NCERT Textbook- 2: Evolution Notes | Study Biology Class 12 - NEET

 Page 1


136
BIOLOGY
Figure 7.8 Diagrammatic representation of the operation of natural selection on different
traits : (a) Stabilising (b) Directional and (c) Disruptive
(a)
(b)
(c)
7.7 HARDY-WEINBERG PRINCIPLE
In a given population one can find out the frequency of occurrence of
alleles of a gene or a locus. This frequency is supposed to remain fixed
and even remain the same through generations. Hardy-Weinberg principle
stated it using algebraic equations.
This principle says that allele frequencies in a population are stable
and is constant from generation to generation. The gene pool (total genes
and their alleles in a population) remains a constant. This is called
genetic equilibrium. Sum total of all the allelic frequencies is 1. Individual
2020-21
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BIOLOGY
Figure 7.8 Diagrammatic representation of the operation of natural selection on different
traits : (a) Stabilising (b) Directional and (c) Disruptive
(a)
(b)
(c)
7.7 HARDY-WEINBERG PRINCIPLE
In a given population one can find out the frequency of occurrence of
alleles of a gene or a locus. This frequency is supposed to remain fixed
and even remain the same through generations. Hardy-Weinberg principle
stated it using algebraic equations.
This principle says that allele frequencies in a population are stable
and is constant from generation to generation. The gene pool (total genes
and their alleles in a population) remains a constant. This is called
genetic equilibrium. Sum total of all the allelic frequencies is 1. Individual
2020-21
137
EVOLUTION
frequencies, for example, can be named p, q, etc. In a diploid, p and q
represent the frequency of allele A and allele a. The frequency of AA
individuals in a population is simply p
2
.  This is simply stated in another
ways, i.e., the probability that an allele A with a frequency of p appear on
both the chromosomes of a diploid individual is simply the product
of the probabilities, i.e., p
2
. Similarly of aa is q
2
, of Aa 2pq. Hence,
p
2
+2pq+q
2
=1. This is a binomial expansion of (p+q)
2
. When frequency
measured, differs from expected values, the difference (direction) indicates
the extent of evolutionary change. Disturbance in genetic equilibrium, or
Hardy- Weinberg equilibrium, i.e., change of frequency of alleles in a
population would then be interpreted as resulting in evolution.
Five factors are known to affect Hardy-Weinberg equilibrium. These
are gene migration or gene flow, genetic drift, mutation, genetic
recombination and natural selection. When migration of a section of
population to another place and population occurs, gene frequencies
change in the original as well as in the new population. New genes/alleles
are added to the new population and these are lost from the old population.
There would be a gene flow if this gene migration, happens multiple times.
If the same change occurs by chance, it is called genetic drift. Sometimes
the change in allele frequency is so different in the new sample of population
that they become a different species. The original drifted population
becomes founders and the effect is called founder effect.
Microbial experiments show that pre-existing advantageous
mutations when selected will result in observation of new phenotypes.
Over few generations, this would result in Speciation. Natural selection is
a process in which heritable variations enabling better survival are enabled
to reproduce and leave greater number of progeny. A critical analysis
makes us believe that variation due to mutation or variation due to
recombination during gametogenesis, or due to gene flow or genetic drift
results in changed frequency of genes and alleles in future generation.
Coupled to enhance reproductive success, natural selection makes it look
like different population. Natural selection can lead to stabilisation (in
which more individuals acquire mean character value), directional change
(more individuals acquire value other than the mean character value) or
disruption (more individuals acquire peripheral character value at both
ends of the distribution curve) (Figure 7.8).
7.8 A BRIEF ACCOUNT OF EVOLUTION
About 2000 million years ago (mya) the first cellular forms of life appeared
on earth. The mechanism of how non-cellular aggregates of giant
macromolecules could evolve into cells with membranous envelop is not
known. Some of these cells had the ability to release O
2
. The reaction
2020-21
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BIOLOGY
Figure 7.8 Diagrammatic representation of the operation of natural selection on different
traits : (a) Stabilising (b) Directional and (c) Disruptive
(a)
(b)
(c)
7.7 HARDY-WEINBERG PRINCIPLE
In a given population one can find out the frequency of occurrence of
alleles of a gene or a locus. This frequency is supposed to remain fixed
and even remain the same through generations. Hardy-Weinberg principle
stated it using algebraic equations.
This principle says that allele frequencies in a population are stable
and is constant from generation to generation. The gene pool (total genes
and their alleles in a population) remains a constant. This is called
genetic equilibrium. Sum total of all the allelic frequencies is 1. Individual
2020-21
137
EVOLUTION
frequencies, for example, can be named p, q, etc. In a diploid, p and q
represent the frequency of allele A and allele a. The frequency of AA
individuals in a population is simply p
2
.  This is simply stated in another
ways, i.e., the probability that an allele A with a frequency of p appear on
both the chromosomes of a diploid individual is simply the product
of the probabilities, i.e., p
2
. Similarly of aa is q
2
, of Aa 2pq. Hence,
p
2
+2pq+q
2
=1. This is a binomial expansion of (p+q)
2
. When frequency
measured, differs from expected values, the difference (direction) indicates
the extent of evolutionary change. Disturbance in genetic equilibrium, or
Hardy- Weinberg equilibrium, i.e., change of frequency of alleles in a
population would then be interpreted as resulting in evolution.
Five factors are known to affect Hardy-Weinberg equilibrium. These
are gene migration or gene flow, genetic drift, mutation, genetic
recombination and natural selection. When migration of a section of
population to another place and population occurs, gene frequencies
change in the original as well as in the new population. New genes/alleles
are added to the new population and these are lost from the old population.
There would be a gene flow if this gene migration, happens multiple times.
If the same change occurs by chance, it is called genetic drift. Sometimes
the change in allele frequency is so different in the new sample of population
that they become a different species. The original drifted population
becomes founders and the effect is called founder effect.
Microbial experiments show that pre-existing advantageous
mutations when selected will result in observation of new phenotypes.
Over few generations, this would result in Speciation. Natural selection is
a process in which heritable variations enabling better survival are enabled
to reproduce and leave greater number of progeny. A critical analysis
makes us believe that variation due to mutation or variation due to
recombination during gametogenesis, or due to gene flow or genetic drift
results in changed frequency of genes and alleles in future generation.
Coupled to enhance reproductive success, natural selection makes it look
like different population. Natural selection can lead to stabilisation (in
which more individuals acquire mean character value), directional change
(more individuals acquire value other than the mean character value) or
disruption (more individuals acquire peripheral character value at both
ends of the distribution curve) (Figure 7.8).
7.8 A BRIEF ACCOUNT OF EVOLUTION
About 2000 million years ago (mya) the first cellular forms of life appeared
on earth. The mechanism of how non-cellular aggregates of giant
macromolecules could evolve into cells with membranous envelop is not
known. Some of these cells had the ability to release O
2
. The reaction
2020-21
138
BIOLOGY
Figure 7.9  A sketch of the evolution of plant forms through geological periods
could have been similar to the light reaction in photosynthesis where water
is split with the help of solar energy captured and channelised by
appropriate light harvesting pigments. Slowly single-celled organisms
became multi-cellular life forms. By the time of 500 mya, invertebrates
were formed and active. Jawless fish probably evolved around 350 mya.
Sea weeds and few plants existed probably around 320 mya.  We are told
that the first organisms that invaded land were plants. They were
widespread on land when animals invaded land. Fish with stout and strong
fins could move on land and go back to water. This was about 350 mya. In
1938, a fish caught in South Africa happened to be a Coelacanth which was
thought to be extinct. These animals called lobefins evolved into the
2020-21
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BIOLOGY
Figure 7.8 Diagrammatic representation of the operation of natural selection on different
traits : (a) Stabilising (b) Directional and (c) Disruptive
(a)
(b)
(c)
7.7 HARDY-WEINBERG PRINCIPLE
In a given population one can find out the frequency of occurrence of
alleles of a gene or a locus. This frequency is supposed to remain fixed
and even remain the same through generations. Hardy-Weinberg principle
stated it using algebraic equations.
This principle says that allele frequencies in a population are stable
and is constant from generation to generation. The gene pool (total genes
and their alleles in a population) remains a constant. This is called
genetic equilibrium. Sum total of all the allelic frequencies is 1. Individual
2020-21
137
EVOLUTION
frequencies, for example, can be named p, q, etc. In a diploid, p and q
represent the frequency of allele A and allele a. The frequency of AA
individuals in a population is simply p
2
.  This is simply stated in another
ways, i.e., the probability that an allele A with a frequency of p appear on
both the chromosomes of a diploid individual is simply the product
of the probabilities, i.e., p
2
. Similarly of aa is q
2
, of Aa 2pq. Hence,
p
2
+2pq+q
2
=1. This is a binomial expansion of (p+q)
2
. When frequency
measured, differs from expected values, the difference (direction) indicates
the extent of evolutionary change. Disturbance in genetic equilibrium, or
Hardy- Weinberg equilibrium, i.e., change of frequency of alleles in a
population would then be interpreted as resulting in evolution.
Five factors are known to affect Hardy-Weinberg equilibrium. These
are gene migration or gene flow, genetic drift, mutation, genetic
recombination and natural selection. When migration of a section of
population to another place and population occurs, gene frequencies
change in the original as well as in the new population. New genes/alleles
are added to the new population and these are lost from the old population.
There would be a gene flow if this gene migration, happens multiple times.
If the same change occurs by chance, it is called genetic drift. Sometimes
the change in allele frequency is so different in the new sample of population
that they become a different species. The original drifted population
becomes founders and the effect is called founder effect.
Microbial experiments show that pre-existing advantageous
mutations when selected will result in observation of new phenotypes.
Over few generations, this would result in Speciation. Natural selection is
a process in which heritable variations enabling better survival are enabled
to reproduce and leave greater number of progeny. A critical analysis
makes us believe that variation due to mutation or variation due to
recombination during gametogenesis, or due to gene flow or genetic drift
results in changed frequency of genes and alleles in future generation.
Coupled to enhance reproductive success, natural selection makes it look
like different population. Natural selection can lead to stabilisation (in
which more individuals acquire mean character value), directional change
(more individuals acquire value other than the mean character value) or
disruption (more individuals acquire peripheral character value at both
ends of the distribution curve) (Figure 7.8).
7.8 A BRIEF ACCOUNT OF EVOLUTION
About 2000 million years ago (mya) the first cellular forms of life appeared
on earth. The mechanism of how non-cellular aggregates of giant
macromolecules could evolve into cells with membranous envelop is not
known. Some of these cells had the ability to release O
2
. The reaction
2020-21
138
BIOLOGY
Figure 7.9  A sketch of the evolution of plant forms through geological periods
could have been similar to the light reaction in photosynthesis where water
is split with the help of solar energy captured and channelised by
appropriate light harvesting pigments. Slowly single-celled organisms
became multi-cellular life forms. By the time of 500 mya, invertebrates
were formed and active. Jawless fish probably evolved around 350 mya.
Sea weeds and few plants existed probably around 320 mya.  We are told
that the first organisms that invaded land were plants. They were
widespread on land when animals invaded land. Fish with stout and strong
fins could move on land and go back to water. This was about 350 mya. In
1938, a fish caught in South Africa happened to be a Coelacanth which was
thought to be extinct. These animals called lobefins evolved into the
2020-21
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EVOLUTION
first amphibians that lived on both land and water. There are no specimens
of these left with us. However, these were ancestors of modern day frogs
and salamanders. The amphibians evolved into reptiles. They lay thick-
shelled eggs which do not dry up in sun unlike those of amphibians.
Again we only see their modern day descendents, the turtles, tortoises
and crocodiles. In the next 200 millions years or so, reptiles of different
Figure 7.10 Representative evolutionary history of vertebrates through geological periods
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Figure 7.8 Diagrammatic representation of the operation of natural selection on different
traits : (a) Stabilising (b) Directional and (c) Disruptive
(a)
(b)
(c)
7.7 HARDY-WEINBERG PRINCIPLE
In a given population one can find out the frequency of occurrence of
alleles of a gene or a locus. This frequency is supposed to remain fixed
and even remain the same through generations. Hardy-Weinberg principle
stated it using algebraic equations.
This principle says that allele frequencies in a population are stable
and is constant from generation to generation. The gene pool (total genes
and their alleles in a population) remains a constant. This is called
genetic equilibrium. Sum total of all the allelic frequencies is 1. Individual
2020-21
137
EVOLUTION
frequencies, for example, can be named p, q, etc. In a diploid, p and q
represent the frequency of allele A and allele a. The frequency of AA
individuals in a population is simply p
2
.  This is simply stated in another
ways, i.e., the probability that an allele A with a frequency of p appear on
both the chromosomes of a diploid individual is simply the product
of the probabilities, i.e., p
2
. Similarly of aa is q
2
, of Aa 2pq. Hence,
p
2
+2pq+q
2
=1. This is a binomial expansion of (p+q)
2
. When frequency
measured, differs from expected values, the difference (direction) indicates
the extent of evolutionary change. Disturbance in genetic equilibrium, or
Hardy- Weinberg equilibrium, i.e., change of frequency of alleles in a
population would then be interpreted as resulting in evolution.
Five factors are known to affect Hardy-Weinberg equilibrium. These
are gene migration or gene flow, genetic drift, mutation, genetic
recombination and natural selection. When migration of a section of
population to another place and population occurs, gene frequencies
change in the original as well as in the new population. New genes/alleles
are added to the new population and these are lost from the old population.
There would be a gene flow if this gene migration, happens multiple times.
If the same change occurs by chance, it is called genetic drift. Sometimes
the change in allele frequency is so different in the new sample of population
that they become a different species. The original drifted population
becomes founders and the effect is called founder effect.
Microbial experiments show that pre-existing advantageous
mutations when selected will result in observation of new phenotypes.
Over few generations, this would result in Speciation. Natural selection is
a process in which heritable variations enabling better survival are enabled
to reproduce and leave greater number of progeny. A critical analysis
makes us believe that variation due to mutation or variation due to
recombination during gametogenesis, or due to gene flow or genetic drift
results in changed frequency of genes and alleles in future generation.
Coupled to enhance reproductive success, natural selection makes it look
like different population. Natural selection can lead to stabilisation (in
which more individuals acquire mean character value), directional change
(more individuals acquire value other than the mean character value) or
disruption (more individuals acquire peripheral character value at both
ends of the distribution curve) (Figure 7.8).
7.8 A BRIEF ACCOUNT OF EVOLUTION
About 2000 million years ago (mya) the first cellular forms of life appeared
on earth. The mechanism of how non-cellular aggregates of giant
macromolecules could evolve into cells with membranous envelop is not
known. Some of these cells had the ability to release O
2
. The reaction
2020-21
138
BIOLOGY
Figure 7.9  A sketch of the evolution of plant forms through geological periods
could have been similar to the light reaction in photosynthesis where water
is split with the help of solar energy captured and channelised by
appropriate light harvesting pigments. Slowly single-celled organisms
became multi-cellular life forms. By the time of 500 mya, invertebrates
were formed and active. Jawless fish probably evolved around 350 mya.
Sea weeds and few plants existed probably around 320 mya.  We are told
that the first organisms that invaded land were plants. They were
widespread on land when animals invaded land. Fish with stout and strong
fins could move on land and go back to water. This was about 350 mya. In
1938, a fish caught in South Africa happened to be a Coelacanth which was
thought to be extinct. These animals called lobefins evolved into the
2020-21
139
EVOLUTION
first amphibians that lived on both land and water. There are no specimens
of these left with us. However, these were ancestors of modern day frogs
and salamanders. The amphibians evolved into reptiles. They lay thick-
shelled eggs which do not dry up in sun unlike those of amphibians.
Again we only see their modern day descendents, the turtles, tortoises
and crocodiles. In the next 200 millions years or so, reptiles of different
Figure 7.10 Representative evolutionary history of vertebrates through geological periods
2020-21
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BIOLOGY
shapes and sizes dominated on earth. Giant ferns (pteridophytes) were
present but they all fell to form coal deposits slowly. Some of these land
reptiles went back into water to evolve into fish like reptiles probably 200
mya (e.g. Ichthyosaurs). The land reptiles were, of course, the dinosaurs.
The biggest of them, i.e., Tyrannosaurus rex was about 20 feet in height
and had huge fearsome dagger like teeth. About 65 mya, the dinosaurs
suddenly disappeared from the earth. We do not know the true reason.
Some say climatic changes killed them. Some say most of them evolved
into birds. The truth may live in between. Small sized reptiles of  that era
still exist today.
The first mammals were like shrews. Their fossils are small sized.
Mammals were viviparous and protected their unborn young inside the
mother’s body. Mammals were more intelligent in sensing and avoiding
danger at least. When reptiles came down mammals took over this earth.
There were in South America mammals resembling horse, hippopotamus,
bear, rabbit, etc. Due to continental drift, when South America joined
North America, these animals were overridden by North American fauna.
Due to the same continental drift pouched mammals of Australia survived
because of lack of competition from any other mammal.
Lest we forget, some mammals live wholly in water. Whales, dolphins,
seals and sea cows are some examples. Evolution of horse, elephant, dog,
etc., are special stories of evolution. You will learn about these in higher
classes. The most successful story is the evolution of man with language
skills and self-consciousness.
A rough sketch of the evolution of life forms, their times on a geological
scale are indicated in (Figure 7.9 and 7.10).
7.9 ORIGIN AND EVOLUTION OF MAN
About 15 mya, primates called Dryopithecus and Ramapithecus were
existing. They were hairy and walked like gorillas and chimpanzees.
Ramapithecus was more man-like while Dryopithecus was more
ape-like. Few fossils of man-like bones have been discovered in Ethiopia
and Tanzania (Figure 7.11). These revealed hominid features leading to
the belief that about 3-4 mya, man-like primates walked in eastern Africa.
They were probably not taller than 4 feet but walked up right. Two mya,
Australopithecines probably lived in East African grasslands. Evidence
shows they hunted with stone weapons but essentially ate fruit. Some of
the bones among the bones discovered were different. This creature was
called the first human-like being the hominid and was called Homo habilis.
The brain capacities were between 650-800cc. They probably did not eat
meat. Fossils discovered in Java in 1891 revealed the next stage, i.e., Homo
erectus about 1.5 mya. Homo erectus had a large brain around 900cc.
2020-21