AQOONSIDE INSTITUTE OF NATURAL SCIENCES

GRADE 12TH BIOLOGY NOTES ON UNIT-4

Unit: 4 Evolution
The unit Content
 4.1. EVOLUTION
 4.1.1. Definition.
 4.1.2. Theories of evolution.
 4.1.3. The evidence for evolution
 4.1.4. Natural selection: Definition, Types & Examples
 4.1.5. Human evolution
 4.1.6. Mutation....
 4.1.7. Genetic Drift
 4.1.8. Gene flow (immigration and emigration).
 4.1.9. Causes of species extinction...
4.1 Evolution
4.1.1 Definition
Evolution
 Is a change in genetic composition of a population over successive generations, which may be caused
by meiosis, hybridization, natural selection or mutation.
 This leads to a sequence of events by which the population diverges from other populations of the
same species and may lead to the origin of a new species.
Theories of the origin of life
 "Origin of Life" is a very complex subject, and oftentimes controversial.
 Two opposing scientific theories that existed on this complex subject for a long time were the so-called
intelligent design and creationism.
 The big bang theory gave new ideas about the topic of biological evolution.
 In the theory it has been hypothesized that complex life-forms on Earth, including humans, arose over a
period of time from simple bacteria like tiny cells by a process of self-organization similar to the evolution
of the Universe of simple material structures toward more and more complex structures.
 There are several theories about the origin of life. Some of them are mentioned below.
1. Special creationism
 It states that at some stage, some supreme being created life on Earth.
 It explained that the formation of life on earth may have been taken place due to supernatural or divine
forces.
 There are fundamental differences between special creationism and scientific thinking in that it is unlikely
that the difference between the two will ever be resolved.
 There are many different versions of special creation, linked with different religions.
2. The theory of spontaneous generation (Abiogenesis):
 It suggests that some life can evolve spontaneously' from non- living objects.
 It was once believed that life could come from non-living things, such as mice from corn, flies from bovine
manure, maggots from rotting meat, and fish from the mud of previously dry lakes.

1
Mukhtaar N. Biology is cool and colourful
  Some of the supporters of abiogenesis were Aristotle, Needham, Van Helmont etc. The theory of spontaneous
generation was first disproved by Francesco Redi.
 Louis Pasteur by his advanced experimental settings disproved the theory for the last time in the way that can
convince scientists to accept biogenesis and to reject abiogenesis.
3. Theory of eternity of life (Biogenesis):
 In this theory of life, there is no beginning and no end to life on Earth and so life neither needs special
creation nor does it need to be generated from non-living matter.
 Supporters of this theory believe that life is an inherent property of the Universe and it has always existed as
has the Universe.
 At the time when such theories were being propounded, many eminent scientists including Albert Einstein
believed that the Universe was unchanging.
 They reasoned that 'if life is found today in an unchanging Universe, then it must always have been there'.
 According to this theory life comes from life never from lifeless things, and different types of organisms have
always existed on earth and shall continue coexist. However, the theory does not explain how life originated
in the beginning.
4. Cosmozoan theory, Panspermia or Spore broth theory
 According to Cosmozoan theory life has reached this planet Earth from other cosmological structures, such as
meteorites, in the form of highly resistant spores.
 This theory was proposed by Richter (1865). According to this theory, 'protoplasm' reached the earth in the
form of spores or germs or other simple particles from some unknown part of the universe with the cosmic
dust, and subsequently evolved into various forms of life.
 Helmholz (1884) speculated that 'protoplasm' in some form reached the earth with falling meteorites.
5. Biochemical theory of evolution (Oparin’s theory):
 Aleksandr Oparin, a Russian biologist and John Haldane, an English biologist independently put forward first
for this theory. .
 Both also suspected that the first life- forms appeared in the warm, primitive ocean and were heterotrophic
rather than autotrophic.
 from sunlight or inorganic, they proposed that common gases in the early Earth atmosphere combined to form
simple organic chemicals, and these in turn combined to form more complex molecules.
 Then, the complex molecules became separated from the surrounding medium, and acquired some of the
characters of living organisms.
 They became able to absorb nutrients, to grow, to divide (reproduce), and so on.
 Later Miller had apparently approved the Oparin-Haldane model by mixing the basic elements to produce
simple organic compounds, and then combining these to produce the building blocks of proteins and nucleic
acid.
 According to these scientists’ thought life originated from water body as the simple form of life.
Autotrophs
 Autotrophs form the basis for all food chains: they are the organisms which create sugars, proteins, lipids, and
other materials for life.
 The first organisms appeared about 4 billion years ago were prokaryotes.
Eubacteria prokaryotes;
 Ordinary bacteria and cyanobacteria (blue-green bacteria and sometimes known as blue-green
algae).
 Eventually evolving into protoctistans, fungi, plants, animals (nearly all are aerobic).

2
Mukhtaar N. Biology is cool and colourful
  One great change that affected the evolution of early life forms was the shift from the reducing
atmosphere to an atmosphere containing oxygen.
 This took place about 2.4 billion years ago. The two major types of autotrophs are
chemoautotrophs and photoautotrophs.
Chemoautotrophs
 Chemoautotrophs are organisms that obtain their energy from a chemical reaction (chemotrophs) but their
source of carbon is the most oxidized form of carbon, carbon dioxide (CO2).
 The best-known chemoautotrophs are Chemolithoautotrophs
 All known chemoautotrophs are prokaryotes, belonging to the Archaea or Bacteria domains.
 They have been isolated in different extreme habitats, associated to deep-sea vents, the deep biosphere or
acidic environments. This form of energy conservation is considered one of the oldest on Earth.
Photoautotrophs
 Photoautotrophs are organisms that use light energy and inorganic carbon to produce organic materials.
Eukaryotic photoautotrophs absorb energy through the chlorophyll molecules in their chloroplasts.
4.1.2 Theories of evolution
Lamarckism
 Was the first scientist to propose that organisms undergo change over time as a result of some natural
phenomenon rather than divine intervention.
 According to Lamarck, a changing environment caused an organism to alter its behavior, thereby
using some organs or body parts more and others less.
He postulated:
A. New Needs:
 Changes in environment factors like light, temperature, medium, food, air etc or migration leads to origin
of new needs in living organisms.
 To fulfil these new needs, living organisms have to exert special efforts like changes in habits or behavior.
B. Use and disuse of organs:
 The new habits involve the greater use of certain organs to meet new needs, and the disuse or
lesser use of certain other organs which are of no use in new conditions.
 In this part of his theory, Lamarck suggests that when a structure or process is continually used,
that structure or process will become enlarged or more developed.
 Conversely, any structure or process that is not used or is rarely used will become reduced in size
or less developed.
 The classic example he used to explain the concept of use and disuse is the elongated neck of the
giraffe.
 According to Lamarck, a given giraffe could, over a lifetime of straining to reach high branches,
develop an elongated neck. However, Lamarck could not explain how this might happen.
C. Inheritance of acquired characters:
 He believed that the favourable acquired characters are inheritable and are transmitted to the
offsprings so that these are born fit to face the changed environmental conditions and the chances
of their survival are increased.
 Lamarck believed that traits changed or acquired during an individual's lifetime could be passed on
to its offspring.
 Giraffes that had acquired long necks would have offspring with long necks rather than the short
necks their parents were born with.

3
Mukhtaar N. Biology is cool and colourful
  This type of inheritance, sometimes called Lamarckian inheritance, has since been disproved by
the discoveries of genetics. However, Lamarck did believe that evolutionary change takes place
gradually of evolution and constantly.
D. Speciation:
 Lamarck believed that in every generation, new characters are acquired and transmitted to next
generation, so that new characters accumulate generation after generation.
 After a number of generations, a new species is formed.
1. Significance of Lamarckism
a) It was first comprehensive theory of biological evolution.
b) It nicely explains the existence of vestigial organs in animals due to their continuous disuse.
c) Vestigial organs are organs that are a part of the organism that are no longer in use that can be used to
determine the relatedness of different species. For example, the bone structures of the front flippers of a
whale contain bones of limbs that exist in mammals such as cows.
d) It explains the development of strong jaw muscles and claws in the carnivores due to their continued extra
use.
e) It stimulated other biologists to look for the mechanism of organic mechanism.
Darwinism (Theory of natural selection)
 Darwin’s theory, known as natural selection believed that organisms possessed variation (each individual
was slightly different from another), and these variations led to some being more likely to survive and
reproduce than others. That is why features that made an organism more likely to survive more likely
appear to each generation.
Basic postulates of Darwinism
Geometric increase:
 All species tend to produce more offspring than can possibly survive.
 However, the space and the availability of food supply is limited to support the number of organisms
that increase in a geometric ratio.
Struggle for existence:
 Since the number of individuals produced is far more than the number that can be supported, there is
an everlasting competition between organisms at all levels of life.
Variation under nature:
 No two individuals of a species are exactly similar and they have some differences.
 These differences are called variations and without evolution is not possible.
 Variations give rise to new characters and heredity passes them on to the next generation (inheritance
of useful variations).
Natural selection or survival of the fittest:
 Due to struggle for existence and useful heritable variations, only those individuals survive which
show high selective value and in the course of time they develop various adaptive modifications to
suit the changed conditions of life.
 Such selection was called natural selection by Darwin.
Origin of species:
 In the course of long periods of time the best fitted and suitable individuals survived and adjust to
the nature.
 As environment is ever changing, further changes occur and thus new adaptations appear in
organisms.
 The later descendants after several generations become quite distinct from their ancestors.
 On this way new species appear.

4
Mukhtaar N. Biology is cool and colourful
Neo-Darwinism theory
 Charles Darwin knew very little of genetics.
 Mendel had not carried out his ground-breaking work on inheritance at the time Darwin published
his book On the Origin of Species.
 However, we can now incorporate our knowledge of genes and gene action into the theory of
natural selection to give a better understanding of what drives evolution.
 Genes determine features. But when we think about how a population might evolve into a new
species, we need to think not just in terms of the alleles each individual might carry, but also in
terms of all the alleles (all the genes) available in the population. We call this the gene pool of the
population.
Postulates of Neo-Darwinism are:
1. Genetic variability
2. Natural selection
3. Reproductive Isolation
4.1.3. The evidence for evolution
4.1.3.1 Comparative anatomy
 Comparative anatomy is one of the strongest forms of evidence for evolution.
 It looks at the structural similarities of organisms and uses these similarities to determine their possible
evolutionary relationships.
 It assumes that organisms with similar anatomical features are closely related evolutionarily, and that
they probably share a common ancestor.
 Some organisms have anatomical structures that are very similar in form, but very different in function.
 We call such structures homologous structures. Because they are so similar, they indicate an
evolutionary relationship and a common ancestor of the species.

 Perhaps the best- known example of homologous structures is the forelimb of mammals.
 When examined closely, the forelimbs of humans, whales, cats and bats are all very similar in
structure.
 Each possesses the same number of bones, arranged in almost the same way while they have different
external features that function in different ways as:
 Arm for manipulation in humans
 leg for running in cats
 flipper for swimming in whales
 wing for flying in bats

5
Mukhtaar N. Biology is cool and colourful
  By comparing the anatomy of these limbs, scientists have determined that the basic pattern (called a
pentadactyl limb) must have evolved just once and that all organisms with this kind of limb were
descended from that original type, which they share a common ancestor
 However, comparative anatomy needs to be used carefully as evidence for evolution.
 This is because while sometimes organisms have structures that function in very similar ways,
morphologically and developmentally these structures are very different. We call these analogous
structures. Because they are so different structurally, even though they have the same function, they
cannot indicate that two species share a common ancestor.
 Although, the wings of a bat, bird and mosquito all serve the same function, yet their anatomies are
very different.
 For example, the bird wing has bones inside and is covered with feathers while the mosquito wing has
neither of these.
 They are analogous structures that have evolved separately.
4.1.3.2 Embryology
 Comparative embryology studies the way in which the embryos of vertebrates develop before they
hatch or born.
 This development shows similarities which supports a common ancestry.

Figure: similarities in development of embryos

 For example, early in development, all vertebrate embryos have gill slits and tails.
 However, the 'gill slits' are not gills; they connect the throat to the outside, but in many species, they
disappear later in the embryonic development.
 However, in fish and larval amphibians they contribute to the development of gills.
 The embryonic tail does not develop into a tail in all species. In humans, it is reduced during development
to the coccyx, or tailbone.
 The more similar the patterns of embryonic development, the more closely related species are assumed to
be.
 The similarity in the development of vertebrates also suggests a common ancestor.
4.1.3.3 Palaeontology (Fossil record):
 The study of fossils is known as palaeontology.
 Fossils are formed when certain remains of organisms or plants get embedded in the soil or water and
are preserved for many hundreds of years.
 They appear either as skeletal remains, footprints, moulds or intact structures as found in the snow.
 By studying fossils, we are able to establish similarities between the organisms in the present to its
ancestor in the past.
 There can be many similarities that prove the common origins between different closely related
animals and the differences can be studied to establish how they differ now and why.
 Fossils are very important evidence to prove the theory of evolution and common ancestry.
 We can group fossils into two categories:
Category 1: The remains of dead animals or plants or the imprint left from the remains, including:
 Bones, teeth, skin impressions, hair
6
Mukhtaar N. Biology is cool and colourful
  The hardened shell of an invertebrate ancient such as a trilobite or an ammonite, an
impression of an animal or plant, even if the actual parts are missing.
Category 2: Something that was made by the animal while it was living and that it has hardened into stone
since then; these are called trace fossils and include:
 Foot prints, burrows, coprolite (Animal faeces)
 Type I fossils can be the actual organism or part of an organism, like a piece of bone or hair or feather
as it actually was.
 For example, this spider has been trapped, completely unchanged, inside the amber for millions of
years.
 Amber is fossilised resin from trees. This spider probably became stuck inside the sticky resin and
could not escape.
 As the amber became fossilised, the spider was protected from micro-organisms and the air which
would have led to its decomposition.
 In many fossils like this, the soft parts of the body have been lost, but the exoskeleton is perfectly
preserved. In some cases, however, the entire body remains.
Dating fossils
 Sedimentary rocks are laid down in layers (strata) which help to deduce how the organisms have
changed over time. This is called stratigraphy.
 The oldest strata and the oldest fossils are found in the lowest layers and more recent rocks and
fossils in the layers above them nearest to the surface.
 Some minerals in rocks and organic matter (e.g., wood, bones, and shells) can contain radioactive
isotopes.
 The abundances of parent and daughter isotopes in a sample can be measured and used to
determine their age. This method is known as radiometric dating.
 The amount of time it takes for half of the parent isotopes to decay into daughter isotopes is known
as the half-life of the radioactive isotope.
 Fossil age can be determined using two ways;
1. Absolute dating which determines the number of years that have elapsed since an event
occurred or the specific time when that event occurred.
2. Relative dating determines the age by analysing rocks and structures placed into
chronological order, establishing the age of one thing as older as or younger than another.
 How do scientists actually date the rocks? How do they find out how old each layer is? To do this,
we scientists use one of two techniques:
1. Radiocarbon dating, or
2. Potassium-argon dating.
 Both these techniques rely on the principle that radioactive atoms decay into other atoms over time.
Radioactive carbon atoms (C14) decay into non-radioactive nitrogen atoms (N).
 Radioactive potassium atoms (K40) decay into argon atoms (A40).
 Each has what is known as a half-life.
 During this period, half of the radioactive atoms decay. So, starting with a certain number of
radioactive potassium atoms, after one half-life, 50% will still be radioactive.
 After a second half-life, 50% of this will have decayed and 25% of the original number will still be
radioactive.
4.1.3.4 Comparative biochemistry
 Organisms that share very similar molecules and biochemical pathways are closely related
evolutionarily.
 Chemicals that have been used in such analysis include DNA protein.
 Species that are closely related are believed to have the most similar DNA and proteins; those that
are distantly related are assumed to share fewer similarities.

7
Mukhtaar N. Biology is cool and colourful
  For example, a comparison of DNA sequences shows that 98% of our DNA is the same as
chimpanzees which confirms that chimpanzees are the closest relatives of humans.
 The haemoglobin molecule is similar in all animals that possess it, but there are differences.
 For example, the haemoglobin of the lamprey (a primitive fish-like animal) has only one
polypeptide chain, not four.
 Most animals have a hemoglobin with four chains but these chains do vary.
4.1.4. Natural selection: Definition, Types & Examples
 Natural selection is the 'driving force' behind evolution.
 It is the process that brings about changes (over time) in populations that can, eventually, lead to different
populations of the same species to become different species.
 Those members of a species which are best adapted to their environment will survive and reproduce in
greater numbers than others that are less well adapted.
 They will pass on their advantageous alleles to their offspring and, in successive generations, the
frequency of these alleles will increase in their gene pool.
 The advantageous types will, therefore, increase in frequency in successive generations.
 To appreciate how natural selection can eventually lead to speciation (the formation of new species), we
must be clear what do we mean by the term species.
 Obviously, humans are different species from chimpanzees. But, the different races of humans are all
members of the same species. Why?
 Species therefore, is a group of similar organisms with a similar biochemistry, physiology and
evolutionary history that can interbreed to produce fertile offspring.
 This explains why all humans are members of the same species, but belongs to a different species from the
chimpanzee. So how can there be different types of natural selection? All types of natural selection work
in the same manner, but their influence on a population is different.
 The different types of natural selection include:
1. Directional selection
2. Stabilising selection and
3. Disruptive selection
Pattern of evolution
 Evolution over time can follow several different patterns. Factors such as environment and predation
pressures can have different effects on the ways in which species exposed to evolve.
 There are three main types of evolution: divergent, convergent, and parallel evolution.

Figure: Types of evolution; A) Divergent B) Convergent C) Parallel

Divergent Evolution

8
Mukhtaar N. Biology is cool and colourful
  When people hear the word "evolution," they most commonly think of divergent evolution, the evolutionary
pattern in which two species gradually become increasingly different.
 This type of evolution often occurs when closely related species diversify to new habitats.
 On a large scale, divergent evolution is responsible for the creation of the current diversity of life on earth
from the first living cells.
 On a smaller scale, it is responsible for the evolution of humans and apes from a common primate ancestor.

Convergent evolution
 Convergent evolution takes place when species of different ancestry begin to share analogous traits because of
a shared environment or other selection pressure.
 For example, whales and fish have some similar characteristics since both had to evolve methods of moving
through the same medium: water.
Parallel Evolution
 Parallel evolution occurs when two species evolve independently of each other, maintaining the same
level of similarity.
 Parallel evolution usually occurs between unrelated species that do not occupy the same or similar niches
in a given habitat.
Types of speciation
 Natural selection provides a mechanism by which new populations of a species can arise. But, at what point
can these populations be considered as distinct species?
 If two populations become SO different, individuals from these different populations cannot interbreed to
produce fertile offspring, then we must think of them as different species.
 There are a number of ways in which this can occur. The two main ways are:
 Allopatric speciation and
 Sympatric speciation.
 As long as two populations are able to interbreed, they are unlikely to evolve into distinct species.
 They must somehow go through a period when they are prevented from interbreeding.
 Both allopatric and sympatric speciation involves isolating mechanisms that prevent different populations
from interbreeding for a period of time.
 During this period, mutations that arise in one population cannot be passed to the other.
 As a result of this, and the different selection pressures in different environments, genetic differences between
the two populations increase.
 Eventually, the two populations will become so different that they will be unable to interbreed or they are
'reproductively isolated'.

9
Mukhtaar N. Biology is cool and colourful
Allopatric Speciation
 The physical isolation of the population due to the extrinsic barrier is called allopatric speciation
 Their differentiation mechanism is Natural selection
 Takes place through geographic isolation
 Emerging new species speed is Slow
 Example: Squirrels in the Grand Canyon, Darwin’s Finches, etc.
Sympatric Speciation
 The evolution of new species from one ancestral species living in the same habitat is called sympatric
speciation.
 Their differentiation mechanism is polyploidy or changes in feeding pattern, etc.
 Do not involve through geographic isolation.
 Emerging new species speed is fast
 Examples include wheat, cultivated corn, tobacco, etc.

4.1.5 Human Evolution

 There has been a 'line of evolution' for millions of years that has given rise to old world monkeys, new world
monkeys, the great apes and the different species of humans that have lived. But we are Homo sapiens and we
are the latest of several humans to live on the planet. We have two features in particular that distinguish us
from other primates. These are:
 A very large brain, and
 Bipedalism: the ability to truly walk on just two legs.

Figure-1: The evolutionary tree for modern primates

 Over the details, they all agreed about the idea - a line evolution that has branched to give the different groups
of primates (including apes and humans) that exist today has existed in not-too-distant past.

10
Mukhtaar N. Biology is cool and colourful
 Figure-2: The evolutionary tree of humans and the great apes
Lucy and Ardy
 Both Lucy and Ardy are important fossils in explaining the evolution of modern humans and chimpanzees
from a common ancestor.
 Lucy was discovered by Donald Johanson and Tom Gray in 1974 at Hadar, Ethiopia.
 Lucy is a fossil dated at about 3.2 million years. She was an adult female of about 25 years and belonged to
the species Australopithecus afarensis.
 Her skeleton was about 40% complete, an unusually high proportion for a fossil skeleton.
 Her pelvis, femur (the upper leg bone) and tibia show that she was bipedal (could walk upright on two legs)
 However, there is also evidence that Lucy was partly arboreal (tree-dwelling).
 At the time she was discovered, Lucy represented one of the oldest fossil hominins.
 The proportions of her humerus and femur were mid-way between those of modern humans and
chimpanzees.
 Lucy had a brain about the same size as that of a chimpanzee, so her discovery was able to settle a debate
amongst biologists at the time which came first, large brain or bipedalism? Clearly bipedalism came before
big brains.

11
Mukhtaar N. Biology is cool and colourful
 Figure 3: A- The original Lucy fossil; B- The Lucy display including reconstructed parts
 The Ardi fossil was first discovered in 1992, in the Afar dessert in Ethiopia, but it was only in 2009 after
many years' analysis, that research papers were finally published that gave Ardi a unique position in the
human evolution.
 Ardi was 1.2 million years older than Lucy, was also female who belonged to the species Ardipithecus
ramidus.
 One significant feature about Ardi was that she was also bipedal.
 At 4.4 million years old, Ardi is the nearest fossil to the 'common ancestor of humans and chimpanzees that
has so far been found.
 This finding finally proved that the common ancestor of humans and chimpanzees could not have resembled a
chimpanzee, as chimpanzees are not truly bipedal.
 However, there was signs of being adapted for both bipedal walking and arboreal life.

Figure 4: A- relatively complete skeleton of Ardipithecus, which lived 4.4 million years ago. B: Ardipithecus shows
signs of being adapted for both bipedal walking and arboreal life.
How brain size changed during human evolution?
 During the course of human evolution, the brain has got bigger. Studies on comparative anatomy of fossils
revealed that the cranial capacity has increased with each new hominid species evolved.
 However, the brain has increased in size as a proportion of body mass. Species of Australopithecus have a
brain that is between 0.7% and 1.0% of their body mass, whereas modern humans have a brain size between
1.8% and 2.3% of their body mass. The brain of Homo sapiens uses 25% of the resting energy requirement,
compared with 8% in the great apes. A larger brain allows humans to:
 Run faster and in
a more upright
posture.
 Plan in advance
to avoid attack.
 Develop and use
tools and
weapons.

12
Mukhtaar N. Biology is cool and colourful
 Figure-5: Brain size in different hominids
Are we still evolving?
 Homo sapiens (modern humans) first appeared in Africa and have since migrated to all other parts of the
world.
 As humans moved from Africa into different areas of the world, they encountered different environments.
 Different selection pressures in the different environments resulted in the different human populations
evolving along different lines.

 For example, as humans encountered colder climates, body features that gave a survival advantage to
conserve heat were selected for. These included:
 A shorter, squatter body shape; this reduces the surface-area-to-volume ratio and so reduces the rate of
heat loss by radiation.
 An increased layer of adipose tissue under the skin to act as insulator.
 increased hairiness; this reduces heat loss by convection.

Figure-6: The migration of modern humans out of Africa it all begins in East Africa. Numbers
indicate the time (in years) since each stage of the migration.
4.1.6. Mutation
 A mutation can be caused by several factors but is divided into two parts.
 If the agent that caused the mutation cannot be identified then it is known as a spontaneous mutation.
13
Mukhtaar N. Biology is cool and colourful
  If the mutation can be identified then it is called an induced mutation.
 Substances that cause mutations are radiation, x-ray, ultra-violet radiation, nuclear radiation and certain
chemical substances.
 These agents can also be called mutagenic agents or mutagens.
 There can be large structural changes involving the whole chromosomes or parts of chromosomes, or changes
that involve only a single base. The changes involving only a single base are called point mutations.
Point mutation
 There are several types of point mutation, in which one of the bases in the DNA sequence of a gene is altered,
usually by being copied wrongly when the DNA replicates.
 The different point mutations are:
 Substitutions
 Addition
 Deletions
 These mutations occur quite randomly when the DNA is replicating and each involves a change to just one
base, but the change to the gene can be dramatic and the result can be that the protein the gene should code for
is not made at all or a different protein is made A point mutation is a change in a single nucleotide in DNA.
 This type of mutation is usually less serious than a chromosomal alteration.
 An example of a point mutation is a mutation that changes the codon UUU to the codon UCU.
Substitution
 Guanine replaces thymine in this substitution. The triplet ATT has been changed to ATG (no other triplet is
affected). The original triplet, ATT, codes for the amino acid isoleucine.

 However, the new triplet, ATG, codes for methionine.

 As a result, a different protein will be synthesised, which may or may not be significantly different from the
original.
 One different amino acid in a protein does not always make a functional change.

Figure-6: A substitution mutation

 If a substitution of just one base in the sixth triplet of the gene coding for one of the four polypeptides in the
haemoglobin molecule alters the triplet from GAG to GTG.
 This results in the amino acid valine replacing glutamate in the polypeptide chain.
 The different haemoglobin molecule formed results in the condition known as sickle-cell anaemia.

14
Mukhtaar N. Biology is cool and colourful
 Figure-7: Sickle-cell anaemia
Addition and deletion
 In a deletion mutation, a base is 'missed out' during replication, whereas in additions, an extra base is
added. Both deletion and additions are more significant mutations than substitutions.
 The reason for this is that they do not just alter the triplet in which the mutation occurs.
 Because there is one fewer or one extra base, the whole sequence after the point of the mutation is
altered.
 We say that there has been a frameshift and these are frameshift mutations.
 A totally different mRNA is produced (if one is produced at all) and a non- functional protein or no
protein at all.
 Sometimes, a whole triplet is missed out or inserted.
 This will result in either one extra or one fewer codon in the mRNA.
 In turn, this will lead to one extra or one fewer amino acid in the polypeptide chain.

Figure- 7 Types of point mutation

Chromosomal mutation
15
Mukhtaar N. Biology is cool and colourful
  A chromosomal mutation is a mutation involving a long segment of DNA.
 Chromosomal mutation occurs when there is any change in the arrangement or structure of the chromosomes.
 Chromosomal mutation occurs most often during meiosis at crossing over in prophase I.
 There are several different mutation types that result in a change in the structure of a chromosome such as
duplication, deletion, inversion and translocation.
 They are much bigger events than point mutations and usually result in the death of a cell.
 They may also affect the whole organism.
 For example, if essential parts of the DNA are affected by chromosomal mutations, a foetus may be aborted.
There are different types of chromosome mutations.

Figure -8: Types of Chromosomal mutations

The Advantages of Chromosomal Mutations
 In some cases, chromosomal mutations can benefit the organism. Some mutations can help the organisms to
survive better than others.
 Lactose tolerance became an advantage to have over others when numerous populations depended on cows
and goats as sources of food.
 On the other hand, chromosomal mutations can be dangerous and even detrimental to the life of living
organisms.
 Some of these can cause numerous problems within animals, plants, and humans.
Genetic Disorder
 These are specific disorders or disabilities caused by mutations within the organism's DNA.
 These can be small genetic issues that may barely affect the individual or larger issues that may bring major
concerns to the individual.
 A chromosomal disorders list can be seen below:
 Trisomy 21: Down Syndrome
 Trisomy 18
 Trisomy 13
 Klinefelter Syndrome
 XYY Syndrome
 Turner Syndrome
 Triple X Syndrome
4.1.7. Genetic Drift
What is genetic drift?
 Allele frequencies can change due to chance alone. This is called genetic drift.

16
Mukhtaar N. Biology is cool and colourful
  Drift is a binomial sampling error of the gene pool. This means, the alleles that form the next generation's
gene pool are a sample of the alleles from the current generation.
 A small percentage of alleles may continually change frequency in a single direction for several generations.
 Alleles can increase or decrease in frequency due to drift.
 Example, we have a very small rabbit population that's made up of 8 brown individuals (genotype BB or Bb)
and 2 white individuals (genotype bb).

Figure-8: Example of genetic drift in a rabbit population

Hardy-Weinberg Equilibrium
 At Hardy-Weinberg Equilibrium, Allele Frequencies do Not Change.
 The study of population genetics relies on the intimate relationship between allele frequencies and
genotype frequencies.
 Each genotype's frequency is the number of individuals with that genotype, divided by the total size of
the population. For example, if 64 of the 100 individuals in a population are homozygous recessive,
then the frequency of that genotype is 64/100, or 0.64.
 Hardy-Weinberg equilibrium is the highly unlikely situation in which allele frequencies and genotype
frequencies do not change from one generation to the next. It occurs only in populations that meet the
following assumptions:
1. Natural selection does not occur;
2. Mutations do not occur, so no new alleles arise;
3. The population is infinitely large, or at least large enough to eliminate random changes in
allele frequencies;
4. Individuals mate at random; and
5. Individuals do not migrate into or out of the population.
 The first equation represents the frequencies of both alleles in the population:
p+ q= 1
 The two frequencies add up to 1 because the two alleles represent all the possibilities in the population. For
example, the frequency of the dark fur allele (D) is 0.6; the frequency of the alternative allele d, which confers

17
Mukhtaar N. Biology is cool and colourful
 tan fur, is 0.4. (Tally the D and d alleles in the picture of the ferrets to verify these numbers.

 At Hardy-Weinberg equilibrium, we can use allele frequencies to calculate genotype frequencies, according to
the equation in p2+2pq+q2=1
 In this equation, the proportion population with genotype DD of the equals p2 (0.36 for our ferrets) and the
proportion with genotype dd equals q2 (0.16). To calculate the frequency of the heterozygous class, multiply
pq by 2 (0.48). Since the homozygotes and the heterozygotes account for all possible genotypes, the sum of
their frequencies must add up to 1.

Hardy-Weinberg Equilibrium. At Hardy-Weinberg equilibrium, allele frequencies remain constant from one
generation to the next; evolution does not occur."
4.1.8. Gene flow (immigration and emigration)
 Gene flow also called migration- is any movement of individuals, and/or the genetic material they carry from
one population to another.
 Gene flow includes lots of different kinds of events, such as pollen being blown to a new destination or people
moving to new cities or countries.
 If gene versions are carried to a population where those gene versions previously did not exist, gene flow can
be a very important source of genetic variation.
 Immigration is when new organisms join a population, changing allele frequencies.
18
Mukhtaar N. Biology is cool and colourful
  Emigration is when members of a population leave, taking with them their genes.
 These phenomena change the overall balance of the gene pool of the populations.
 Gene transfer is the flow of alleles from one species to another.
 Horizontal gene transfer is especially common in bacteria.
…………………………………THE END…………………………………………

19
Mukhtaar N. Biology is cool and colourful