr
male carrying an r allele (event 1) fertilizing an egg with an r allele that is
produced by an Rr female (event 2)—is given by the product of the probabilities
(chances) of each event. In this case, this is ½ * ½, or ¼.
The Principle of Independent Assortment: The alleles of different genes segregate, or as
we sometimes say, assort, independently of each other. This principle is another rule of genetic
transmission.

This analysis is predicated on two assumptions:

(1) That each gene segregates its alleles, and
(2) That these segregations are independent of each other. The second assumption implies that there
is no connection or linkage between the segregation events of the two or more genes.

For example, a gamete that receives W through the segregation of the texture gene is just as likely
to receive G as it is to receive g through the segregation of the color gene.

The law of independent assortment, is that the alleles of a gene pair located on one pair of
chromosomes are inherited independently of the alleles of a gene pair located on
another chromosome pair and that the sex cells containing various assortments of these genes fuse
at random with the sex cells produced by the other parent.
 Mutation theory of evolution : Hugo de vries
1. --Lamarck or Darwin is based on the population study but both theories could not be
able to explain the origin of variation and their mode of transmission from one
generation to next generation.
2. --Hugo de Vries (1848-1935) - Dutch botanist, put forward mutation theory in 1901.
3. --He proposed his views regarding “the formation of new species”. He proposed the
concept of Genes.
4. --Mutation is sudden change occurring in the genetic material
5. --Rediscovered the laws of heredity (unaware of Mendel's work)
6. --New species are not formed by continuous variations but by sudden appearance of
variations, which he named as mutations.
7. --Hugo de Vries stated that mutations are heritable and persist in successive
generations.
The mutation theory
1. The new species arise from the pre-existing one in a single generation by sudden
appearance of new features through a genetic variation known as mutations. Mutations
are ultimate source of variation.
2. Contrast to earlier theories (Lamarck or Darwin (species evolved through the gradual
accumulation of variation over vast epochs)), De Vries proposed that evolution is a
sudden, discontinuous and jerky process rather than continuous and gradual.
3. He termed the process as Saltation ie single step large mutation.
4. In addition, natural selection works on mutation, preserve the mutations found useful
and eliminates the mutants with harmful mutation. But he didn’t support the struggle
of co-existenance between the mutated organism and the parent species (non-
mutated species).
EXPERIMENT:
1. Conducted experiment on plant called as Evening Primrose
(Oenothera lamarckiana).

2. He took normal plant and collected the seeds through self pollination.

3. F1: when seed were allowed to grow, majority of plants were

normal like their parents except few one. These plants were quite different from there parents. (significantly in
leaf traits and plant overall size)

4. F2: Different plants were also self-pollinated and when their seeds were sown. In 2nd generation the majority
of the plants were similar to the parents while a few were still more different. This continued generation after
generation.

5. Hence, he observed that in each generation, majority of the offsprings are similar to their parents but it gives
few off springs with variants. In addition, he found that mutations appeared suddenly and were inherited by
offsprings.
De Vries found 4 different types of plants:
1. Progressive, with newer traits.

2. Retrogressive, that had reduced or lost traits compared to their parents

3. Degressive, plants with weak with low survival.

4. Inconstant, these plants are unstable and they resembles parents as well as at times produced
variants.
Salient features
1. Mutations or discontinuous variations are the raw material of evolution.
2. Mutations appear all of a sudden. They become operational immediately.
3. Unlike Darwin’s continuous variations or fluctuations, mutations do not revolve
around the mean or normal character of the species.
4. The same type of mutations can appear in a number of individuals of a species.
5. All mutations are inheritable.
6. Mutations appear in all conceivable directions.
7. Useful mutations are selected by nature. Lethal mutations are eliminated. However,
useless and less harmful ones can persist in the progeny.
8. Accumulation of variations produce new species. Sometimes a new species is pro­
duced from a single mutation.
Evidence for the theory: There are experimental evidences to support
the Huge De Vries mutation theory
Reproduced: The experiment performed by De Vries was reproduced by several other scientist and they
came to similar conclusions.
Mutations found spontaneously in nature and these sudden appearance of mutant varities has strengthen
the mutation theory. These evidences found in nature are as follows:
1. A). The Ancon sheep was produced from an ordinary sheep in single step in 1891. The mutated
sheep was short height and it was useful for farmers as they could not be able to jump from low
stone fences.
2. B). Hornless or polled Hereford cattle were produced in single step in 1889 from normal parents.
3. C). The hairless cats, dog and mice were produced from normal parents in single step.
4. D). Oenothera lamarckiana has 14 chromosomes but the mutants were having 16,20,22,24,28 and 30
chromosomes.
Mutations are genetically linked and inheritable. A single large mutation can produce new species in
plant. For example, Delicious apple.
Evidence against the theory: There are evidences against the mutation theory proposed by
Huge De Vries. These objections are as follows:

The mutation are of rare occurrence. Hence it is difficult to assume that if all animal or plant
species could appear solely by mutation.

Mutations are recessive where as it is generally the dominant mutations that bring about
evolution.

This theory does not explain the role of nature. The relationship between flower and the insect
(such as length of proboscis in insect and position of nectar in flower) can not be explained by
mutation theory.

Significance of Hugo de Vries’ Mutation Theory:

This Theory gives direct attention to the mutations. But later on it was thought that evolution
cannot occur by mutations alone. Natural selection and isolation of mutants were also essential
for evolution.
 Neo Darwinism
Darwin developed the concept of Darwinism elucidating the origin of new speciation. As per him, in
individuals, accumulation of fittest phylogenetic variations is the major driving force of speciation, hence
the natural selection is the survival of the fittest in the environment.
Neo Darwinism is a modified theory of Darwinism explaining the origin of species on a genetic basis, hence
the main driving force of Neo Darwinism is genetic variation. Consequently, the main difference lies in the
variation type and type of natural selection.
Darwinism
1. Darwinism, it is the theory of biological evolution
2. It explains that species tend to develop via some natural selection of suitable phenotypic variations
which can be inherited. This expands the ability of an individual to survive, compete and also to
reproduce.
3. Fundamental views of Darwinism are – entities reproduce more which exceeds the extent of individuals
capable of surviving. Such individuals possess phenotypic variations that are inheritable. Amongst these
various phenotypes, few are favorable to the conditions compared to others. Fittest phenotypes gather
Neo-Darwinism
1. This is an altered explanation of Darwin’s theory with regards to modern synthesis of natural
selection and Mendelian genetics.
2. The main force driving speciation is the gathering of genotypic variations in a gene pool
3. Sometimes this theory is also referred to as the Modern or synthetic theory of natural selection.
4. Reproductive isolation has a major part in speciation allowing differential amplification of the fittest
genes in a gene pool. Thus, natural selection of the most suitable genes is associated with the
origination of new species.
 Five Kingdom Classification:
1. Very early on, scientists began grouping the living organisms under different categories.
2. Some biologists (Greek philosopher Aristotle) classified organisms into plants and animals.
3. Amongst these, the Five Kingdom Classification proposed by Robert Whittaker stood out and is
widely used.
4. In 1969, Robert H. Whittaker proposed a Five- Kingdom System of Classification, in which all
organisms are placed into five kingdoms.
5. This process of classification of living organism is known as taxonomy.
6. Closely related organisms (i.e., organisms having similar characteristics) are placed into the same
taxon.
7. Classification is the arrangement of organisms into taxonomic groups known as taxa on the basis of
similarities or relationships.
8. Five kingdoms: monera (the prokaryotes): protista (the single-celled eukaryotes); fungi (fungus and
related organisms); plantae (the plants); animalia (the animals).
Classification basis for Five Kingdom Classification
Characteristics of the five kingdoms of living things all the species in a particular kingdom have similar
characteristics in terms of their growth and the way they function. The organisms are divided into the
five kingdoms based on their general features like:
Nutrition. autotrophic (makes its own food) or heterotrophic (feeds on other living things), saprophytic
(feeds on dead and decaying ). the plantae kingdom consists of autotrophs, i.e., they make their food.
fungi and animalia kingdom consist of heterotrophs, i.e., organisms under this kingdom depend on
others for food. monera and protista consist of both- autotrophs and heterotrophs.

Cell organisation. unicellular (having only one cell) or multicellular (having two or more cells).

Cell type. eukaryotes (the genetic material is surrounded by a membrane) or prokaryotes (lacking a
membrane). . the kingdom monera is the only kingdom that consists of prokaryotes, as the other four
kingdoms have eukaryotic organisms.
Cell wall: the kingdoms monera, fungi, and plantae consist of organisms that have cell walls in
their cells. some of the protists also have cell walls. but, the cells of the organisms of the animalia
kingdom do not have cell walls.

Nuclear membrane: the organisms under the protists kingdom have cells without a nuclear
membrane, while the others have a nuclear membrane.

Respiration. aerobic (needs oxygen) or anaerobic (does not use oxygen).

Reproduction. sexual, asexual or through spores.

The kingdoms include:

1. Bacteria and archaea are in the kingdom prokaryotae (or monera)

2. Algae and protozoa are in the kingdom protista (organisms in this
kingdom are referred to as protists)
3. Fungi are in the kingdom fungi
4. Plants are in the kingdom plantae
5. Animals are in the kingdom animalia
Kingdom Monera

 This is the kingdom of microscopic living things and groups together the prokaryotes-
Bacteria belong to kingdom monera (archaea and bacteria).
 Made up of single-cell things with no defined nucleus and also lack cell organelles.
 Some organisms show the presence of cell wall while there are others without a cell wall.
Consequently, some organisms are autotrophic and others are heterotrophic.
 Most bacteria are aerobic and heterotrophic, while the archaea are usually anaerobic and
their metabolism is chemosynthetic.
 This group is present in all habitats. They can be able to grow from soil to extreme
conditions such as hot springs, snow and deep ocean.
The selected examples of kingdom monera are:
1. Archaebacteria:
The bacteria belongs to this group has distinct cell wall structure to allow their survival into
extreme conditions. These bacterial species live in harsh habitats such as extreme salty areas
(halophiles), hot springs (thermoacidophiles) and marshy areas (methanogens).

2. Eubacteria:
These are the true bacteria due to presence of rigid cell wall and a well defined flagellum for
motility. Several species belonging to this group contain chlorophyll a and are photosynthetic
autotrophs. In addition, few of the chemosynthetic autotrophs oxidize various inorganic
substances such as nitrates, nitrites and ammonia to generate ATP.
Different Shapes
KINGDOM PROTISTA:
 All unicellular eukaryotes belongs to kingdom Protista.

 The organisms in this group has well defined nucleus and other membrane bound
organelles
 These are the simplest forms of eukaryotes that exhibit either autotrophic or
heterotrophic mode of nutrition.
 These organisms have mixed properties of plant, fungi, animals and considered to be
responsible for linking organisms in other kingdoms.
 Protista reproduce mostly asexually and sexually with cell fusion and zygote formation.
 Some organisms have appendages such as cilia or flagella or pseudopodia to move
around.
 Some examples are Diatoms and golden-brown algae, slime moulds, Protozoans like
Amoeba, Paramecium
kingdom protista is categorized into subsequent groups:
Chrysophytes: the golden algae (desmids) and diatoms fall under this group. they are
found in marine and freshwater habitats.
Dinoflagellates: They are usually photosynthetic and marine. The colour they appear
is dependent on the key pigments in their cells; they appear red, blue, brown, green or
yellow.
Euglenoids: most of them live in freshwater habitation in motionless water. The cell
wall is absent in them, instead, there is a protein-rich layer called a pellicle.
Slime moulds: these are saprophytic. the body moves along putrefying leaves and
twigs and nourishes itself on organic material. under favourable surroundings, they
form an accumulation and were called plasmodial slime moulds.
Protozoans: they are heterotrophs and survive either as parasites or predators.
Kingdom Fungi
 Heterotrophic, and are unicellular (yeast) to muticellular (pencillium) organism Eukaryotic
organisms are grouped under Kingdom Fungi.
 They have cell walls, which are made up of a substance called Chitin.
 Their mode of nutrition is saprophytic as they use decaying organic matter as food.
 They are universally present in air, water, soli, plant and on animal body. These organisms
are infectious (candida) and noninfectious (yeast).
 In addition, they live as symbionts: lichens (algae with fungi) and mycorrhiza (plant and
fungi).
 Reproduction in fungi is mostly by vegetative method such as fragmentation, fission,
budding and asexual reproduction by spores.
 Yeast, Mushroom, Aspergillus are examples of Fungi.
 These organisms are commercially being used in fermentation to produce alchol, antibiotics,
vitamin and other products. Mushroom is used as edible fungi.
KINGDOM PLANTAE:
All multicellular green plants are part of kingdom plantae.
Their cell wall is made up of cellulose.
Mostly these organisms are photosynthetic but few insect eater plants are parasitic in
nature.
Plants reproduce through asexual vegetative propogation and sexual method. Plants
are either bisexual or unisexual with separate male and female.
Subgroups: Algae, Bryophytes, Pteridophytes, Gymnosperm and Angiosperm are the
main groups present in kingdom plant.
KINGDOM ANIMALIA:
 These organisms are heterotrophic multicellular eukaryotic without thick cell wall.
 They depends on plants for nutrition.
 They have digestive system, circulatory system and well defined sensory and
neuromotor mechanism.
 These organisms have a definitive development pattern to acquire defined shape and
size in adults.
 Reproduction in these organism is mostly by sexual mechanism with separate male and
female.
OTHER ORGANISMS: There are organisms which doesn’t fall into five kingdoms due to their
unique features. These organisms are not placed and placed into their own group separately.

These organisms are as follows:

1. Virus: These are non-nucleated and acellular organisms. Virus can be stored for year as particle
and they are considered to be false living organism. They depends on host organisms to provide
cellular machinery for multiplication. They contains genetic material either as DNA or RNA and
protein coat. Post infection, they integrate genetic material into the host genome to control the
cellular machinery of host. Virus can use bacteria, plant or animal cells as host. They are mostly
pathogenic and be responsible for disease in bacteria, plant and animals.

2. Viroids: T.O. Diener discovered these organisms which contains only free RNA. They are smaller
than virus. It doesn’t contain protein coat as found in virus.
3. Symbiotic associations are not considered in this classification system. For example, lichens are
organisms wch are formed by the symbiotic association between fungi and algae.
Taxonomic Categories: Each kingdom is a group of similar organism
but these organism are further dividied into different taxonomic
categories
Kingdom →Phylum → Class → Order → Family → Genus →
Species
Taxonomic Hierarchy
 Species: Organisms sharing a set of biological traits and reproducing
only their exact kind. Species is the fundamental unit in taxonomy
 Genus: The genus are the closely related species.
 Family: The family are the closely related genera.
 Order: The order are the closely related families.
 Class: The class are the closely related order.
 Division: The division are the related classes.
 Kingdom: The most basic category of organisms is called a kingdom.