CHAPTER > 07
Evolution
NEET KEY NOTES
The origin of life is considered a unique event in the history In 1953, SL Miller and his student H Urey created primitive
of universe. The study of history of life forms on the earth is earth conditions in the laboratory and provided
called evolutionary biology. experimental evidence for chemical evolution. They clearly
Evolution is the process that results in heritable changes in demonstrated that electrical discharges or UV radiations or
population spread over many generations leading to both can create organic compounds from mixture of gases
, NH 3and H2in the ratio of 2 : 1 : 2 and water vapours)
diversity of organisms on earth. ( CH 4
in the experiment.
Origin of Life Origin of First Cell
It is considered a unique event in the history of our universe.
First non-cellular life form originated 3 billion years ago.
Our universe is about 20 billion years old. It comprises of These molecules were like RNA, protein and polysaccharides
huge clusters of galaxies. inside a self-replicating metabolic capsule which later
Various theories given by different thinkers and scientists to reproduced its molecules.
explain origin of life are described below Cellular life form first evolved about 2000 million years ago.
The Big Bang theory attempted to explain the origin of These were single-celled and formed in aquatic environment.
universe. According to this theory, a single huge explosion This form of abiogenesis, i.e. the first form of life arose
resulted in universe expansion, decreased temperature, slowly through evolutionary forces from non-living
condensation of gases, formation of hydrogen and helium, molecules, was accepted by many scientists.
etc., which resulted in formation of galaxies.
Earth is about 4.5 billion years old. Biological Evolution
Life appeared 500 million years after the formation of earth. It is the process of inheritance of genetic changes over
several generations. Unlike chemical evolution, biological
Theory of special creation states that God created life by his
divine act of creation. evolution happens when organisms could replicate. It took
place in following steps – Coacervates formation, formation
Theory of cosmic panspermia (by early Greek thinkers) of protobionts, origin of prokaryotes and eukaryotes.
states that the spores or panspermia came from outer space
Coacervates were aggregate of organic biomolecules,
and developed into living forms.
surrounded by film of water molecules. They lack definite
Theory of spontaneous generation states that the life membrane, but do exhibit some life like characters.
originated from decaying and rotting matter like straw, mud, Microspheres were the non-living collections of organic
etc. Louis Pasteur rejected the theory of spontaneous
macromolecules double-layered and can be formed from
generation and demonstrated that life came from
proteinoids, by heating a mixture of amino acids at 180ºC.
pre-existing life and gave theory of biogenesis.
The first organisms originated in reducing (oxygen free)
Theory of chemical evolution or Oparin-Haldane theory
atmosphere were anaerobes (able to respire in the absence of
states that life originated from pre-existing non-living
oxygen). They were called heterotrophs because they were
organic molecules (e.g. RNA, protein, etc.) and the formation
dependent on pre-existing organic molecules for obtaining
of life was preceded by chemical evolution.
nutrition. Some of them might have evolved into autotrophs.
The conditions on the earth that favoured chemical evolution
The doctrine of organic evolution states that the present
were very high temperature (800ºC), volcanic storms and
complex organisms (eukaryotes) have been produced by
reducing atmosphere that contained CH , NH 3, water
gradual changes in the earlier simpler forms (prokaryotes) of
vapours, etc.
life over ages.
, Charles Darwin during a sea voyage on his ship HMS Beagle I Adaptive convergence/convergent evolution When
concluded that existing living forms share similarities among more than one adaptive radiation appeared to have
themselves and with life forms that existed million of years occurred in an isolated geographical area
(representing different habitats) and two or more
groups of unrelated animals come to resemble each
Evidences of Evolution other for similar mode of life or habitat, it is called
Evidences that the evolution of life forms took place on earth adaptive convergence or convergent evolution.
came from palaeontology, comparative anatomy and I Adaptive convergence in mammals Australian
morphology, biochemical evidences and biogeographical marsupials and placental mammals in North
America provide another example of convergent
evolution. Despite the temporal and geographical
Palaeontology is the study of fossils in different sedimentary
separation, marsupials in Australia and placental
layers, indicating the geological period in which they existed.
mammals in North America have produced
Geological time scale is a chronological order or history of varieties of species living in similar habitat with
evolution based upon the study of fossils. It has been divided similar ways of living. They are not related, but
resemble each other because they fill similar niches
Embryological evidences These were given by Ernst Haeckel. in each continent.
He observed structures common in embryonic stage of all
vertebrates, like presence of gill slits in all vertebrates embryo Theories of Evolution
Various theories regarding evolution of species, i.e.
biological evolution have been postulated by different
evolutionists. Some of them are as follows
ago. Lamarckism
Lamarckism (‘Inheritance of Acquired Characters in
Organisms’) is given by Lamarck in 1809. It states that
changes in structure or function of organ acquired
during an individual’ lifetime in response to changes
in the environment are inherited by its offspring.
evidences.
Major postulates of this theory are as follows
Internal vital force
Effect of environment and new needs
Use and disuse of organs
Inheritance of acquired characters
into eras, periods and epochs.
This theory was rejected by August Weismann
(1892) who proposed the ‘Theory of Continuity of
Germplasm’ that states characters influencing
germ cells are only inherited.
Neo-Lamarckism proposed that environment
influences an organism directly or indirectly (i.e.
produced variation) and changes its heredity, only
those variations are passed on to next generation
which affects germ cells.
Darwinism
Darwinism (Theory of Natural Selection) is given by
times. Charles Darwin in 1859. The essence of this theory is
natural selection. The environment selects those
individuals which are better fitted, and have adaptive
ability to survive and reproduce.
divergent evolution.
evolution.
, The two key concepts of Darwinian theory of evolution are I Sum total of all allelic frequencies is 1. It can be
described as follows expressed by the following equation
2
(i) Branching descent is defined as a process by which new p +
species originate from a single common descent.
where, p and q are individual frequencies of
(ii) Natural selection states that organisms better adapted to different alleles. This is a binomial expansion of
environment are selected by nature and they survive, ( p + q ) 2. When frequency measured, differs from
reproduce and pass their characters to next generations.
expected values, the difference indicates the extent
It leads to speciation (formation of new species).
of evolutionary change.
I An example of natural selection is industrial melanism in
I Any fluctuation in genetic equilibrium leads to
peppered moths of England. evolution.
Before industrialisation (in 1850s), white peppered moths
were able to escape predation due to trees covered by Factors Affecting
white coloured lichen, but after industrialisation, black
peppered moths increased and selected by nature due to Hardy-Weinberg Equilibrium
trees covered by black soot present in that region, i.e. the The factors that affect Hardy-Weinberg equilibrium are
proportion was reversed. given below
Gene migration or Gene flow Due to migration, new
Mutation Theory of Evolution genes or alleles are added to the new population and
(Mechanism of Evolution)
In 1901, Hugo de Vries carried out experiments on evening
primrose plant (Oenothera lamarckiana) and proposed the
mutation theory of evolution.
This theory states that the evolution occurs by the sudden
large differences in the population. He believed that mutation is the
only cause of evolution and not the minor heritable
variations that Darwin talked about.
He stated that the mutation caused speciation and named it founder.
saltation (single step large mutation). Mutations are random
and without directions, while, Darwinian variations are small Mutations These occur randomly and at very slow
and directional. rates. They lead to new phenotypes and due to the
considerable genetic variation, speciation occurs.
Modern Synthetic Theory Genetic recombination During gametogenesis,
of Evolution crossing over in meiosis leads to new combination of
With progress in genetics, the sources of variation were genes.
explained and Darwin’s theory was modified. Natural selection It occurs due to the inheritance of
variations, which leads to the survival of the fittest. So,
Now, the most accepted theory of evolution is known as
if above five factors are absent in a population it means
modern synthetic theory of evolution, designated by Huxley in 1942
there is no evolution going on in that population. It has
in which the origin of species is based on the
following effects
interaction of genetic variation in a population, natural
selection and reproductive isolation. I Stabilisation Large number of individuals acquire
Hardy-Weinberg Principle I Directional change Large number of individuals
This principle states that allele frequencies in a population are stable
and are constant from generation to generation, i.e.
gene pool is constant.
It occurs when population is large and there is random I Disruption More individuals acquire peripheral
mean character value.
mating among members of the population. This is called
genetic equilibrium or Hardy-Weinberg equilibrium.
acquire value other than mean character value. It
means natural selection is taking place in one
direction.
character value at both ends of the distribution
curve.
2 pq + q 2 = 1
Evolution
NEET KEY NOTES
The origin of life is considered a unique event in the history In 1953, SL Miller and his student H Urey created primitive
of universe. The study of history of life forms on the earth is earth conditions in the laboratory and provided
called evolutionary biology. experimental evidence for chemical evolution. They clearly
Evolution is the process that results in heritable changes in demonstrated that electrical discharges or UV radiations or
population spread over many generations leading to both can create organic compounds from mixture of gases
, NH 3and H2in the ratio of 2 : 1 : 2 and water vapours)
diversity of organisms on earth. ( CH 4
in the experiment.
Origin of Life Origin of First Cell
It is considered a unique event in the history of our universe.
First non-cellular life form originated 3 billion years ago.
Our universe is about 20 billion years old. It comprises of These molecules were like RNA, protein and polysaccharides
huge clusters of galaxies. inside a self-replicating metabolic capsule which later
Various theories given by different thinkers and scientists to reproduced its molecules.
explain origin of life are described below Cellular life form first evolved about 2000 million years ago.
The Big Bang theory attempted to explain the origin of These were single-celled and formed in aquatic environment.
universe. According to this theory, a single huge explosion This form of abiogenesis, i.e. the first form of life arose
resulted in universe expansion, decreased temperature, slowly through evolutionary forces from non-living
condensation of gases, formation of hydrogen and helium, molecules, was accepted by many scientists.
etc., which resulted in formation of galaxies.
Earth is about 4.5 billion years old. Biological Evolution
Life appeared 500 million years after the formation of earth. It is the process of inheritance of genetic changes over
several generations. Unlike chemical evolution, biological
Theory of special creation states that God created life by his
divine act of creation. evolution happens when organisms could replicate. It took
place in following steps – Coacervates formation, formation
Theory of cosmic panspermia (by early Greek thinkers) of protobionts, origin of prokaryotes and eukaryotes.
states that the spores or panspermia came from outer space
Coacervates were aggregate of organic biomolecules,
and developed into living forms.
surrounded by film of water molecules. They lack definite
Theory of spontaneous generation states that the life membrane, but do exhibit some life like characters.
originated from decaying and rotting matter like straw, mud, Microspheres were the non-living collections of organic
etc. Louis Pasteur rejected the theory of spontaneous
macromolecules double-layered and can be formed from
generation and demonstrated that life came from
proteinoids, by heating a mixture of amino acids at 180ºC.
pre-existing life and gave theory of biogenesis.
The first organisms originated in reducing (oxygen free)
Theory of chemical evolution or Oparin-Haldane theory
atmosphere were anaerobes (able to respire in the absence of
states that life originated from pre-existing non-living
oxygen). They were called heterotrophs because they were
organic molecules (e.g. RNA, protein, etc.) and the formation
dependent on pre-existing organic molecules for obtaining
of life was preceded by chemical evolution.
nutrition. Some of them might have evolved into autotrophs.
The conditions on the earth that favoured chemical evolution
The doctrine of organic evolution states that the present
were very high temperature (800ºC), volcanic storms and
complex organisms (eukaryotes) have been produced by
reducing atmosphere that contained CH , NH 3, water
gradual changes in the earlier simpler forms (prokaryotes) of
vapours, etc.
life over ages.
, Charles Darwin during a sea voyage on his ship HMS Beagle I Adaptive convergence/convergent evolution When
concluded that existing living forms share similarities among more than one adaptive radiation appeared to have
themselves and with life forms that existed million of years occurred in an isolated geographical area
(representing different habitats) and two or more
groups of unrelated animals come to resemble each
Evidences of Evolution other for similar mode of life or habitat, it is called
Evidences that the evolution of life forms took place on earth adaptive convergence or convergent evolution.
came from palaeontology, comparative anatomy and I Adaptive convergence in mammals Australian
morphology, biochemical evidences and biogeographical marsupials and placental mammals in North
America provide another example of convergent
evolution. Despite the temporal and geographical
Palaeontology is the study of fossils in different sedimentary
separation, marsupials in Australia and placental
layers, indicating the geological period in which they existed.
mammals in North America have produced
Geological time scale is a chronological order or history of varieties of species living in similar habitat with
evolution based upon the study of fossils. It has been divided similar ways of living. They are not related, but
resemble each other because they fill similar niches
Embryological evidences These were given by Ernst Haeckel. in each continent.
He observed structures common in embryonic stage of all
vertebrates, like presence of gill slits in all vertebrates embryo Theories of Evolution
Various theories regarding evolution of species, i.e.
biological evolution have been postulated by different
evolutionists. Some of them are as follows
ago. Lamarckism
Lamarckism (‘Inheritance of Acquired Characters in
Organisms’) is given by Lamarck in 1809. It states that
changes in structure or function of organ acquired
during an individual’ lifetime in response to changes
in the environment are inherited by its offspring.
evidences.
Major postulates of this theory are as follows
Internal vital force
Effect of environment and new needs
Use and disuse of organs
Inheritance of acquired characters
into eras, periods and epochs.
This theory was rejected by August Weismann
(1892) who proposed the ‘Theory of Continuity of
Germplasm’ that states characters influencing
germ cells are only inherited.
Neo-Lamarckism proposed that environment
influences an organism directly or indirectly (i.e.
produced variation) and changes its heredity, only
those variations are passed on to next generation
which affects germ cells.
Darwinism
Darwinism (Theory of Natural Selection) is given by
times. Charles Darwin in 1859. The essence of this theory is
natural selection. The environment selects those
individuals which are better fitted, and have adaptive
ability to survive and reproduce.
divergent evolution.
evolution.
, The two key concepts of Darwinian theory of evolution are I Sum total of all allelic frequencies is 1. It can be
described as follows expressed by the following equation
2
(i) Branching descent is defined as a process by which new p +
species originate from a single common descent.
where, p and q are individual frequencies of
(ii) Natural selection states that organisms better adapted to different alleles. This is a binomial expansion of
environment are selected by nature and they survive, ( p + q ) 2. When frequency measured, differs from
reproduce and pass their characters to next generations.
expected values, the difference indicates the extent
It leads to speciation (formation of new species).
of evolutionary change.
I An example of natural selection is industrial melanism in
I Any fluctuation in genetic equilibrium leads to
peppered moths of England. evolution.
Before industrialisation (in 1850s), white peppered moths
were able to escape predation due to trees covered by Factors Affecting
white coloured lichen, but after industrialisation, black
peppered moths increased and selected by nature due to Hardy-Weinberg Equilibrium
trees covered by black soot present in that region, i.e. the The factors that affect Hardy-Weinberg equilibrium are
proportion was reversed. given below
Gene migration or Gene flow Due to migration, new
Mutation Theory of Evolution genes or alleles are added to the new population and
(Mechanism of Evolution)
In 1901, Hugo de Vries carried out experiments on evening
primrose plant (Oenothera lamarckiana) and proposed the
mutation theory of evolution.
This theory states that the evolution occurs by the sudden
large differences in the population. He believed that mutation is the
only cause of evolution and not the minor heritable
variations that Darwin talked about.
He stated that the mutation caused speciation and named it founder.
saltation (single step large mutation). Mutations are random
and without directions, while, Darwinian variations are small Mutations These occur randomly and at very slow
and directional. rates. They lead to new phenotypes and due to the
considerable genetic variation, speciation occurs.
Modern Synthetic Theory Genetic recombination During gametogenesis,
of Evolution crossing over in meiosis leads to new combination of
With progress in genetics, the sources of variation were genes.
explained and Darwin’s theory was modified. Natural selection It occurs due to the inheritance of
variations, which leads to the survival of the fittest. So,
Now, the most accepted theory of evolution is known as
if above five factors are absent in a population it means
modern synthetic theory of evolution, designated by Huxley in 1942
there is no evolution going on in that population. It has
in which the origin of species is based on the
following effects
interaction of genetic variation in a population, natural
selection and reproductive isolation. I Stabilisation Large number of individuals acquire
Hardy-Weinberg Principle I Directional change Large number of individuals
This principle states that allele frequencies in a population are stable
and are constant from generation to generation, i.e.
gene pool is constant.
It occurs when population is large and there is random I Disruption More individuals acquire peripheral
mean character value.
mating among members of the population. This is called
genetic equilibrium or Hardy-Weinberg equilibrium.
acquire value other than mean character value. It
means natural selection is taking place in one
direction.
character value at both ends of the distribution
curve.
2 pq + q 2 = 1
