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 1. INTRODUCTION
 1.1. Definition and scope of evolution
 1.2. Why we study evolution?

 2. HISTORICAL DEVELOPMENT OF EVOLUTION

 2.1. Evolutionary and non evolutionary ideas before Darwin
 2.2. Darwin’s Theory (1859)
 2.3. The modern synthesis (Neo-Darwinism)
 2.4. Development within the modern synthesis (1960s – present)
 3. THEORIES OF THE ORIGIN OF LIFE
 3.1. Special creation theory
 3.2. Spontaneous generation theory
 3.3. Steady-state theory
 3.4. Cosmozoan theory
 3.5. Biochemical evolution 2
 4. EVIDENCES FOR THE THEORY OF EVOLUTION
 4.1. Paleontological evidences
 4.2. Biogeographical evidences
 4.3. Classification
 4.4. Plant and Animal Breeding
 4.5. Comparative Anatomy and Morphology
 4.6. Adaptive Radiation
 4.7. Evidences from Embryology
 4.8. Evidences from Molecular Biology
 4.9. Evidences from contemporary processes
 5. MECHANISM OF EVOLUTION (GENETIC BASIS OF EVOLUTION)
 5.1. Populations and demes
 5.2. Genetic variation
 5.3. Sources of genetic variation
 5.4. Gene pool
 5.5. Allele frequency
 5.6. Genotypic frequency
 5.7. The Hardy-Weinberg equation
 5.8. Factors producing gene pool change in populations

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 6. SPECIATION
 6.1 Species concepts
 6.2 Modes of Speciation
 6.3. Rate of speciation
 6.4. Development of reproductive isolating mechanisms
 7. HUMAN EVOLUTION
 7.1. The taxonomic position of humans in the animal kingdom
 7.2. Extinct and extant hominids

 7.3. Important hominid features

 7.4. The evolutionary relationships among the hominids
 7.5. Migration of hominids out of Africa
 7.6. The origin of Homo sapiens (single versus multiregional origi
n)
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 1.1 Definition and scope of evolution
 “Evolution” in its most general sense, is the inference that th
e universe has changed over time - that stars and galaxies and pl
anets and living things on Earth are different now than they were
in the past.
 In biology, evolution is the principle that all life is related t
hrough descent with modification from common ancestors.
 Organic evolution or biological evolution is often used to distin
guish this meaning from other usages.
 Charles Darwin used the term "descent with modification” to ref
er to biological evolution.
 This definition encompasses:
 small-scale evolution (changes in gene frequency), &
 large-scale evolution (descent of d/t species from a common ance
stor) 5
 Biological evolution is not simply a matter of change over time.
 For example, trees lose their leaves, mountain ranges rise and erode, but they are not
examples of biological evolution because they don't involve descent through genetic
inheritance.

Leaves on trees change color and fall over several weeks. Mountain ranges erode over millions of years.

A genealogy illustrates change with Over a large number of years, evolution produces tremendous diversity in forms
inheritance over a small number of years. of life.

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 The central idea of biological evolution is that all life on Earth shares a c
ommon ancestor, just as you and your cousins share a common grandmother.

 The development of the modern theory of evolution

began with the introduction of the concept of natural
selection in a joint 1858 paper by Charles Darwin
and Alfred Russel Wallace.
 This theory achieved a wider readership in Darwin's
1859 book, The Origin of Species.
 Today, evolution is an extremely active field of research.
Russel Wallace Charles
 Understanding evolution is essential to achieving a full
Darwinunderstanding of the
variety, relationships, & functioning of living things.
 “Nothing in biology makes sense except in the light of evolution” Theodosiu
s Dobzhansky (1973). 7
 1.2. Why we study evolution?
 Over time, two observations have proved to be perplexing.

 The 1st of these has to do with the diversity of life:

 Why are there so many different kinds of plants and animals?

 The number of species on Earth is estimated to be more than 5 million.

 The 2nd question involves the inverse of life’s diversity, similarities

among organisms:

 How can the similarities among organisms be explained?

 Humans have always noticed the similarities among closely related species.

 Even distantly related species share many anatomical and functional chara
cteristics.

 From bacteria to humans, all living things use the same biochemical machi
neries. 8
 The concept of biological evolution addresses both of these fundamental question
s.

 Organisms in nature typically produce more offspring than can survive and reprod
uce given the constraints of food, space, and other resources in the environment.

 These offspring often differ from one another in ways that are heritable - that
is, they can pass on the differences genetically to their own offspring.

 If competing offspring have traits that are advantageous in a given environment,

they will survive and pass on those traits.

 As differences continue to accumulate over generations, populations of organisms

diverge from their ancestors and they can be considered as different species.

 Though humans, fish, and bacteria would seem to be so different, they all share
some of the characteristics of their common ancestors.

 Thus, the millions of different species of plants, animals, and microorganisms t

hat live on earth today are related by descent from common ancestors—like dista
nt cousins.

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 Evolution also explains many features of the physical world we inhabit:

 Life has played an important role in altering the planet’s physical enviro
nment.

 For example, composition of our atmosphere is partly a consequence of livin

g systems.

 Studying evolution also has great practical value:

 Evolution explains why many human pathogens have been developing resistance.

 Evolutionary biology has contributed to agricultural advances by explaining

the relationships among wild and domesticated plants and animals and their
natural enemies.

 Understanding of evolution has been essential in finding and using natural

resources, such as fossil fuels, and it will be crucial as human societies
strive to establish sustainable relationships with the natural environment.
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 2.1. Evolutionary and non evolutionary ideas before Darwin
 The emergence of scientific transformism in the nineteenth century was the pro
duct of a complex historical development of theories about the nature of organ
ic life, the classification of forms, the relation of time to the world order,
and the relation of the natural world to theories of origin.
 Georges Cuvier (1769–1832): had a brilliant insight into the nature of fossi
ls by noticing the strong connection between an animal’s diet and its anatomy.

 The bodies of living carnivores were entirely designed to capture and devour a
nimal prey, he wrote, and this gave them a structure entirely different than t
hat of plant-eating animals.
 Fossil animals should obey the same principles, so Cuvier began an intensive s
tudy of the new finds.
 He became an expert of looking at single teeth or fragments of bones and drawi
ng conclusions about their origins.
 Cuvier was equally interested in geology.
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 Jean-Baptist Lamarck (1744–1829): Cuvier’s older colleag
ue is a curious figure of history who is remembered best f
or being wrong; his name usually appears as a footnote to
the story of Charles Darwin and evolution.

 Lamarck made the famous claim that giraffes got their long
necks by stretching for the most luscious leaves, high in
the treetops.

 He believed that each generation passed its slightly elong

ated neck to the next, finally achieving the towering heig
ht of today’s animals.

 Lamarck’s assumptions were wrong, but his theory that spe

cies could transform themselves was extremely influential.

 Charles Lyell (1797–1875): In the early 19th century, Hut

ton’s hypothesis that the surface of the Earth had been r
eshaped by natural forces over long periods of time—possi
bly including the biblical flood—had spread among the sci
entific community.

 But creationists still had the upper hand on most of the f

ield of geology.
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 Lyell, who would later become one of Charles Darwin’s closest friends and
a strong supporter of evolution, changed that.

 He became convinced of Hutton’s theories through careful studies of volcan

oes in Scotland and France.

 Lyell proposed that a volcanic event could simultaneously raise masses of l

and and depress surrounding areas, perhaps as much as hundreds of feet.

 Such upheavals could significantly alter a coastline, quickly making freshw

ater lakes of areas that had been ocean and vice versa.

 That, in turn, would explain why fossils of freshwater and ocean creatures
could be found in alternating strata.

 Lyell presented his ideas on the formation of the Earth’s surface in a ser
ies of books entitled Principles of Geology, published in the 1830s.

 Robert FitzRoy (1805–65), captain of the HMS Beagle, gave Charles Darwin t
he first volume as a present when the young naturalist joined the ship’s c
rew.

 His ideas had a strong influence on Darwin. 13

 2.2. Darwin’s Theory (1859)
 From the 16th century onward, English ships traveled to the far reaches
of the globe on expeditions of discovery and colonization.
 At the beginning of the 19th century, scientists often accompanied naval
voyages in hopes of finding plants that could be sources of new foods an
d textiles or other valuable resources.
 Robert FitzRoy, captain of the British navy ship Beagle, took Charles Da
rwin in his journey of an expedition to South America.
 During his five years on the Beagle, Darwin investigated everything: geo
logical formations, the physical characteristics of plants and animals,
the origins of coral, the social behavior of insects and other creatures.
 Everywhere the Beagle sailed, Darwin witnessed how completely species we
re dependent on each other and their environments.
 The Galapagos archipelago (a volcanic cluster of rocks several hundred m
iles off the west coast of South America) has been called “evolution’s
laboratory” because its special geographical situation simplified the p
roblem of the development of new species to just a few variables.
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 Figure. The route taken by the HMS Beagle from 1831–1836
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 The Galapagos were far from the mainland, so they were rarely influenced
by the arrival of new species from the outside.
 At the same time, they were just far enough from each other to isolate s
ome of their populations, which is a key factor in evolution.
 The Galapagos were full of species of birds, lizards, and turtles so sim
ilar to each other that without careful study it was hard to tell them a
part.
 Alferd Russel Wallace (1823–1913): Like Darwin, Wallace had almost univ
ersal interests.
 The incredible variety of highly similar species in Spice Islands, a lab
yrinth that stretches between Indonesia and Malaysia, which had never be
en thoroughly explored struck Wallace just as it had Darwin in the Galap
agos.
 In 1855 he wrote a paper called “On the Law Which Has Regulated the Int
roduction of Species,” published in the Annals and Magazine of Natural
History.
 In it he stated that species arose through gradual change.
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 This map from Wallace's 1876 book shows his Oriental biogeograph
ic region.
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 In 1858, Wallace packed his new ideas into a paper called “On the Tend
ency of Varieties to Depart Indefinitely from the Original Type ” and s
ent it to Charles Darwin.

 The next year (1859), Darwin finished and published his book On the Ori
gin of Species by Means of Natural Selection, which spelled out the the
ory in detail and presented an enormous amount of evidence in support o
f it.

 Darwin proposed that evolution could be explained by the differential s

urvival of organisms following their naturally occurring variation—a p
rocess he termed "natural selection" .

 According to this view, the offspring of organisms differ from one anot
her and from their parents in ways that are heritable—that is, they ca
n pass on the differences genetically to their own offspring.

 Furthermore, organisms in nature typically produce more offspring than

can survive and reproduce given the constraints of food, space, and oth
er environmental resources.
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 If a particular offspring has traits that give it an advantage in a part
icular environment, that organism will be more likely to survive and pas
s on those traits.

 As differences accumulate over generations, populations of organisms div

erge from their ancestors.

 The most serious difficulty facing Darwin's evolutionary theory was the
lack of an adequate theory of inheritance that would account for the pre
servation through the generations of the variations on which natural sel
ection was supposed to act.

 Ernst Haeckel (1834-1919): As soon as Darwin and Wallace made their idea
s public, embryologists such as Ernst Haeckel began trying to understand
development in terms of evolution.

 Haeckel believed that evolution offered an explanation, and he formulate

d a hypothesis called recapitulation: As an individual organism develops
(ontogeny), it replays the evolutionary history of its species ( phylogen
y).

 All life began as a single cell; so does an individual.

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 August Weismann (1834–1914): A German biologist who disproved Lamarck’s co
ncept of the inheritance of acquired traits.
 He cut off the tails of mice for generation after generation. No matter how
long he did so, none of his animals gave birth to a litter of mice lacking t
ails.
 The experiment has been repeated by many other scientists & has always led t
o the same conclusion.
 Hugo de Vries (1848–1935): rediscovered the 3:1 ratio and proved that the p
rinciples held for 20 other plants, drawing conclusions almost identical to
those of Mendel.
 In 1901 de Vries proposed an important mechanism for variation: a sudden, di
scontinuous change which he called mutation.
 William Bateson (1861–1926): believed that understanding Mendel’s principl
es might provide a way of linking heredity to evolution.
 He used chickens to test whether the laws also worked in animals and found t
hat they did.
 More work turned up traits that did not obey the 3:1 ratio; particular color
s in flower petals, for example, sometimes appeared only once in 16 flowers.
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 Bateson had Mendel’s paper translated into English so that it could be read
by many more scientists, and in 1902 he published a book called Mendel’s Pr
inciples of Heredity: A Defence.
 However, Mendelism was not universally accepted in the early twentieth centu
ry.

 Karl Pearson was one of the leading figures who rejected Mendelism.

 Though Mendelism eventually allowed a revival of Darwin’s theory, the early

Mendelians such as Hugo de Vries and William Bateson (1900-20), all opposed
Darwin’s theory of natural selection.

 2.3. The modern synthesis (Neo-Darwinism)

 By the second decade of the twentieth century, within the theory of evolutio
n, the main problem was to reconcile the Mendelian theory of genetics with t
he biometrician’s description of continuous variation in real populations.

 This reconciliation was achieved by several authors in many stages.

 The theoretical work was mainly done, independently, by R.A. Fisher, J.B.S.
Haldane, and Sewall Wright.
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 Their synthesis of Darwin’s theory of natural selection with the
Mendelian theory of heredity established what is known as neo-Darw
inism, or the synthetic theory of evolution, or the modern synthes
is, after the title of a book by Julian Huxley, Evolution: the Mod
ern Synthesis (1942).
 Darwin’s theory now possessed what it had lacked for half a centu
ry: a firm foundation in a well tested theory of heredity.
 The ideas of Fisher, Haldane, & Wright are known mainly from their
 works published
Fisher written his
around 1930.
book The Genetical Theory of
Natural Selection in 1930.
 Wright published a long paper on “Evolution in
Mendelian populations” in 1931. Fisher
 Haldane published a more popular book, The Causes
of Evolution, in 1932.
 These classic works demonstrated that natural selection
could work with the kinds of variation observable in Wright
natural populations and the laws of Mendelian
inheritance. Haldane 22
 2.4. Development within the modern synthesis (1960s – present)
 The reconciliation between Mendelism and Darwinism soon inspired new genetic resear
ches.

 Today, the modern synthesis incorporates our expanding knowledge in genetics, syste
matics, paleontology, developmental biology, behavior, and ecology.

 It explains Darwin’s observation of variation among offspring in terms of mutation.

 Mutations provide the genetic variability on which natural selection acts during ev
olution.

 The modern synthesis has dominated the thinking and research of biologists working
in many areas and has resulted in an enormous accumulation of new discoveries that
validate evolution by natural selection.

 Most biologists not only accept the basic principles of modern synthesis but also t
ry to better understand the causal processes of evolution.

 For example, what is the role of chance in evolution ? How rapidly do new species ev
olve? These and other questions have arisen in part from a re-evaluation of the fos
sil record and in part from new discoveries in molecular aspects of inheritance.
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 Motoo Kimura (1968, 1983) thought that the rate of molecular
evolution, and the amount of molecular variation, was too hi
gh for a process driven by natural selection.
 Kimura introduced the neutral theory of molecular evolution.

 Evolution should explain both life on Earth today and its hi

story, and fossils have been the starting point for some con
troversies within the theory.
 So few fossils have survived that they provide only a tiny g
limpse into the past; even so, if evolution was a slow, stea
dy process, as Darwin believed, they ought to reflect that.
 Yet there seemed to have been epochs, hundreds of millions o
f years ago, in which “bursts” of evolution took place.
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 To account for this, in 1972 Stephen Jay Gould and Niles El
dredge, two American paleontologists, proposed a concept ca
lled punctuated equilibrium.

 However, the new science of molecular biology showed that D

arwin had been right after all: evolution usually happens t
hrough incremental changes.
 A vast body of scientific evidence supports evolution, incl
uding observations from the fossil record, comparative anat
omy, biogeography, developmental biology, and molecular bio
logy.
 In addition, evolutionary hypotheses are increasingly being
tested experimentally.
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 The question of the origin of life is not easy to answer since it is impossi
ble to go back in time and observe life beginnings nor are there any witness
es.

 3.1. Special creation theory

 Life forms may have been put on earth by supernatural or divine forces.

 The hypothesis of special creation that a divine God created life is at the
core of most major religions.

 3.2. Spontaneous generation theory

 Life may have evolved from inanimate matter, as association among molecules
became more and more complex.

 Most scientists tentatively accept this hypothesis of spontaneous origin.

 In this view, the force leading to life was selection.

 As changes in molecules increased their stability and caused them to persist

longer, these molecules could initiate more and more complex associations, c
ulminating in the evolution of cells.
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 3.3. Steady-state theory
 The Steady State theory is a now-obsolete theory and model alternative to th
e Big Bang theory of the universe's origin.
 In steady state views, new matter is continuously created as the universe ex
pands.
 3.4. Cosmozoan theory
 Life may have an extraterrestrial origin.
 Life may not have originated on earth at all; instead, it may have infected
earth from some other planet.
 3.5. Biochemical evolution
 In the late 1920s the astronomer Edwin Hubble made a very interesting and im
portant discovery that the universe is expanding.
 Hubble's hypothesis of an expanding universe leads to certain deductions.
 One is that the universe was more condensed at a previous time.
 From this deduction came the suggestion that all the currently observed matt
er and energy in the universe were initially condensed in a very small and i
nfinitely hot mass.
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 A huge explosion, known as the Big Bang, then sent matter and energy expanding
in all directions.
 As the universe expanded, according to current scientific understanding, matter
collected into clouds that began to condense and rotate, forming the forerunner
s of galaxies.
 Within galaxies, including our own Milky Way galaxy, changes in pressure caused
gas and dust to form distinct clouds.
 In some of these clouds, where there was sufficient mass and the right forces,
gravitational attraction caused the cloud to collapse.
 If the mass of material in the cloud was sufficiently compressed, nuclear react
ions began and a star was born.
 Some proportion of stars, including our sun, formed in the middle of a flattene
d spinning disk of material.
 In the case of our sun, the gas and dust within this disk collided and aggregat
ed into small grains, and the grains formed into larger bodies called planetesi
mals ("very small planets"), some of which reached diameters of several hundred
kilometers.
 In successive stages these planetesimals coalesced into planets and their numer
ous satellites.
 The rocky planets, including Earth, were near the sun, and the gaseous planets
were in more distant orbits.
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 The best estimate of Earth's age and of the solar system i
s 4.54 billion years.
 Origins of life cannot be dated as precisely, but there is
evidence that bacteria-like organisms lived on Earth 3.5 b
illion years ago, and they may have existed even earlier,
when the first solid crust formed, almost 4 billion years
ago.
 There are many models for the origin of life, all based on
an understanding of how the simplest living organisms toda
y operate.
 The first ‘modern’ model for the origin of life was pres
ented in the 1920s independently by two scientists, A. I.
Oparin (1894–1980) and J. B. S. Haldane (1892–1964).

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 Oparin and Haldane share the distinction of being independ
ent co-founders of the so-called biochemical theory for th
e origin of life.
 According to the Oparin–Haldane model, life could have ar
isen through a series of organic chemical reactions that p
roduced ever more complex biochemical structures.
 They proposed that common gases in the early Earth atmosph
ere combined to form simple organic chemicals, and that th
ese in turn combined to form more complex molecules.
 Then, the complex molecules became separated from the surr
ounding medium, and acquired some of the characters of liv
ing organisms.
 They became able to absorb nutrients, to grow, to divide
(reproduce), and so on.
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 The Oparin–Haldane model was not tested until the 1950s.

 In 1953, Stanley Miller (1920–2007), made a model of the

Precambrian atmosphere and ocean in a laboratory glass vessel.

 He exposed a mixture of nitrogen, hydrogen, methane, ammonia, and water to e

lectrical sparks, to mimic lightning, and found a brownish sludge containing
a variety of simple organic compounds such as amino acids in the bottle afte
r a few days.

 So Miller had apparently recreated the first two steps in the Oparin–Haldan
e model, mixing the basic elements to produce simple organic compounds, and
then combining these to produce the building blocks of proteins and nucleic
acids.

 Oxygen in the Earth’s atmosphere is thought to have evolved late, after the
activity of microorganisms had significantly changed the environment.

 “Oxygen-breathing” organisms could only survive once that had happened.

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 Figure. Stanley Miller’s prebiotic soup experiment .
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 Since Darwin's time, massive evidence has accumulated supporting the fact of evolut
ion that all living organisms present on earth today have arisen from earlier forms
in the course of earth's long history.
 The scientific community has discovered that no known hypothesis other than univers
al common descent can account scientifically for the unity, diversity, and patterns
of life forms.
 Many of the predictions of common descent have been confirmed from independent area
s of science.
 4.1. Paleontological evidences
 Fossils are recognizable fragment, or an impression left by an animal or plant pres
erved.
 Fossils formed only if some matter covered it and protected from carrion eaters, mi
crobes and others.
 The fossil record provides important evidence for evolution.
 Hundreds of thousands of fossil organisms, found in well-dated rock sequences, repr
esent successions of forms through time and manifest many evolutionary transitions.
 Microbial life of the simplest type was already in existence 3.5 billion years ago.
 The oldest evidence of more complex organisms (i.e, Eukaryotic cells, which are mor
e complex than bacteria) has been discovered in fossils sealed in rocks approximate
ly 2 billion years old.
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 • Cast fossil • Amber

• Body fossil

• Petrified wood
• Fossil mold

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 Multicellular organisms, which are the familiar fungi, plants, a
nd animals, have been found only in younger geological strata.

 So many intermediate forms have been discovered between fish and

amphibians, between amphibians and reptiles, between reptiles an
d mammals, and along the primate lines of descent that it often
is difficult to identify categorically when the transition occur
s from one to another particular species.

 Actually, nearly all fossils can be regarded as intermediates in

some sense; they are life forms that come between the forms that
preceded them and those that followed.

 The fossil record thus provides consistent evidence of systemati

c change through time—of descent with modification.

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36
From this huge body of evidence, it can be predicted that no reversals will be found in
future paleontological studies.
•That is, amphibians will not appear before fishes, nor mammals before reptiles, and no
complex life will occur in the geological record before the oldest eukaryotic cells.
•This prediction has been upheld by the evidence that has accumulated until now: no
reversals have been found.
•What would make you change your mind about Evolution?
The famous biologist, J.B.S. Haldane, answered, “Find me a rabbit fossil in Pre-
Cambrian Rock.”

(nobody has!)
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 Geological time scale: sequence of geological periods
formulated by comparing many different sites.

 These periods are grouped into eras with boundaries be

tween the eras marking major transitions in the life f
orms fossilized in the rocks.

 Periods within each era are subdivided into shorter i

ntervals called epochs.

 The divisions are not arbitrary, but are associated wi

th boundaries that correspond to times of change.

 This record of the rocks presents a history of the rel

ative age of fossils, showing the order in which speci
es groups evolved.
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39
 By the mid-19th century geologists had wo
rked out the major geological periods in
the history of the Earth and assembled di
agrams like this one showing strata in th
eir historical order. Each layer containe
d a unique set of fossils, which helped b
iologists understand how the types and fo
rms of life had changed over immense stre
tches of geological time.

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 Dating of Fossils: There are two types of methods to determine the age of fossils: rel
ative dating and absolute dating.
 Relative dating: is the inference of the age of fossils from that of surrounding rocks.
 Absolute Dating: gives the age in years rather than in relative terms (e.g., before, a
fter).
 Radiometric dating determines the age of rocks and fossils on an absolute time scale.
 Fossils usually contain isotopes of elements that accumulated in the living organisms.
 Since each radioactive isotope has a fixed half-life (the number of years it takes for
50% of the original sample to decay), it can be used to date fossils by comparing the
ratio of certain isotopes (e.g., 14C, and 12C) in a living organism to the ratio of the
same isotopes in the fossil.
 For example, carbon-14 has a half-life of 5600 years, meaning that one-half of the car
bon-14 in a specimen will be gone in 5600 years; half of the remaining carbon-14 would
disappear from the specimen in the next 5600 years; this would continue until all of t
he carbon-14 had disappeared.
 Thus, a sample beginning with 8g of carbon-14 would have 4g left after 5600 years and
2g after 11,200 years.
 However, carbon-14 is useful in dating fossils less than 50,000 years old due to its r
elatively short half-life.
 Thus, Paleontologists use other radioactive isotopes with longer half-lives to date ol
der fossils.
 For example, Uranium-238 has a half-life of 4.5 billion years and is reliable for dati
ng rocks (and fossils within those rocks) hundreds of millions of years old.

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  4.2. Biogeographical evidences
 Biogeography (distribution of species) also has contributed evidence for
descent from common ancestors.

 Some species, such as human beings and our companion, the dog, can live u
nder a wide range of environments.

 Others are amazingly specialized. For example, one species of a fungus (L

aboulbenia) grows exclusively on the rear portion of the covering wings o
f a single species of beetle (Aphaenops cronei) found only in some caves
of southern France.

 How can we make clear the existence of such extraordinary, seemingly unus
ual creatures as the fungus described above?

 Why are island groups like the Galápagos so often inhabited by forms simi
lar to those on the nearest mainland but belonging to different species?

 Evolutionary theory explains that biological diversity results from the d

escendants of local or migrant predecessors becoming adapted to their div
erse environments.
42
 This explanation can be tested by examining present species and local fossils
to see whether they have similar structures, which would indicate how one is d
erived from the other.

 There should also be evidence that species without an established local ancest
ry had migrated into the locality.

 Wherever such tests have been carried out, these conditions have been confirme
d.

 Drifting of continents is a major geographical factor correlated with the spat

ial distribution of life.

 At one time, all the continents occurred as one large continuous land mass cal
led Pangea.

 A continental drift theory suggests that in the paleozoic and much of the meso
zoic era, there were two large continents, Laurasia in the northern and equato
rial region, and Gondwana in the south.

 During the Cretaceous period, these continents shifted north ward and break up:
Laurasia into much of Eurasia and North America, and Gondwana into Antarctica,
Australia, India, Arabia, Africa, and South America.

 This isolated the fauna and flora occupying different plates.

43
 In the 20th C, scientists recognized that biogeography has been far more dynamic over the c
ourse of life's history.

 In 1915 the German geologist Alfred Wegener was struck by the fact that identical fossil p
lants and animals had been discovered on opposite sides of the Atlantic.

 Since the ocean was too far for them to have traversed on their own, Wegener proposed that
the continents had once been connected.

 Wegener found that the distributions of fossils of several organisms supported his theory
that the continents were once joined together.

 In the 1960s, scientists were able to demonstrate the mechanism that made continental drif
t possible- plate tectonics. 44
 Figure. Migrating contin
ents.
 As the Earth’s major pl
ates move relative to ea
ch other, continents rid
e passively on them.
 Through Earth history, t
he form of landmasses ha
s changed constantly, so
metimes with dramatic im
pact on the biota living
on them.

45
 Figure. The Earth's crust has been found to be composed of several dist
inct plates.
46
 The biogeographical realms were formed and divergence of organisms in the different re
alms continued.
 The evidence that Darwin found for the influence of geographical distribution on the e
volution of organisms has become stronger with advancing knowledge.
 For example, approximately 2,000 species of flies belonging to the genus Drosophila ar
e now found throughout the world. About one-quarter of them live only in Hawaii.
 More than a thousand species of snails and other land mollusks also are found only in
Hawaii.
 The explanation for the multiplicity of related species in remote localities is that s
uch great diversity is a consequence of their evolution from a few common ancestors th
at colonized an isolated environment.
 The Hawaiian Islands are far from any mainland or other islands, and on the basis of g
eological evidence they never have been attached to other lands.
 Thus, the few colonizers that reached the Hawaiian Islands found many available ecolog
ical niches, where they could, over numerous generations, undergo evolutionary change
and diversification.
 No mammals other than one bat species lived in the Hawaiian Islands when the first hum
an settlers arrived; similarly, many other kinds of plants and animals were absent.
 The explanation for the absence of many kinds of organisms, and the great multiplicati
on of a few kinds, is that many sorts of organisms never reached the islands, because
of their geographic isolation.
 Those that did reach the islands diversified over time because of the absence of relat
ed organisms that would compete for resources.
47
  4.3. Classification
 The taxonomic system used today was developed by Linnaeus in the eighteenth century.
 The two main objectives of taxonomy are to sort out and identify closely related species and
to order species into the broader taxonomic categories.
 The goal of systematics is to have classification reflect the evolutionary affinities of the
species.
 The taxonomic hierarchy is set up to fit evolutionary history.
 In general, groups subordinate to other groups in the taxonomic hierarchy should represent fi
ner and finer branching of phylogenetic tree.
 Classification schemes and phylogenetic trees are hypotheses of history based on current data.

 Therefore, phylogenetics is the scientific discipline concerned with describing and reconstru
cting the patterns of genetic relationships among species and among higher taxa.
 Phylogenetic trees are a convenient way of visually representing the evolutionary history of
life.
 The phylogenetic trees illustrate the inferred relationships between organisms and the order
of speciation events that led from earlier common ancestors to their diversified descendants.
 If modern species have descended from ancestral ones in this tree-like, branching manner, it
should be possible to infer the true historical tree that traces their paths of descent.

48
49
  4.4. Plant and Animal Breeding
 Crossing two different kinds of organisms sometimes produce offspring whic
h are unlike either parent.
 Some hybrids, such as mules and hybrid corn, have characteristics not only
different from the parents, but in some way better adapted.
 Similar selections have led to our present varieties of apples, wheat, ros
es, chickens, cattle and horses.
 If man can establish varieties from a common stem, certainly natural selec
tion can also produce varieties.
 Moreover, nature has had much more time than man has, and could well produ
ce different species, genera, families, and so on.
 Artificial selection operating over a long period of time can give rise to
varieties markedly different from the starting generation.
 The above evidence clearly shows that the characters of populations are no
t rigidly fixed, but rather can be altered through breeding experiments.
 If human beings are able to alter the characteristics of organisms in orde
r to produce new varieties, then it can be argued that nature can do even
much more through natural selection.
50
51
 4.5. Comparative Anatomy and Morphology
 Inferences about common descent derived from paleontology are reinforced
by comparative anatomy.
 For example, the skeletons of humans, mice, and bats are strikingly simi
lar, despite the different ways of life of these animals and the diversi
ty of environments in which they flourish.
 The correspondence of these animals, bone by bone, can be observed in ev
ery part of the body, including the limbs.
 Scientists call such structures homologies and have concluded that they
are best explained by common descent.
 Comparative anatomists investigate such homologies, not only in bone str
ucture but also in other parts of the body, working out relationships fr
om degrees of similarity.
 Their conclusions provide important inferences about the details of evol
utionary history, inferences that can be tested by comparisons with the
sequence of ancestral forms in the paleontological record.

52
 4.6. Adaptive Radiation
 Adaptive radiation is the evolution of many diversely adapted species from a com
mon ancestor.
 The evolution of modern life has included long, relatively sluggish periods punc
tuated by briefer intervals of more extensive turnover in species composition.
 These intervals of extensive turnover included explosive adaptive radiations of
major taxa as well as mass extinctions.
 The evolution of some novel characteristics opened the way to new adaptive zones
allowing many taxa to diversify greatly during their early history.
 For Example, evolution of wings allowed insects to enter an adaptive zone with a
bundant new food sources and adaptive radiation resulted in hundreds of thousand
s of variations on the basic insect body plan.
 Examples of adaptive radiation are the endemic species of the Galapagos Island w
hich descended from small populations which floated, flew, or blown from South A
merica to the islands.
 Darwin’s finches can be used to illustrate a model for such adaptive radiation
on island chains.
 A single dispersal event may have seeded one island with a peripheral isolate of
the ancestral finch which diverged as it underwent allopatric speciation.
53
 A few individuals of these new species may have reached neighboring islands, f
orming new peripheral isolates which also speciated.
 After diverging on the island it invaded, a new species could recolonize the i
sland from which its founding population emerged and coexisted with the ancest
ral species or form still another species.
 Multiple invasions of islands are distant enough from each other to permit geo
graphic isolation, but near enough for occasional dispersal.

 The different species of finches on the Galápagos Islands, now known as Darwi
n's finches, have different-sized beaks that have evolved to take advantage of
distinct food sources.
54
 4.7. Evidences from Embryology
 Embryology, the study of biological development from the time of conception, i
s another source of independent evidence for common descent.

 Barnacles, for instance, are sedentary crustaceans with little apparent simila
rity to such other crustaceans as lobsters, shrimps, or copepods.

 Yet barnacles pass through a free-swimming larval stage in which they look lik
e other crustacean larvae.

 The similarity of larval stages supports the conclusion that all crustaceans h
ave homologous parts and a common ancestry.

 Similarly, a wide variety of organisms from fruit flies to worms to mice to hu

mans have very similar sequences of genes that are active early in development.

 These genes influence body segmentation or orientation in all these diverse gr

oups.

 The presence of such similar genes doing similar things across such a wide ran
ge of organisms is best explained by their having been present in a very early
common ancestor of all of these groups.
55
56
 4.8. Evidences from Molecular Biology
 The unifying principle of common descent that emerges from all the foregoing lines of
evidence is being reinforced by the discoveries of modern biochemistry and molecular
biology.

 The code used to translate nucleotide sequences into amino acid sequences is essentia
lly the same in all organisms.

 Moreover, proteins in all organisms are invariably composed of the same set of 20 ami
no acids.

 This unity of composition and function is a powerful argument in favor of the common
descent of the most diverse organisms.

 As the ability to sequence the nucleotides making up DNA has improved, it also has be
come possible to use genes to reconstruct the evolutionary history of organisms.

 Because of mutations, the sequence of nucleotides in a gene gradually changes over ti

me.

 The more closely related two organisms are, the less different their DNA will be.

 Because there are tens of thousands of genes in humans and other organisms, DNA conta
ins a tremendous amount of information about the evolutionary history of each organis
m.
57
58
 4.9. Evidences from contemporary processes
 Evolution by natural selection is not only a historical process—it still operate
s today.
 For example, the continual evolution of human pathogens has come to pose one of t
he most serious public health problems now facing human societies.
 Many strains of bacteria have become increasingly resistant to once-effective ant
ibiotics as natural selection has amplified resistant strains that arose through
naturally occurring genetic variation.
 The microorganisms that cause malaria, gonorrhea, tuberculosis, & other diseases
have demonstrated greatly increased resistance to the antibiotics & other drugs u
sed to treat them in the past.
 The continued use and overuse of antibiotics has had the effect of selecting for
resistant populations because the antibiotics give these strains an advantage ove
r nonresistant strains.
 Similar episodes of rapid evolution are occurring in many different organisms.
 Rats have developed resistance to the poison warfarin.
 Many hundreds of insect species and other agricultural pests have evolved resista
nce to the pesticides used to combat them—and even to chemical defenses genetica
lly engineered into plants.
 Species of plants have evolved tolerance to toxic metals and have reduced their i
nterbreeding with nearby non-tolerant plants—an initial step in the formation of
separate species.
 New species of plants have arisen through the crossbreeding of native plants with
plants introduced from elsewhere in the world.
59
 No longer appropriate to claim that Darwin's theory of natural selection
is the best theory of the mechanism of evolution.
 The classic view has been replaced by a new concept called modern synthe
sis, which includes several other mechanisms in addition to natural sele
ction,.
 The modern synthesis understands evolution to be a change in the frequen
cy of alleles within a population from one generation to the next.
 5.1. Populations and demes
 Population is a group of organisms of the same species occupying a parti
cular space at a particular time.
 Deme (local population) is a group of organisms of the same species wher
e breeding is random.
 Population is a collection of demes with strong connections between adja
cent demes.

60
 5.2. Genetic variation
 The members of natural populations vary with respect to characteristics
at all levels.
 They differ in their morphology, their microscopic structure, their chro
mosomes, the amino acid sequences of their proteins, and in their DNA se
quences.
 The genetic variation among individuals at different levels can be inves
tigated by employing a variety of genetic markers.
 A genetic marker is a measurable character that can detect variation in
a DNA sequence.
 Three types of genetic markers namely: morphological, biochemical (prote
in/allozyme) and molecular (DNA) have been developed to assess genetic v
ariation among individuals.
 Genetic variation is the raw material for evolutionary change.

61
 5.3. Sources of genetic variation
 Evolution consists of two basic types of processes: those that introduce new genetic
variation into a population, and those that affects the frequencies of existing vari
ation.
 Mutation, recombination, and gene flow can create genetic variation in a population.

 Selection and genetic drift affect existing variation.

 The magnitude and distribution of genetic diversity within a species depends on the
effects and interactions of several evolutionary forces over the long evolutionary h
istory of the species.
 5.4. Gene pool
 Gene pool is the sum total of genes of all the individuals of a Mendelian population.
 A Mendelian population or genetic population is a group of sexually interbreeding or
ganisms.
 If the gene pool of a population is described completely, it tells us not only the k
inds of genes present in the population but also the proportions of the different ki
nds of genes and the way in which these are distributed among the individuals of the
population in question.
62
 5.5. Allele frequency
 Allelic frequency refers to the proportion of an allele in the gene pool as comp
ared with other alleles at the same locus, with no regard to their distribution
in organisms.
 For example, in a hypothetical population with two alleles A & a on a particular
locus, will have three types of individuals: ¼ AA, ½ Aa, and ¼ aa.
 Allelic frequencies may be calculated in either of the two ways: from observed n
umber of different genotypes or from the genotypic frequencies.
 Allelic frequencies from observed number of different genotypes:

 Suppose there are 100 individuals in a population out of which there are 40 homo
zygous dominant (AA), 40 heterozygous (Aa) and 20 homozygous recessive (aa), the
n:-
80  40 40  40
 Frequency of allele A will200be  0.6 , and frequency
200
of0.4allele a w

ill be
 Allelic frequencies from genotypic frequencies:
Allelic frequency = frequency of the homozygote genotype + ½ frequency of the heterozygote genotype

 If the frequency of allele A is represented by p and that of allele a by q, then

at gene equilibrium condition their total frequency is one (i.e., p + q =1).
63
 5.6. Genotypic frequency
 Genotype frequency is the total number of a kind of individuals from a populati
on all of which exhibit similar genotype with respect to the locus under consid
eration.
 Suppose in a population there are two alleles at one gene locus (A & a) and the
y are related as dominant and recessive.
 Naturally, three kinds of individuals- homozygous dominant, heterozygous, and h
omozygous recessive will occur in the population.
 If N = total number of individuals in the population, D = number of homozygous
dominants, H = number of heterozygotes, R = number of homozygous recessives, th
en
 Genotype frequency of AA individuals = D/N
 Genotype frequency of Aa individuals = H/N
 Genotype frequency of aa individuals = R/N
 Thus, genotype frequency for a particular type of gene combination on the same
locus can be determined by dividing the number of individuals with that genotyp
e by the total number of individuals in the population.
 Note that the total genotypic frequency should be one (i.e., D/N + H/N + R/N =
1).
64
 5.7. The Hardy-Weinberg equation
 Based on Mendel's principles of inheritance, Hardy and Weinberg developed the concept th
at is known today as the Hardy-Weinberg Principle, which states: "In a large, randomly b
reeding (diploid) population, genotype and allelic frequencies will remain the same from
generation to generation; assuming no unbalanced mutation, gene migration, selection or
genetic drift."
 When a population meets all of these conditions, it is said to be in Hardy-Weinberg equi
librium.
 This equilibrium can be mathematically expressed based on simple binomial (for two allel
es) or multinomial (multiple allele) distribution of the allele frequencies as: p2 + 2pq
+ q2= 1
 The Hardy-Weinberg law has three important properties:
 Allele frequencies predict genotype frequencies.
 At equilibrium, allele and genotype frequencies do not change from generation to generat
ion.
 Equilibrium is reached in one generation of random mating.
 In the Hardy-Weinberg law, the following conditions are the basic assumptions:
 The population is infinitely large (sampling errors and random effects are negligible).
 Mating within the population occurs at random.
 There is no selective advantage, no mutation, and no gene flow.
65
 5.8. Factors producing gene pool change in populations
 Hardy-Weinberg’s law provides a situation where the genes in the population
have reached the equilibrium and the gene pool is constant.
 If this is the condition naturally, there will be no change and no evolution.
 But it has been observed in nature that over a long period of time this equil
ibrium is disturbed and changes do occur on account of several forces.
 In other words, factors, which change genetic equilibrium, can bring about ev
olution.
 Mechanisms that produce the changes in allelic as well as genotypic frequency
are selection, genetic drift, mutation, non-random mating, and gene flow.
 Non-random mating
 Mating is rarely at random and the nature of mating system is an important ev
olutionary force.
 Any departure from random mating upsets the Hardy-Weinberg equilibrium distri
bution of genotypes in a population.
 A single generation of random mating will restore genetic equilibrium if no o
ther evolutionary mechanism is operating on the population.
66
 Differential sexual selections (positive and negative assortative matings) are the
evolutionary forces, resulting in the abundance of certain genotypes at the expense
of others.
 Positive assortative mating is a type of non-random mating pattern in which individ
uals mate with others who are like themselves for selected traits.
 The net effect of positive assortative mating is a progressive increase in the numb
er of homozygotes (AA & aa) and a corresponding decrease in the number of heterozyg
otes (Aa) in a population.
 Inbreeding is a form of positive assortative mating that involves preferential mati
ng between close relatives. The most extreme case of inbreeding is self fertilizati
on.
 Negative assortative mating is another type of non-random mating pattern is in whic
h individuals only select mates who are different from themselves for selective tra
its.
 The net effect is a progressive increase in the frequency of heterozygous genotypes
(Aa) and a corresponding decrease in homozygous (AA and aa) ones in a population.
 Out breeding is a form of negative assortative mating that involves preferential ma
ting between non-related individuals.
 NB. Neither positive nor negative assortative mating affects the allelic frequencie
s of a population, but they may influence the genotypic frequencies if the phenotyp
es are genetically determined.
67
 Random genetic drift
 Genetic drift is the term used in population genetics to refer to random fluctuation
s in allele frequencies due to chance events in a population due to random sampling
effects in the formation of successive generations.

 Genetic drift is an important force in evolution in small size populations and it is

not a potent evolutionary force in very large randomly mating populations.
 These gene frequencies continue to fluctuate until one allele is lost and the other
is fixed.
 The speed or intensity of genetic drift is actually determined by the effective popu
lation size (Ne).
 This may differ from the total population size (N) if some individuals do not breed.
 When the number of breeding males (Nm)  number of breeding females (Nf), the effect
ive population size can be quite different from the actual population size.
 The mathematical relation is
 If Nm = Nf = N/2, then Ne = N. But if a single male does all the mating then Ne = 4.
68
 There are several ways in which sampling error occurs in natural populations.
 For example, it may occur due to founder effect, bottleneck effect or when pop
ulation size remains continuously small over many generations.
 The Founder Effect: a small group breaks off from a larger population and form
s a new population.
 Bottleneck Effect: In many species, there have been catastrophic periods cause
d by rapid dramatic changes, during which most individuals died without passin
g on their genes.
 The few survivors of these evolutionary bottlenecks then were reproductively v
ery successful, resulting in large populations in subsequent generations.
 The consequence of this bottleneck effect is the extraordinary reduction in ge
netic diversity.
 Gene flow (Migration)
 In population genetics, the term migration is really meant to describe gene fl
ow, defined as the movement of alleles from one area (deme, population, and re
gion) to another.
 Gene flow can be a very important source of genetic variation if genes are car
ried to a population where those genes did not exist previously.
69
 It is a source of variation in the recipient population, but serves to homoge
nize the species as a whole, preventing speciation events between otherwise i
solated groups.
 Consider a single pair of alleles, A (p) and a (q).
 The change in the frequency of A in one generation can be expressed as:- ∆p =
m(pm – p)
 Where; p = frequency of A in existing population, pm = frequency of A in immigra
nts, ∆p = change in one generation, and m = coefficient of migration
 Example:
 Assume that p = 0.4 and pm = 0.6, and that 10% of the parents giving rise to t
he next generation are immigrants (m = 0.1).
 Then the changes in the frequency of A in one generation is
∆p = m(pm – p)
= 0.1(0.6 – 0.4)
= 0.02
 In the next generation, the frequency of A (p1) will increase as follows:-
p1 = p + ∆p
= 0.4 + 0.02
= 0.42
70
 Natural selection
 Natural selection is the principal force that shifts allele frequencies
within large populations and is one of the most important factors in evo
lutionary change.
 The basic concept of natural selection is that nature selects individual
s that are endowed with traits, which improve survival and reproduction,
and selects against individuals burdened by traits that are unfavorable.
 Natural selection is not random process as it preferentially selects for
different mutations based on differential fitnesses, however, it only se
lects among the existing variation already in a population.
 Natural selection is measured in terms of Darwinian fitness (w), which i
s defined as the relative reproductive ability of a genotype.
 Usually, an adaptive value (fitness) of 1 is assigned to a genotype that
produces most of the offspring.
 A related measure is the selection coefficient (s), which is the measure
of the relative intensity of selection against a genotype. Thus, s = 1-w

71
 Selective sweeps describe the effect of selection acting on linked alleles.
 It comes in two forms as background selection and genetic hitchhiking.
 Background selection occurs when a deleterious mutation is selected against
and linked mutations are eliminated along with the deleterious variant, res
ulting in lower genetic polymorphism in the surrounding region.
 Genetic hitchhiking occurs when a positive mutation is selected for, and li
nked mutations are pushed towards fixation along with the positive variant.
 Natural selection on mutations can also be stabilizing, directional or disr
uptive.
 Stabilizing or purifying selection favors average characteristics in a popu
lation, thus reducing gene variation but retaining the mean , i.e., it favor
s individuals with intermediate characteristics and genetic diversity decre
ases as the population stabilizes on a particular trait value.
 The extreme values of the character are selected against.
 Directional or positive selection occurs when characteristics lie along a p
henotypic spectrum and the individuals at one end are more successful.
 Directional selection increases frequency of a beneficial mutation, or push
es mean in either direction.
 Directional selection occurs when natural selection favors a single allele
and therefore allele frequency continuously shifts in one direction.
72
Disruptive selection favors those with extreme characteristics, i.e., it favors both ex
tremes, and results in a bimodal distribution of gene frequency.
Disruptive selection is a type of natural selection that simultaneously favors individu
als at both extremes of the distribution.
When disruptive selection operates, individuals at the extremes contribute more offspri
ng than those in the center, producing two peaks in the distribution of a particular tr
ait.

The diagram shows the three types of selection operating within populations. 0 indicate
s the original coincidence between optimum phenotype and optimum environmental conditio
ns; N indicates the new position of coincidence of between optimum phenotype and optimu
m environmental conditions. Organisms possessing characteristics in the shaded portions
of the normal distribution are at a selective disadvantage and are eliminated by select
ive pressure. 73
 Mutation
 When a cell reproduces, its genes (DNA) are physically replicated.
 Normally an exact copy of the parental DNA is produced, but sometimes
a copying error happens.
 The set of enzymes that replicate the DNA include proof-reading and r
epair enzymes.
 These enzymes detect and correct most of the copying errors, but some
errors persist even after proof-reading and repair. These errors are
called mutations.
 The new sequence of DNA that results from a mutation may code for a f
orm of protein with different properties from the original.
 Mutations can happen in any cell, but the most important mutations, f
or evolution, are those occurring in the production of the gametes.
 These mutations are passed on to the offspring, who may differ from t
heir parents because of the mutation.
74
 Mutations are considered the driving force of evolution.

 Most mutations probably do not confer a significant advantage or disadvan

tage because they occur in non-gene coding regions of DNA molecules. They
are relatively neutral in their effect.

 However, some mutations are extremely serious and can result in death bef
ore birth.

 Less favorable (or deleterious) mutations are removed from the gene pool
by natural selection, while more favorable (beneficial or advantageous) o
nes tend to accumulate.

 In a population the mutations disturb the genetic equilibrium and the gen
e frequency.

 Suppose if allele A mutates to allele a, the gene pool of the population

is altered due to a decrease in the number of A genes and increase in the
number of a genes.

 If we represent the frequency of gene A by p and its mutation rate by µ,

the change in the frequency of A in the gene pool of the population will
be ∆p = p- µp.
75
 6.1 Species Concepts
 What is a species?
 Defining the term species has been problematic for evolutionary biologists for dec
ades.

 The difficulty in determining what, exactly, a species is, arises because speciati
on is a continuous, ongoing process.

 Often, there is a continuum of variation between populations and species.

 When, in that gradual continuum populations of one species are genetically differe
nt enough to merit the distinction of being separate species is not always clear.

 A species concept is a way of defining a species, and there are more than 20 diffe
rent species concepts.

 The most famous, and the one that most biologists use today, is the biological spe
cies concept, which states that species are groups of actually or potentially inte
rbreeding populations, which are reproductively isolated from other such groups.

 This definition works well for most animals; however, it has limitations: it does
not always work with plants, and extinct species (e.g. fossils) or asexually repro
ducing species (e.g. bacteria).

76
 What is Speciation?
 Speciation is the origin of new species.

 Generally, this entails one species changing over time and eventually becoming two
species.

 It consists of the evolution of biological barriers to gene flow (reproductive iso

lation) between two populations of the same species.

 In other words, speciation is the evolution of reproductive isolation between two

populations.

 A genetic difference that brings change in the timing, location, or rituals of mat
ing could be enough.

 This change might evolve by natural selection or genetic drift.

 Most evolutionary changes consist of two types of processes: anagenesis and cladog
enesis.
 Anagenesis, also known as "phyletic change", is the evolution of species involving
a change in gene frequency in an entire population rather than a branching event.

 A key point is that the entire population is different from the ancestral populati
on such that the ancestral population can be considered extinct.
77
 Cladogenesis describes the splitting of a species into two or more group
s that subsequently diverge in their traits.
 This event usually occurs when a few organisms end up in new, often dist
ant areas or when environmental changes cause several extinctions, openi
ng up ecological niches for the survivors.
 Darwin’s finches exemplify cladogenesis, a single South American specie
s having multiplied into several species after reaching the Galapagos Is
lands.

 6.2 Modes of Speciation

 Modes of speciation are often classified according to how much the geogr
aphic separation of incipient species can contribute to reduced gene flo
w.
 TheMode of speciation
following table compares some New species formed
of these from modes.
speciation
Allopatric (allo = other, patric = place) geographically isolated populations
Sympatric (sym = same) within the range of the ancestral population
Parapatric (para = beside) a continuously distributed population

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 Allopatric Speciation
 In this mode of speciation, something extrinsic to the organisms prevents two
or more groups from mating with each other regularly, eventually causing that
lineage to speciate.

 Isolation might occur because of great distance or a physical barrier, such a

s a desert or river.

 According to this concept, spatial isolation or physical separation of breedi

ng populations of the same species is a necessary prerequisite to speciation.
 An essential element in allopatric speciation is geographic variation.

 Allopatric speciation often occurs in three steps as follows:

 1st , the populations become physically separated, often by a long, slow geolo
gical process like

 Formation of glaciers (huge mountains of ice) during the ice ages

 Continental drift

 Changes in water position and level

79
 2nd, the separated populations diverge, through changes in mating strategy or
use of their habitat.

 Three forces are playing on these samples which may lead to the uniqueness o
f these groups from the parental population:

 Genetic drift:- populations will be small for several generations, genetic

drift may take place.

 Natural selection:- physical and biological environment will be different

on the other side of the barrier, so selection will operate in different d
irections than in the parental population.

 Mutation:- mutations occur at random and these will be different in the tw

o groups.

 3rd , they become reproductively separated such that they cannot interbreed a
nd exchange genes.

 An excellent example of the result of allopatric (geographic) speciation is

seen in Darwinian finches that inhabit the Galapagos Islands.

 There are now 14 species of finches in the Galapagos Islands, differing prim
arily in feeding habits and in bill size and shape.
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 Sympatric Speciation

 Sympatric speciation is speciation without geographical isolation.

 Unlike allopatric speciation, sympatric speciation does not require large-sc

ale geographic distance to reduce gene flow between parts of a population.

 Even though, many sympatric species arise by the secondary overlap of specie
s that originally diverged geographically, it is also theoretically possible
that sympatric species can arise in one geographic area due to different con
ditions.

 Different modes of sympatric speciation , including the following, have been

observed.

 Polyploidy- one very rapid and well documented mode of sympatric speciation
is by polyploidy.

 Strong disruptive selection (Effect of microhabitat condition)- extremely st

rong disruptive selection lead to reproductive isolation between populations
of different microhabitats.

 Host preference and survival- merely exploiting a new niche may automaticall
y reduce gene flow with individuals exploiting the other niche.
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 Effects of change in behavior or change in the courtship pattern- change
s in behavior are thought to lead to reproductive isolation.

 For example, a simple change in the pheromone molecule, changes in the c

ourtship pattern could could lead to sympatric isolation.

 Parapatric Speciation

 In parapatric speciation, the new species evolve from adjacent populatio

ns, rather than completely separate ones, as in allopatric speciation.

 The full process could occur as follows:

 Initially, one species is distributed in space.

 The species evolves a “stepped cline” pattern of geographic variation.

 The stepped cline could exist because of an abrupt environmental change:

one form of the species would be adapted to the conditions on one side o
f the boundary, the other form to the conditions on the other side of th
e boundary.
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 6.3. Rate of Speciation
 In studying the fossil record, it becomes apparent that the pace of evolutio
n varies from one group of organism to another and it poses the question tha
t does evolution occur in rapid bursts or gradually?

 Regarding this question, several mechanisms were forwarded, but the major on
es are gradualism and punctuated equilibrium.

 Gradualism

 Gradualism is a model of evolution that assumes slow, steady rates of change.

 Charles Darwin's original concept of evolution by natural selection assumed

gradualism.

 Gradualism shares the notion that past changes must not be explained by invo
king unusual catastrophic events that are not observable today.

 If new species evolved in single, catastrophic events, examples should be ap

parent today and are not.
83
 Instead we observe small, continuous changes in the phenotypes present i
n natural populations.

 Such continuous changes can produce major differences among species only
by accumulating over many thousands to millions of years.

 A simple statement of Darwin’s theory of gradualism is accumulation of

quantitative changes leads to qualitative changes.

 What should we observe in the fossil record if evolution is slow and ste
ady?

 If evolution is slow and steady, we'd expect to see the entire transitio
n, from ancestor to descendent, displayed as transitional forms over a l
ong period of time in the fossil record.

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 In the above example, the preservation of many transitional forms, throu
gh layers representing a length of time, gives a complete record of slow
and steady evolution.

 In fact, we see many examples of transitional forms in the fossil record.

 For example, just a few steps in the evolution of whales from land-dwell
ing mammals, highlighting the transition of the walking forelimb to the
flipper is shown below.

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 Punctuated Equilibrium
 The punctuated equilibrium theory of Niles Eldredge and Stephen Jay Goul
d was proposed as a criticism of the traditional Darwinian theory of evo
lution.
 Eldredge and Gould observed that evolution tends to happen sometimes mov
ing very fast, sometimes moving very slowly or not at all.
 If you study the fossils of organisms found in subsequent geological lay
ers, you will see long intervals in which nothing changed ("equilibriu
m"), "punctuated" by short, revolutionary transitions, in which species
became extinct and replaced by wholly new forms.
 What would we observe in the fossil record if evolution happens in "quic
k" jumps?
 If evolution happens in "quick" jumps (100, 000 years for significant ch
anges), we would expect to see big changes happen quickly in the fossil
record, with little transition between ancestor and descendent.

86
 When evolution is rapid, transitional forms may not be preserved, even if fo
ssils are laid down at regular intervals.
 But, does a jump in the fossil record necessarily mean that evolution has ha
ppened in a "quick" jump?
 We expect to see a jump in the fossil record if evolution has occurred as a
"quick" jump, but a jump in the fossil record can also be explained by irre
gular fossil preservation.

 This possibility (irregular fossil preservation) can make it difficult to co

nclude that evolution has happened rapidly.
 In conclusion, we observe examples of both slow, steady change and rapid, pe
riodic change in the fossil record.
 Both happen, i.e., evolutionary pace may be quite erratic (irregular), fast
at times, slow at others.
87
 6.4. Development of reproductive isolating mechanisms
 Speciation, or the evolution of reproductive isolation, occurs as a by-product of gene
tic changes that accumulate between two previously interbreeding populations of the sa
me species.

 The specific genetic differences that confer reproductive isolation are called reprodu
ctive isolating mechanisms.

 A number of such isolating mechanisms have been identified. These are generally divide
d into two major groups: Prezygotic and post zygotic reproductive isolating mechanisms.

 Prezygotic Reproductive Isolating Mechanisms

 Prezygotic reproductive isolating mechanisms are mechanisms, which prevent zygote form
ation.
 It includes temporal, habitat, behavioral, structural, and chemical isolating mechanis
ms.

 a. Temporal (Seasonal) Isolation

 It prevents fertilization because the two different species reproduce at different tim
es.

 Example: Spring vs fall, diurnal differences.

88
 b. Habitat (Ecological) Isolation

 In this type of reproduction isolation populations occupy the same general

area but potential mates do not meet because of differences in the formatio
n of habitat requirements.

 c. Behavioral (Ethological) Isolation

 In this type of reproductive isolation, potential mates meet but do not mat
e because courtship behavior patterns do not match.

 d. Structural (Mechanical or Morphological) Isolation

 Copulation in animals or pollen transfer in plants may be prevented or rest

ricted by differences in the structure of genitalia or the flower parts.

 e. Chemical Isolation

 In species with external fertilization, even if the eggs and sperm of two d
ifferent closely related species are shed at the same time, fertilization m
ay be rare or absent because of a lack of attraction between the sperm and
egg.
89
 Post zygotic Reproductive Isolating Mechanisms
 Post zygotic isolating mechanisms prevent the proper functioning of zygotes a
fter they form by reducing their viability or fertility.

 Post zygotic isolating mechanisms include hybrid inviability, hybrid sterilit

y, and hybrid breakdown.

 a. Hybrid Inviability

 Sometimes mating and fertilization may take place successfully but the hybrid
fails to develop into adult because the F1 hybrid zygotes are inviable or hav
e reduced viability.

 b. Hybrid Sterility

 In this type of post zygotic isolation, F1 hybrids are viable but sterile. E.
g.- mule

 c. Hybrid (F2) Breakdown

 In this type of post zygotic isolating mechanism, the F 1 hybrids are fertile,
but the F2 or back cross individuals have reduced viability.
90
 7.1. The taxonomic position of humans in the animal kingdom
 Humans belong to the Kingdom: Animalia, Phylum: Chordata, Class:
Mammalia, Order: Primates
 There are three main groups of living primates:
 1. the lemurs of Madagascar and the lorises and pottos of tropical Africa and
southern Asia
 2. the tarsiers, which live in Southeast Asia; and
 3. the anthropoids, which include monkeys and apes and are found worldwide.
 Humans are members of the ape group.
 Many characters distinguish humans from other apes.
 Most obviously, humans stand upright and are bipedal (walk on two
legs).
 Humans have a much larger brain and are capable of language, symb
olic thought, and the manufacture and use of complex tools.
 Humans also have reduced jawbones and jaw muscles , along with a s
horter digestive tract.
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92
 7.2. Extinct and extant hominids
 The study of human origins is known as paleoanthropolo
gy.
 Paleoanthropologists have unearthed fossils of approx
imately 20 extinct species that are more closely relat
ed to humans than to chimpanzees.
 These species are known as hominins (its older synonym
is hominid).
 Since 1994, fossils of four hominin species older than
4 million years have been discovered.
 The oldest of these hominins, Sahelanthropus tchadmsis,
lived about 6-7 million years ago.
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 Figure: A timeline for some selected hominin species. Most of these fossils come from sites in
eastern and southern Africa. Note that at most times in hominin history, two or more hominin sp
ecies were contemporaries. Some of the species are controversial, reflecting phylogenetic debat
es about the interpretation of skeletal details and biogrography.
94
 Australopiths
 The fossil record indicates that hominin diversity increased dramatically be
tween 4 million and 2 million years ago.

 Many of the hominins from this period are collectively called australopiths.

 Australopiths got their name from the 1924 discovery in South Africa of Aust
ralopithecus africanus ("southern ape of Africa), which lived between 3 and
2.4 million years ago.

 With the discovery of more fossils, it became clear that A. africanus walked
fully erect (was bipedal) and had human-like hands and teeth.

 However, its brain was only about one-third the size of the brain of a prese
nt-day human.

 In 1974, “Lucy” (3.2-million-year-old Australopithecus skeleton that was

40% complete) was discovered in the Afar region of Ethiopia.

 Lucy and similar fossils have been designated as a separate species, Austral
opithecus afarensis.
 A. afarensis existed as a species for at least 1 million years.
95
 Another lineage of australopiths consisted of the "robust" australopiths.

 These hominins, which included species such as Paranthropus boisei, had

sturdy skulls with powerful jaws and large teeth, adapted for grinding an
d chewing hard, tough foods.

 Early Homo

 The earliest fossils that paleoanthropologists place in our genus, Homo,

are those of the species Homo habilis.

 These fossils, ranging in age from about 2.4 to 1.6 million years, show c
lear signs of certain derived hominin characters above the neck.

 Compared to the australopiths, H. habilis had a shorter jaw and a larger

brain volume, about 600-750 cm3.

 Sharp stone tools have also been found with some fossils of H. habilis (t
he name means "handy man”).

 Fossils from 1.9 to 1.5 million years ago mark a new stage in hominin evo
lution.
96
 A number of paleoanthropologists recognize these fossils as those of a distinct sp
ecies, Homo ergaster.
 Homo ergaster had a substantially larger brain than H. habilis (over 900 cm3), as
well as long, slender legs with hip joints well adapted for long-distance walking.
 Homo ergaster fossils have been discovered in far more arid environments than earl
ier hominins and have been associated with more sophisticated stone tools.
 Neanderthals
 In 1856, miners discovered some mysterious human fossils in a cave in the Neander
Valley in Germany.
 The 40,000-year-old fossils belonged to a thick-boned hominin with a prominent bro
w.
 The hominin was named Homo neanderthalensis and is commonly called a Neanderthal.
 Neanderthals were living in Europe and the Near East by 200, 000 years ago, but ne
ver spread outside that region.
 They had a brain as large as that of present-day humans, buried their dead, and ma
de hunting tools from stone and wood.
 But despite their adaptations and culture, Neanderthals apparently became extinct
about 28,000 years ago.
97
 Homo sapiens
 Evidence from fossils, archaeology, and DNA studies has led to a convincing hyp
othesis about how our own species, Homo sapiens, emerged and spread around the
world.
 The discovery of preserved bones of several ancient humans on the island of Flo
res in Indonesia in 2004 suggested a possible real and recent coexistence of tw
o types of people.
 A female skeleton found 17 feet beneath a cave floor with pieces of others near
by was named Homo floresiensis, popularly called the Hobbit.
 She was about half as tall as a modern human, with a brain about a third of the
size.
 She lived about 50,000 years ago.
 But who were the Hobbits? At first, some researchers thought that Hobbits were
direct descendants of Homo erectus, a species that lived before our own.
 Then, analysis of limb bones of the skeletons revealed feet and proportions lik
e those of an ape, despite the more human-like skull.
 This analysis suggests that the Hobbits may have been direct descendants of a p
rimate even older than Homo erectus.
 They evolved in a different direction thanks to the isolation of their island.
98
 7.3. Important hominid features
 Sahelallthropus and other early hominins shared some of the derived characters of
humans.

 For example, they had reduced canine teeth, and they had relatively flat faces.

 They also show signs of having been more upright and bipedal than other apes.

 In chimpanzees, the foramen magnum (the hole at the base of the skull through whi
ch the spinal cord exits) is relatively far back on the skull, while in early hom
inins (and in humans), it is located underneath the skull.

 Bipedalism

 Our anthropoid ancestors of 30-35 million years ago were still tree-dwellers.

 But by about 10 million years ago, the Himalayan mountain range had formed, thrus
t up in the aftermath of the Indian plate's collision with the Eurasian plate.

 The climate became drier, and the forests of what are now Africa and Asia contrac
ted.

 For decades, paleoanthropologists have seen a strong connection between the rise
of savannas and the rise of bipedal hominins.
99
 Tool Use

 The manufacture and use of complex tools is a derived behavioral character

of humans.

 Determining the origin of tool use in hominin evolution is one of paleoanth

ropology's great challenges.

 Other apes are capable of surprisingly sophisticated tool use.

 The oldest generally accepted evidence of tool use by hominins is 2.5-milli

on-year-old cut marks on animal bones found in Ethiopia.

 These marks suggest that hominins cut flesh from the bones of animals using
stone tools.

 If Australopithecus garhi, were indeed the creators of the stone tools used
on the bones, that would suggest that stone tool use originated before the
evolution of large brains in hominins.
100
 7.4. The evolutionary relationships among the hominids
 It's important to avoid two common misconceptions when considering these early hominin
s.

 One is to think of them as chimpanzees.

 Chimpanzees represent the tip of a separate branch of evolution, and they acquired der
ived characters of their own after they diverged from their common ancestor with human
s.

 Another misconception is to think of human evolution as a ladder leading directly from

an ancestral ape to Homo sapiens.

 But when the characteristics of all hominins that lived over the past 6 million years
are considered, H. sapiens appears not as the end result of a straight evolutionary pa
th, but rather as the only surviving member of a highly branched evolutionary tree.

101
 Figure: Phylogenetic relations among the species that are shown is in
most cases uncertain.
102
 7.5. Migration of hominids out of Africa
 Homo erectus originated in Africa and was the first hominin to migrate out of Afric
a.

 The oldest fossils of hominins outside Africa, dating back 1.8 million years, were
discovered in the former Soviet Republic of Georgia.

 Homo erectus eventually migrated as far as the Indonesian archipelago.

 Comparisons of H. erectus fossils with humans and studies of human DNA indicate tha
t H. erectus became extinct sometime after 200,000 years ago.

 The oldest known evidence for anatomically modern humans (as of 2018) are fossils f
ound at Jebel Irhoud, Morocco, dated about 300,000 years old.

 (The oldest known fossils of our own species have been found at two different sites
in Ethiopia and include specimens that are 195,000 and 160,000 years old.)

 Oldest fossils of H. sapiens outside Africa are from Middle East and date back abou
t 115,000 years.

 Studies of the human Y chromosome suggest that humans spread beyond Africa in one o
r more waves, first into Asia and then to Europe and Australia.

 The date of the first arrival of humans in the New World is uncertain, although the
oldest generally accepted evidence puts that date at sometime before 15,000 years a
go.
103
 7.6. The origin of Homo sapiens (single versus multiregional origin)
 Paleoanthropologists refer to the kind of human beings that live tod
ay a Homo sapiens a as “anatomically modern humans.”
 Anatomically modern humans differ from the fossils discussed so far
in a series of details in skull anatomy.
 Anatomically modern humans were fully established in Africa, Europe,
and Asia by 30,000–40,000 years ago.
 When and where did anatomically modern humans originate?
 For years, this question has been the topic of a debate between two
hypotheses.
 Some paleoanthropologists argue that anatomically modern humans evol
ved independently in Asia, Europe, and Africa; this is the “multire
gional” hypothesis.
 Others argue that modern humans originated only in Africa, and then
emigrated to Asia and Europe, replacing the indigenous people, with
little or no interbreeding; this is the “out of Africa” hypothesis.
104
• However, fossil as well as genetic evidences have tended to favor the out
of Africa hypothesis.
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