BIOLOGY CLASS XI & XII
GLOSSARY OF WORDS
CHAPTER : 10
BIOLOGY
MICROBES IN HUMAN WELFARE
MICROBES: Organisms which are too
small to be seen with naked eyes.
INOCULUM: Cells added to start
bacterial culture. Or Biological material injected in human to induce or
increase immunity to a disease.
BEVERAGE: A drink other than water.
ALCOHOLIC BEVERAGE: A drink
containing alcohol.
FERMENTATION: The chemical breakdown
of a substance by bacteria or yeast or any other microorganisms, typically
involving effervescence & giving off of heat. The process involved in
alcoholic beverage formation.
ANTIBIOTICS: Substances which are
used to kill or suppress the growth of other disease causing microbes.
BROAD SPECTRUM ANTIBIOTIC: Antibiotics
effective against wide range of bacteria or microbes.
BACILLUS: Rod like in shape.
COCCUS: Spherical in shape.
ENZYME: Enzyme is a substance which
act as catalyst in living organisms, regulating the rate of reaction without being used up or
consumed.
CHEESE: A food made from pressed curds
of milk, firm and elastic or soft and semiliquid in texture.
TODDY: Fresh or fermented sap of palm
used as drink.
IMMUNOSUPPRESSIVE: A substance which
suppresses the immune system.
CLOTBLUSTER: A drug such as
streptokinase used to dissolve the clot.
ORGANIC ACIDS: Acids of plant or
animal origin like citric acid etc.
SEWAGE: Waste water.
FLOCS: Masses of bacteria associated
with fungal filament to form a mesh like structure called flocs. Flocs are in
mud composition.
METHANOGEN: Collective name for the
bacteria which act on cellulose to produce Methane gas along with CO2 and H2.
BIOFERTILIZERS: Biofertilizers are
substances used enrich the nutrient quality of soil.
MYCORRHIZA : Mycorrhiza
is fungal symbiotic association with plants which help in enriching the
nutrient content of soil.
Bt-TOXIN: A crystalline poisonous
substance released by Bacillus thuringiensis bacteria which kills the insects.
Following is the link of video on Microbes In Human Welfare
https://youtu.be/jz4z6lw6YKM
https://youtu.be/jz4z6lw6YKM
video on MICROBES IN HUMAN WELFARE
Following are the 11 links for PPT of UNIT -II, Class XII Biology:-
PRINCIPLES OF INHERITANCE (1).pptx
Evidence_of_Evolution (1).pptx
Hardy Weinberg Equilibrium.pptx
HOMOLOGOUS AND ANALOGOUS ORGANS^.pptx
THEORIES OF ORIGIN OF LIFE.pptx
Following is the notes of PRINCIPLES OF INHERITANCE , BIOLOGY, CLASS XII
BASICS OF BIOLOGY
BY- G. K. SINGH
PGT (BIOLOGY)
UNIT- VI
CLASS: XII
CHAPTER-5
PRINCIPLES
OF INHERITANCE AND VARIATION
G.J.Mendel is known as
father of genetics.
Mendel was born on 22nd
July 1822.
He died in 1884 till his death his work remain urecognised.
Later 16 years after
his death his work was recognised by
Hugo De Vres , Correns and Tscermak.
Q.Mendel’s work was
unrecognized for long period because:
i)
Because it could not be published in any international
general.
ii)
Because biologist did not accept his work.
iii)
Factors which according to Mendel were responsible for
inheritance of character from next generation to next was not understood how
they can pass.
Q.WHY
DID MENDEL SELECTED PEA PLANT FOR HIS STUDIES OF INHERITANCE?
Mendel
Selected pea palnt (Pisum sativum) for his studies of inheritance.
Because:
i)Pea
plant has short life cycle.
ii)They
have 7 pairs of physically distinct contrasting character.
iii)They
were easy to grow.
iv)They
can be both self as well as cross pollinated.
v)Pea
seeds do not show dormancy.
vi)Pea
plants produce large number of seeds.
Q.WHY
WAS MENDEL SUCCESSFUL IN HIS WORK:
i)
Selection of pea plant which has seven pairs of
contrasting characters.
ii)
Pea plant produce large no. of seeds so he got large
sample size for his studies.
iii)
Pea plant has short life span so he could study many
generation.
iv)
He analysed the data properly mathematically as he was
a mathematician.
v)
No gene of pea plant were linked.
Q..
WHAT WERE THE SEVEN PAIRS OF CONTRASTING CHARACTERS IN PES?
Seven
pairs of contrasting characters in pea are:
CHARACTER DOMINANT RECESSIVE
i)Plant Height Tall Dwarf
ii)Flower Position Axial
Terminal.
iii)Flower colour Purple White
iv)Pod shape
Inflated
Constricted
v)Pod colour Green Yellow
vi)Seed shape Round Wrinkled
vii)Seed colour Yellow Green.
GENE
the term Gene was given by Johannsen.
Gene is the functional unit of
inheritance.
Genes
were called Factor by the Mendel.
Alleles:
Alleles are the factors which code
for a pair of contrasting character.
Alleles have two forms:
i)
DOMINANT: The form which expresses in Heterozygous
condition. Dominant allele is always written in capital letter.
ii)
RECESSIVET: The form which can not express in
heterozygous condition. Recessive allele is always written in small letter.
Homozygous:
If both the alleles of a character
are of the same type.
Eg. (TT) dominant or (tt) Recessive.
Heterozygous:
If both the alleles of a character
are of different types.
Eg.
(Tt)
Phenotype:
Externally visible character.
Eg.
Tall or dwarf.
Genotype:
Genetic makeup of an organism.
Eg.
Homozygous or Heterozygous /dominant or recessive
I,e.
(Tt) , (TT), (tt)
Monohybrid
cross:
Cross for only one pair of
contrasting character .
Eg.Plant
height (short or Tall),seed colour or seed shape etc.
T T x t t
T T t t in F1 PR = GR = 4:0
 |
Tt Tt Tt Tt------ F-1 all tall , all
Heterozygous.
On SELFING THE F-1 INDIVIDUALS
Tt x Tt
T t T t
 |
TT Tt Tt tt F2
In
F2 Phenotype is 3 tall and one dwarf but
Genotype
is 1 Homozygous tall(TT), 2 Heterozygous tall (Tt) and 1 Recessive(tt)
In
F2
PR = 3:1
F2
GR = 1:2:1
In monohybrid number of gametes
produced = 2
 |
Dihybrid
cross:
Cross for two pairs of contrasting
characters.
Eg.
Seed colour and seed shape together.
Tall palnt with Axial flower Dwarf plant with terminal flower
TTAA X ttaa
TA (4 Gamete) ta (
all 4 gamete)
F1 TtAa ( all
16 F1 plants Heterozygous, tall and axial )
F1 PR = GR = 16:0
ON SELFING F1 GENERATION
TtAa X TtAa
Four gametes formed are TA, Ta,
tA, ta
|
|
TA |
Ta |
tA |
ta |
|
TA |
TTAA |
TTAa |
TtAA |
TtAa |
|
Ta |
TTAa |
TTaa |
TtAa |
Ttaa |
|
tA |
TtAA |
TtAa |
ttAA |
ttAa |
|
ta |
TtAa |
Ttaa |
ttAa |
ttaa |
Tall
& Axial = 9
Tall & Terminal = 3
Dwarf and Axial = 3
Dwarf and Terminal = 1
So in F2 of dihybrid cross PR= 9:3:3:1
F2
of dihybrid cross Gr=1:2:1:2:4:2:1:2:1
Trihybrid Cross:
TTAAPP
X ttaapp
Dominant
Character:
The character which is expressed in
heterozygous condition is called Dominant character. or the character which
expresses in F1 generation of a monohybrid cross.
Recessive
Character:
The character which is not expressed
in heterozygous condition. Or the character which fails to express itself in F1
generation of a monohybrid cross.
FILIAL: Generation
SELFING:CROSSING TWO INDIVIDUALS OF THE SAME GENERATION
Phenotypic
ratio of F2 generation of a monohybrid cross is 3:1(Tall:dwarf)
Genotypic
ratio of F2 Generation of a monohybrid cross is 1:2:1
(Homozygous
tall: Heterozygous tall: Dwarf )
Test
Cross:
The
cross of any individual with homozygous recessive individual for the trait is
known as test cross .
Test
cross helps us to know whether an individual is homozygous or heterozygous for
the trait.
If
the result of test cross is all dominant character than the individual must be
Homozygous for the trait but if the result is 50% dominant and 50% recessive it
must be Heterozygous..
Back
Cross:
Crossing any F1 individual with any
one of the parent is called back cross.
Back
cross is done to for breed improvement in both plants and animals.
ON THE
BASIS OF MONOHYBRID CROSS MENDEL GAVE TWO LAWS OF INHERITANCE:
I)LAW OF
DOMINANCE.
II)LAW OF
SEGREGATION .
Law of
Dominance and Law of segregation can be explained by both Monohybrid as well as
Dihybrid corss
ON
THE BASIS OF DIHYBRID CROSS MENDEL GAVE
I)LAW
OF INDEPENDENT ASSORTMENT.
Law
of Independent assortment can only be explained by Dihybrid cross.
1 INCOMPLETE DOMINANCE
When experiments on peas were repeated sometimes the F1 had a phenotype
that did not resemble either of the two parents and was in between the two. The
inheritance of flower colour in the dog flower (snapdragon or Antirrhinum sp.)
is a good example to understand incomplete dominance. In a cross between
true-breeding red-flowered (RR) and truebreeding white-flowered plants (rr),
the F1 (Rr) was pink . When the F1 was self-pollinated the F2 resulted in the
following ratio 1 (RR) Red: 2 (Rr) Pink: 1 (rr) White. Here the genotype ratios
were exactly as we would expect in any mendelian monohybrid cross, but the
phenotype ratios had changed from the 3:1 dominant : recessive ratio. What
happened was that R was not completely dominant over r and this made it
possible to distinguish Rr as pink from RR (red) and rr (white) .
INCOMPLETE DOMINANCE:
Inheritance of flower colour in Dog
flower (snap dragon or antirrhinum) shows beautiful example of incomplete
dominance. When red coloured (RR) Flower were crossed with white (rr) coloured
flowersthe F1 progenys were all Pink (Rr) but
F2 progenys were
1 red (RR) :2
Pink (Rr) :1 white (rr)
In the F1
progeny none of the alleles were fully dominant or able to suppress the other
and an intermediate pink colour appeared.
2. CO-DOMINANCE
in the case of co-dominance the F1 generation resembles both parents. A
good example is different types of red blood cells that determine ABO blood
grouping in human beings.
ABO blood groups are controlled by
the gene I. The plasma membrane of the red blood cells has sugar polymers that
protrude from its surface and the kind of sugar is controlled by the gene. The
gene (I) has three alleles I A , I B and i. The alleles I A and
I B produce a slightly different form of the sugar while allele i does not
produce any sugar. Because humans are diploid
organisms, each person possesses any two of the three I gene alleles. I A and I
B are completely dominant over i, in other words when I A and i are present
only I A expresses (because i does not produce any sugar), and when I B and i
are present I B expresses. But when I A and I B are present together they both
express their own types of sugars: this is because of co-dominance. Hence red
blood cells have both A and B types of sugars.
Since there are three different
alleles, there are six different combinations of these three alleles that are
possible, and therefore, a total of six different genotypes of the human ABO
blood types .
MULTIPLE ALLELISM
-We realise that the example of ABO blood
grouping also provides a good example of multiple alleles? Here you can see
that there are more than two, i.e., three alleles, governing the same
character. Since in an individual only two alleles can be present, multiple
alleles can be found only when population studies are made.
ATAVISM:
If an ancestral
trait reappears after having being disappeared or lost in few earlier
generation the phenomenon is called Atavism.
CHROMOSOMAL
THEORY OF INHERITANCE :
Chromosomal Theory
Of Inheritance was an extension of MENDEL’S laws and it was proposed by Walter Sutton and Theodore Boveri.
In 1865 Mendels
work was published but remain unrecognized .
|
|
Mendels Theory |
Sutton and Boveri |
|
1 |
Factors are in pairs . |
chromosomes
which have these genes are also in homologouspairs |
|
2 |
Factors separate at the time Of gamete formation |
Chromosomes
of homologous pair separate during gamete Formation |
|
3 |
These factors or alleles assort Independently. |
Chromosomes
also assort independently. |
T. H.
Morgan gave the experimental proof of the chromosomal theory of inheritance.
Morgan
worked with the tiny fruit fly Drosophila Melanogaster.
LINKAGE:
Physical association of genes on a chromosome.Closer the genes on a
chromosome the more they will be linked i,e. the less will be the chances of
their recombination or separation.
Recombination frequency is more if the genes located away from each
other.
LINKAGE
AND RECOMBINATION
Morgan carried out several dihybrid crosses in Drosophila to study genes
that were sex-linked. When Morgan
hybridised yellow-bodied, white-eyed females to brown-bodied, red-eyed males
and intercrossed their F1 progeny. He observed that the two genes did not
segregate independently of each other and the F2 ratio deviated very
significantly from the 9:3:3:1 ratio .
Morgan and his group knew that the genes were located on the X
chromosome and saw quickly that when the
two genes in a dihybrid cross were situated on the same chromosome, the
proportion of parental gene combinations were much higher than the non-parental
type. Morgan attributed this due to the physical association or linkage of the
two genes. and coined
The term linkage to describe this physical association
of genes on a chromosome and the term recombination to describe the generation
of non-parental gene combinations .
Morgan and his group also found that even when genes were grouped on the
same chromosome, some genes were very tightly linked (showed very low
recombination) while others were loosely
linked (showed higher recombination) .
For example he found that the genes
white and yellow were very tightly linked and showed only 1.3 per cent
recombination while white and miniature wing showed 37.2 per cent
recombination.
Mendel’s student Alfred Sturtevant used the frequency of
recombination between gene pairs on the same chromosome as a measure of the
distance between genes and ‘mapped’ their position on the chromosome.
COMPLETE
LINKAGE :
If the genes are located very close to each other they are said to be
completely linked and chances of recombination is less. Complete linked genes
are so close that there is no chance of their separation because crossing over
is not possible.
INCOMPLETE
LINKAGE:
If the genes which are not so close
they may get separated in next generation they are supposed to be incompletely
linked.
SIGNIFICANCE
OF LINKED GENES IS THAT THEY ARE USED AS MARKERS.
Linkage
group is same as no of chromosomes in one set.
In human
the no. of chromosomes is n=23 so there are 23 linkage groups.
In
Drosophila n=4 so linkage group is 4
GENE
MAPPING:
Recombination Frequency:
Recombination
frequency is directly proportional to map unit.
Map Unit:
If gene A and B are
10 map unit apart than crossing over frequency will be 10%.
Polygenic traits:
If a character is
controlled by three or more genes iit is known as Polygenic traits .
e.g .Human skin
colour . The effect of each allele is additive.
PLEIOTROPY
Occasionally, a single gene product may produce more than one effect.
For
example, starch synthesis in pea seeds is controlled by one gene. It has two
alleles (B and b).
Dominance
is not an autonomous feature of a gene or the product that it has information
for. It depends as much on the gene product and the production of a particular
phenotype from this product as it does on the particular phenotype that we
choose to examine, in case more than one phenotype is influenced by the same
gene.
PLEIOTROPY
:
The phenomenon in which a single
gene exhibit more than one phenotypic expression .
e.g .Disease
Phenylketonuria.
SEX
DETERMINATION:
Human beings have 22pair of autosomes and 1(23rd
pair) pair of sex chromosomes.XX is female And XY is male. So males are
Heterogamety.
IN BIRDS
ZZ is male and ZW is female. So females
are heterogamety.
SEX
DETERMINATION IN HONEY BEES.
In honey bees sex determination
is based on the number of sets of chromosomes.
An individual developed
from fertilized egg is female (queen or worker) are diploid 2n=32
An individual developed
from an unfertilized egg by parthenogenesis is male with haploid n=16
chromosomes.
POLYGENIC INHERITANCE
Mendel’s studies mainly described those traits that have distinct
alternate forms such as flower colour which are either purple or white. But if
you look around you will find that there are many traits which are not so
distinct in their occurrence and are spread across a gradient.
Example, in humans
we don’t just have tall or short people as two distinct alternatives but a whole
range of possible heights. Such traits are generally controlled by three or
more genes and are thus called as polygenic traits. Besides the involvement of
multiple genes polygenic inheritance also takes into account the influence of
environment. Human skin colour is another classic example for this.
In a polygenic trait the phenotype reflects the
contribution of each allele, i.e., the effect of each allele is additive.
To understand this better let us
assume that three genes A, B, C control skin colour in human with the dominant
forms A, B and C responsible for dark skin colour and the recessive forms a, b
and c for light skin colour. The genotype with all the dominant alleles
(AABBCC) will have the darkest skin colour and that with all the recessive
alleles (aabbcc) will have the lightest skin colour.
As expected the genotype with three dominant alleles and three recessive
alleles will have an intermediate skin colour. In this manner the number of
each type of alleles in the genotype would determine the darkness or lightness
of the skin in an individual.
PLEIOTROPY
A single gene can exhibit multiple phenotypic expression. Such a gene is
called a pleiotropic gene and the phenomenon is called Pleiotropy.
The underlying mechanism of
pleiotropy in most cases is the effect of a gene on metabolic pathways which
contribute towards I phenotypes.
An example of
this is the disease phenylketonuria, which occurs in humans. The disease is
caused by mutation in the gene that codes for the enzyme phenyl alanine
hydroxylase (single gene mutation). This manifests itself through phenotypic
expression characterised by mental retardation and a reduction in hair and skin
pigmentation.
MENDELIAN DISORDERS
Broadly, genetic disorders may be grouped into two categories –
(1)Mendelian disorders and
(2) Chromosomal disorders.
Mendelian disorders are mainly
determined by alteration or mutation in the single gene. These disorders are
transmitted to the offspring on the same lines as we have studied in the
principle of inheritance. The pattern of inheritance of such Mendelian disorders
can be traced in a family by the pedigree analysis.
Most common and
prevalent Mendelian disorders are Haemophilia, Cystic fibrosis, Sicklecell
anaemia, Colour blindness, Phenylketonuria, Thalassemia, etc.
These Mndelian disorders may be
dominant or recessive.
By pedigree analysis one can easily understand whether the trait in
question is dominant or recessive. Similarly, the trait may also be linked to
the sex chromosome as in case of haemophilia. It is evident that this X-linked
recessive trait shows transmission from carrier female to male progeny
COLOUR BLINDNESS :
It is a sex-linked recessive
disorder due to defect in either red or green cone of eye resulting in failure
to discriminate between red and green colour. This defect is due to mutation in
certain genes present in the X chromosome. It occurs in about 8 per cent of
males and only about 0.4 per cent of females. This is because the genes that
lead to red-green colour blindness are on the X chromosome. Males have only one
X chromosome and females have two. The son of a woman who carries
the gene has a 50 per cent chance of being colour blind. The mother is
not herself colour blind because the gene is recessive. That means that its
effect is suppressed by her matching dominant normal gene.
A daughter will
not normally be colour blind, unless her mother is a carrier and her father is
colour blind.
HAEMOPHILIA :
This sex linked recessive disease, which shows its transmission from
unaffected carrier female to some of the male progeny has been widely studied. In
this disease, a single protein that is a part of the cascade of proteins
involved in the clotting of blood is affected. Due to this, in an
affected individual a simple cut will result in non-stop bleeding.
The heterozygous female (carrier)
for haemophilia may transmit the disease to sons.
The possibility
of a female becoming a haemophilic is extremely rare because mother of such a
female has to be at least carrier and the father should be haemophilic (unviable
in the later stage of life).
The family pedigree of Queen Victoria shows a number of haemophilic
descendents as she was a carrier of the disease.
SICKLE-CELL ANAEMIA :
This is an autosome linked
recessive trait that can be transmitted from parents to the offspring when both
the partners are carrier for the gene (or heterozygous).
The disease is controlled by a single pair of allele,
HbA and HbS . Out of the three possible genotypes only homozygous individuals
for HbS (HbSHbS ) show the diseased phenotype.
Heterozygous (HbAHbS ) individuals appear apparently unaffected but they
are carrier of the disease as there is 50 per cent probability of transmission
of the mutant gene to the progeny, thus exhibiting sickle-cell trait .
The defect is caused by the substitution of Glutamic
acid by Valine At 6th position of β chain of haemoglobin.
CHROMOSOMAL DISORDERS
The chromosomal disorders on the other hand are caused due to absence or
excess or abnormal arrangement of one or more chromosomes. Failure of
segregation of chromatids during cell division cycle results in the gain or
loss of a chromosome(s), called aneuploidy.
For example,
Down’s syndrome results in the gain of extra copy of chromosome 21.
Turner’s syndrome results due to
loss of an X chromosome in human females.
POLYPLOIDY
Failure of cytokinesis after
telophase stage of cell division results in an increase in a whole set of
chromosomes in an organism and, this phenomenon is known as polyploidy. This
condition is often seen in plants.
The total number of chromosomes in a normal human cell is 46 (23 pairs).
Out of these 22 pairs are autosomes and one pair of chromosomes are sex
chromosome.
TRISOMY AND MONOSOMY
Sometimes, though rarely, either an additional copy of a chromosome may
be included in an individual or an individual may lack one of any one pair of
chromosomes. These situations are known as trisomy or monosomy of a chromosome,
respectively.
Such a situation leads to very
serious consequences in the individual.
Down’s syndrome,
Turner’s syndrome, Klinefelter’s syndrome are common examples of chromosomal
disorders.
DOWN’S
SYNDROME :
The cause of this genetic disorder is the presence of an additional copy
of the chromosome number 21 (trisomy of 21). This disorder was first described
by Langdon Down (1866).
EFFECTS OF DOWN’S SYNDROME
The affected individual is short
statured with small round head, furrowed tongue and partially open mouth. Palm
is broad with characteristic palm crease. Physical, psychomotor and mental
development is retarded.
KLINEFELTER’S SYNDROME :
This genetic disorder is also caused due to the presence of an additional
copy of Xchromosome resulting into a karyotype of 47, XXY. Such an individual
has overall masculine development, however, the feminine development
(development of breast, i.e., Gynaecomastia) is also expressed (Figure 5.17 a).
Such individuals are sterile.
TURNER’S
SYNDROME :
Such a disorder is caused due to the absence of one of the X chromosomes,
i.e., 45 with X0, Such females are sterile as ovaries are rudimentary besides
other features including lack of other secondary sexual characters .
CHAPTER 7
EVOLUTION
ORIGIN OF LIFE
Stellar distances are
measured in light years. What we see today is an object whose emitted light
started its journey millions of year back and from trillions of kilometres away
and reaching our eyes now. However, when we see objects in our immediate surroundings
we see them instantly and hence in the present time.
UNIVERSE
The origin of life is
considered a unique event in the history of universe. The universe is vast. The
universe is very old – almost 20 billion years old. Huge clusters of galaxies
comprise the universe. Galaxies contain stars and clouds of gas and dust.
Considering the size of universe, earth is indeed a speck.
THE BIG BANG THEORY
The Big Bang theory attempts to explain the
origin of universe. It talks of a singular huge explosion unimaginable in
physical terms. The universe expanded and hence, the temperature came down.
Hydrogen and Helium formed sometime later. The gases condensed under
gravitation and formed the galaxies of the present day universe.
In the solar system of the
milky way galaxy, earth was supposed to have been formed about 4.5 billion
years back. There was no atmosphere on early earth. Water vapour, methane,
carbondioxide and ammonia released from molten mass covered the surface. The UV
rays from the sun broke up water into Hydrogen and Oxygen and the lighter H2
escaped. Oxygen combined with ammonia and methane to form water, CO2 and
others. The ozone layer was formed. As it cooled, the water vapor fell as rain,
to fill all the depressions and form oceans. Life appeared 500 million years
after the formation of earth, i.e., almost four billion years back.
DID LIFE COME FROM OUTERSPACE?
THEORIES OF ORIGIN OF
LIFE
(1)THEORY
OF PANSPERMIA
(1)Some scientists believe
that it came from outside. Early Greek thinkers thought units of life called
spores were transferred to different planets including earth. ‘Panspermia’ is
still a favourite idea for some astronomers.
(2)THEORY
OF SPONTANEOUS GENERATION.
(2)it
was also believed that life came out of decaying and rotting matter like straw,
mud, etc. This was the theory of spontaneous generation.
(3)THEORY
OF BIOGENESIS
(3)Louis
Pasteur by careful experimentation demonstrated that life comes only from
pre-existing life. He showed that in pre-sterilised flasks, life did not come
from killed yeast while in another flask open to air, new living organisms
arose from ‘killed yeast’. Spontaneous generation theory was dismissed once and
for all.
(4) THEORY OF CHEMICAL
EVOLUTIONOF LIFE
Oparin of Russia and Haldane of England
proposed that the first form of life could have come from pre-existing
non-living organic molecules (e.g. RNA, protein, etc.) and that formation of
life was preceded by chemical evolution, i.e., formation of diverse organic
molecules from inorganic constituents. The conditions on earth were – high
temperature, volcanic storms, reducing atmosphere containing CH4 , NH3 , etc.
S.L. MILLER’S
EXPERIMENT FOR CHEMICAL ORIGIN OF LIFE
In
1953, S.L. Miller, an American scientist created similar conditions in a
laboratory scale (Figure 7.1). He created electric discharge in a closed flask
containing CH4 , H2 , NH3 and water vapour at 8000C. He observed formation of
amino acids. In similar experiments others observed, formation of sugars,
nitrogen bases, pigment and fats. Analysis of meteorite content also revealed
similar compounds indicating that similar processes are occurring elsewhere in
space. With this limited evidence, the first part of the conjectured story,
i.e., chemical evolution was more or less accepted. The first non-cellular
forms of life could have originated 3 billion years back. They would have been
giant molecules (RNA, Protein
Polysaccharides, etc.).
These capsules reproduced their molecules perhaps. The first cellular form of
life did not possibly originate till about 2000 million years ago. These were
probably single-cells. All life forms were in water environment only. This
version of a biogenesis, i.e., the first form of life arose slowly through
evolutionary forces from non-living molecules is accepted by majority.
EVOLUTION OF LIFE
FORMS – A THEORY
(5) THE THEORY OF
SPECIAL CREATION.
Conventional religious literature
tells us about the theory of special creation. This theory has three
connotations. One, that
(1) That all living organisms (species or
types) that we see today were created as such by the supreme power God
(2) That the diversity was always the same since
creation and will be the same in future also.
(3) That earth is about 4000 years old.
All these ideas were strongly challenged
during the nineteenth century. Based on observations made during a sea voyage
in a sail ship called H.M.S. Beagle round the world, Charles
Darwin concluded that existing living forms share similarities to varying
degrees not only among themselves but also with life forms that existed
millions of years ago.
There has been
gradual evolution of life forms. Any population has built in variation in
characteristics. Those characteristics which enable some to survive better in
natural conditions (climate, food, physical factors, etc.) would outbreed
others that are less-endowed to survive under such natural conditions. Another
word used is fitness of the individual or population. The fitness, according to
Darwin, refers ultimately and only to reproductive fitness.
THEORY OF NATURAL
SELECTION
Hence, those who are
better fit in an environment, leave more progeny than others. These, therefore,
will survive more and hence are selected by nature. He called it natural
selection and implied it as a mechanism of evolution. Let us also remember that
Alfred Wallace, a naturalist who worked in Malay Archipelago had also come to
similar conclusions around the same time. In due course of time, apparently new
types of organisms are recognisable periods in the history of earth (epochs,
periods and eras). The geological history of earth
closely correlates with the biological history of earth. A
common permissible conclusion is that earth is very old, not thousand of years
as was thought earlier but billions of years old.
WHAT ARE THE EVIDENCES FOR
EVOLUTION?
Evidence that evolution of life
forms has indeed taken place on earth has come from many quarters.
PALEONTOLOGICAL
EVIDENCE.
Fossils are remains of hard parts of
life-forms found in rocks. Rocks form sediments and a cross-section of earth's
crust indicates the arrangement of sediments one over the other during the long
history of earth. Different-aged rock sediments contain fossils of different
life-forms who probably died during the formation of the particular sediment. A
study of fossils in different sedimentary layers indicates the geological
period in which they existed. The study showed that life-forms varied over time
and certain life forms are restricted to certain geological timespans. Hence,
new forms of life have arisen at different times in the history of earth. All
this is called paleontological evidence.
EMBRYOLOGICAL
EVIDENCES FOR EVOLUTION
Embryological support for
evolution was also proposed by Ernst Heckel based upon the observation of
certain features during embryonic stage common to all vertebrates that are
absent in adult. For example, the embryos of all vertebrates including human
develop a row of vestigial gill slit just behind the head but it is a
functional organ only in fish and not found in any other adult vertebrates.
However, this o proposal was disapproved on
careful study performed by Karl Ernst von Baer. He noted that embryos never
pass through the adult stages of other animals.
ANATOMICAL AND
MORPHOLOGICAL EVIDENCES.
TEY ARE OF FOLLOWING
TYPES
1. Homologous
organs.
2. Analogous
organs.
3. Vestigeal
Organs
HOMOLOGOUS
ORGANS AND DIVERGENT EVOLUTION
Whales, bats, Cheetah and
human (all mammals) share similarities in the pattern of bones of forelimbs .
Though these forelimbs perform different functions in these animals, they have
similar anatomical structure – all of them have humerus, radius, ulna, carpals,
metacarpals and phalanges in their forelimbs. Hence, in these animals, the same
structure developed along different directions due to adaptations to different
needs. This is divergent evolution and these structures are homologous.
Homology indicates common ancestry. Other examples are vertebrate hearts or
brains. In plants also, the thorn and tendrils of Bougainvillea and Cucurbita
represent homology (Figure 7.3a). Homology is based on divergent evolution
whereas analogy refers to a situation exactly opposite.
ANALOGOUS ORGANS AND
CONVERGENT EVOLUTION
Wings of butterfly and of birds look alike.
They are not anatomically similar structures though they perform similar
functions. Hence, analogous structures are a result of convergent evolution -
different structures evolving for the same function and hence having
similarity. Other examples of analogy are the eye of the octopus and of mammals
or the flippers of Penguins and Dolphins. One can say that it is the similar
habitat that has resulted in selection of similar adaptive features in
different groups of organisms but toward the same function: Sweet potato (root
modification) and potato (stem modification) is another example for analogy. In
the same line of argument, similarities in proteins and genes performing a
given function among diverse organisms give clues to common ancestry.
EVIDENCE SUPPORTING
EVOLUTION BY NATURAL SELECTION
Another interesting observation supporting
evolution by natural selection comes from England. In a collection of moths
made in 1850s, i.e., before industrialisation set in, it was observed that
there were more white-winged moths on trees than dark-winged or melanised
moths. However, in the collection carried out from the same area, but after
industrialisation, i.e., in 1920, there were more dark-winged moths in the same
area, i.e., the proportion was reversed.
The explanation put forth
for this observation was that ‘predators will spot a moth against a contrasting
background’. During postindustrialisation period, the tree trunks became dark
due to industrial smoke and soots. Under this condition the white-winged moth
did not survive due to predators, dark-winged or melanised moth survived.
Before industrialisation set in, thick growth of almost white-coloured lichen
covered the trees - in that background the white winged moth survived but the
dark-coloured moth were picked out by predators. lichens can be used
as industrial pollution indicators. They will not grow in areas that are
polluted. Hence, moths that were able to camouflage
themselves, i.e., hide in the background, survived (Figure 7.4). This
understanding is supported by the fact that in areas where industrialisation
did not occur e.g., in rural areas, the count of melanic moths was low. This
showed that in a mixed population, those that can better-adapt, survive and
increase in population size. Remember that no variant is completely wiped out.
Similarly, excess use of herbicides, pesticides, etc., has only resulted in
selection of resistant varieties in a much lesser time scale.
Evolution is not a
directed process in the sense of determinism. It is a stochastic process based
on chance events in nature and chance mutation in the organisms.
WHAT IS ADAPTIVE
RADIATION?
ADAPTIVE RADIATION
Small black birds
later called Darwin’s Finches amazed him. He realised that there were many
varieties of finches in the same island. All the varieties, he conjectured,
evolved on the island itself. From the original seed-eating features, many
other forms with altered beaks arose, enabling them to become insectivorous and
vegetarian finches (Figure 7.5). This process of evolution of different species
in a given geographical area starting from a point and literally radiating to
other areas of geography (habitats) is called adaptive radiation.
Example: Darwin’s
finches and Australian marsupials represent one of the best examples of ADAPTIVE
RADIATION
When more than one
adaptive radiation appeared to have occurred in an isolated geographical area
(representing different habitats), one can call this convergent evolution. Placental
mammals in Australia also exhibit adaptive radiation in evolving into varieties
of such placental mammals each of which appears to be ‘similar’ to a
corresponding marsupial (e.g., Placental wolf and Tasmanian wolf-marsupial).
BIOLOGICAL EVOLUTION
The essence of Darwinian theory about evolution is natural selection. The rate of appearance of new forms is
Comments
Post a Comment