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ALL INDIA SENIOR SECONDRY
CERTIFICATE EXAMINATION
2011-12
INVESTIGATRY PROJECT REPORT
ON THE TOPIC
“EVOLUTION”
Submitted To:-
Submitted By:-
Mr. N.K. AggarwalAyushKhichar
PGT Biology
12TH Science
K.V. Sikar
Roll No:-
CONTENT
1) Introduction
2) Genetic basis of evolution
A) Genetic variation in population
A1)Gene Flow
A2) Mutation
3) Natural selection in populations
A) Directional Selection
B) Stabilizing Selection
C)Disruptive Selection
D) Sexual Selection
4) Genetic drift
5) Origin of new species
A) Allopatric Speciation
B) Sympatric Speciation
C) Gradual Change
6)Patterns of descent
A) Divergent Evolution
B) Adaptive Radiation
C) Convergent Evolution
D) Co-evolution
7)How scientists study evolution
A) Fossils
B) Distribution of Species
C) Anatomical Similarities
D) Molecular Similarities
E) Direct Observation
8) Development of evolutionary theory
A) Linnaeus and Scientific Classification
B) 19th-Century Foundations
C) Darwin and Natural Selection
D) Mendel and Early Genetics
E) Population Genetics and the Modern Synthesis
F)New Techniques in Molecular Biology
G) Punctuated Equilibria
9)Role of Extinction
10) Human Evolution
11) Common misconceptions
12) References
INTRODUCTION
Evolution, in biology is a complex process by which the characteristics of living organisms
change over many generations as traits are passed from one generation to the next. Evolution
provides an essential framework for studying the ongoing history of life on Earth.
A central and historically controversial component of evolutionary theory is that all living
organisms, from microscopic bacteria to plants, insects, birds, and mammals share a common
ancestor. Species that are closely related share a recent common ancestor, while distantly related
species have a common ancestor further in the past. The animal most closely related to humans,
for example, is the chimpanzee. The common ancestor of humans and chimpanzees is believed to
have lived approximately 6 million to 7 million years ago.
Purple and Yellow Tube Sponge
Sponges, considered to be the most primitive of the multicellular animals, are represented in the
fossil record back to the Cambrian Period, at least 600 million years ago. The purple and yellow
tube sponge displays one of the many different body forms typical of sponges.
EVOLUTIONARY HISTORY OF LIFE:
Origin of life :The current scientific consensus is that the complex biochemistry that makes up
life came from simpler chemical reactions.The beginning of life may have included selfreplicating molecules such as RNA, and the assembly of simple cells.
All organisms on Earth are descended from a common ancestor or ancestral gene pool. Current
species are a stage in the process of evolution, with their diversity the product of a long series of
speciation and extinction events. Recently, evidence for common descent has come from the
study of biochemical similarities between organisms. For example, all living cells use the same
basic set of nucleotides and acids.
The development of molecular genetics has revealed the record of evolution left in
organisms' genomes: dating when species diverged through the molecular clock produced by
mutations. For example, these DNA sequence comparisons have revealed that humans and
chimpanzees share 96% of their genomes and analyzing the few areas where they differ helps
shed light on when the common ancestor of these species existed.
In the mid-19th century aBritish naturalist Charles Darwin in his book On the Origin of Species
by Means of Natural Selection, described the evolution of life as a process of natural selection.
Life, he suggested, is a competitive struggle to survive, often in the face of limited resources.
Living things must compete for food and space.Darwin offered that, within a given population in
a given environment, certain individuals possess characteristics that make them more likely to
survive and reproduce. These individuals will pass these critical characteristics on to their
offspring. The number of organisms with these traits increases as each generation passes on the
advantageous combination of traits. Outmatched, individuals lacking the beneficial traits
gradually decrease in number.
Animal Kingdom
Kingdom Animalia includes more than one million living species, grouped into more than 30
phyla. Vertebrates, members of the phylum Chordata, comprise only one percent of these
organisms. Phylum Arthropoda is more successful in sheer numbers, total mass, and distribution
than all other groups of animals combined. The remaining animal phyla are composed of mostly
marine-dwelling organisms. Illustrated here is the evolutionary relationship between all of these
groups.
GENETIC BASIS OF EVOLUTION
Natural selection is tied to traits that organisms pass from one generation to the next. In humans,
these traits include hundreds of features such as eye color, blood type, and height. Nature offers
countless other examples of traits in living things, such as the pattern on a butterfly’s wings, the
markings on a snail’s shell, the shape of a bird’s beak, or the color of a flower’s petals.
Such traits are controlled by specific bits of biochemical instructions known as genes. Genes are
composed of individual segments of the long, coiled molecule called deoxyribonucleic acid
(DNA). They direct the synthesis of proteins, molecular laborers that serve as building blocks of
cells, control chemical reactions, and transport materials to and from cells. Proteins are
themselves composed of long chains of amino acids, and the biochemical instructions found in
DNA determine the arrangement of amino acids in a chain. The specific sequence of amino acids
dictates the structure and resulting function of each protein.
All genetic traits result from different combinations of gene pairs, one gene inherited from the
mother and one from the father. Each trait is thus represented by two genes, often in different
forms. Different forms of the same gene are called alleles. Traits depend on very precise rules
governing how genetic units are expressed through generations.
 Genetic Variation in Populations
Evolutionary change takes place in populations over the course of many generations. Since
individual organisms cannot evolve in a single lifetime, evolutionary science focuses on a
population of interbreeding individuals. All populations contain some variations in traits. In
humans, for example, some people are tall, some are short, and some are of medium height.
In interbreeding populations, genes are randomly shuffled among members of the population
through sexual reproduction, the process that produces genetically unique offspring. Individuals
of different sexes develop specialized sex cells called gametes. In humans and other vertebrates
(animals with backbones), these gametes are sperm in males and eggs in females. When males
and females mate, these sex cells join in fertilization. A series of cell divisions creates
individuals with a unique assembly of genes. No individual members of any population (except
identical twins, which develop from a single egg) are exactly alike in their genetic makeup. This
diversity, referred to as genetic diversity or variation, is essential to evolution
A) Gene Flow
When individuals move between one population and another, new genes may be introduced to
populations. This phenomenon, known as gene flow, results from chance dispersal as well as
intentional migration. For example, two populations of related wildflowers, one red and one
white, separated by a large tract of land. Under normal circumstances, the two groups do not
interbreed because the wind does not blow hard enough to carry pollen between the populations
so that pollination can occur. If one day an unusually strong wind carries pollen from the red
wildflower population to the white wildflower population, the gene for red flowers may be
introduced to the white population’s gene pool. In many animals, gene flow results when
individuals from one population migrate to another population.
B) Mutation
Genes themselves are constantly being modified through a process called mutation—a change in
the structure of the DNA in an individual's cells. Mutations can occur during replication, the
process in which a cell splits itself into two identical copies known as daughter cells. Normally
each daughter cell receives an exact copy of the DNA from the parent cell. Errors results in a
change in the gene. Such a change may affect the protein that the gene produces and, ultimately
change an individual’s traits. Some mutations occur spontaneously, others are caused by factors
in the environment, known as mutagens. Mutagens that affect human DNA include ultraviolet
light, X rays, and various chemicals.
NATURAL SELECTION IN POPULATIONS
Natural selection sorts out the useful changes in the gene pool. When this happens, populations
evolve. Beneficial new genes quickly spread through a population because members who carry
them have a greater reproductive success, or evolutionary fitness, and consequently pass the
beneficial genes to more offspring. Over the course of several generations, the gene and most of
its carriers are eliminated from the population. Severely detrimental genes may persist at very
low levels in a population, however, because they can be reintroduced each generation by
mutation.
Natural selection only allows organisms to adapt to their current environment. Should
environmental conditions change, new traits may prevail. Moreover, natural selection does not
always favor a single version of a trait. Nor does natural selection always favor change. If
environmental conditions so dictate, natural selection maintains the status quo by eliminating
extreme versions of a particular trait from the population.
Stonefish have evolved an ornate coloration that
blends well against the coral and mud of their ocean bottom habitat. Natural selection may have
favored this coloration in stonefish because well camouflaged members of the population were
better able to surprise small fish and other prey.
A) Directional Selection
Often, shifts in environmental conditions, such as climate change or the presence of a new
disease or predator, can push a population toward one extreme for a trait. In periods of prolonged
cold temperatures, for example, natural selection may favor larger animals because they are
better able to withstand extreme temperatures. This mode of natural selection, known as
directional selection, is evident in cheetahs. About 4 million years ago, cheetahs were more than
twice as heavy as modern cheetahs. But quicker and lighter members of the population had
greater reproductive success than did larger members of the population. Over time, natural
selection favored smaller and smaller cheetahs.
B) Stabilizing Selection
Sometimes natural selection acts to preserve the status quo by favoring the intermediate version
of a characteristic instead of one of two extremes. An example of this selective force, known as
stabilizing selection, was evident in a study of the birth weight of human babies, in the middle of
the 20th century babies of intermediate weight, about 3.5 kg (8 lb), were more likely to survive.
Babies with a heftier birth weight had lower chances for survival because they were more likely
to cause complications during the delivery process, and lightweight babies were often born
premature or with other health problems. Babies of intermediate birth weight, then, were more
likely to survive to reproductive age.
c) Disruptive Selection
Occasionally natural selection favors two extremes, causing alleles for intermediate forms of a
trait to become less common in the gene pool. The African Mocker swallowtail butterfly has
undergone this form of selection, known as disruptive selection. The Mocker swallowtail evades
its predators by resembling poisonous butterflies in its ecosystem. Predators have learned to
avoid these poisonous butterflies and also to steer away from the look-alike Mocker
swallowtails. The Mocker swallowtail has a large range, and in different regions, the Mocker
swallowtail looks very different, depending on which species of poisonous butterfly it mimics. In
some areas the butterfly displays black markings on a white background; in others the markings
float on an orange background. As long as a Mocker swallowtail appears poisonous to predators,
it has a greater chance of survival and therefore a higher evolutionary fitness. Mocker
swallowtails that do not look poisonous have a much lower evolutionary fitness because
predators quickly eat them. Disruptive selection, then, favors the extreme color patterns of white
or orange, and nothing in between.
D) Sexual Selection
Sexual selection operates on factors that contribute to an organism's mating success. In many
animals, sexual attractiveness is an important component of selection because it increases the
likelihood of mating. Sexual selection rarely affects females, because the duration of pregnancy
and infant care limits the number of babies they can have. Males, on the other hand, have few
limitations on the number of offspring they can father, and a male who produces many offspring
has a high level of evolutionary fitness. Males of many species, then, must compete with other
males to mate with females. Some males win females’ attention more often than others and, as a
result, pass their genes to more offspring.
GENETIC DRIFT
Natural selection is not the only force that changes the ratio of alleles present in a population.
Sometimes the frequency of particular alleles may be altered drastically by chance alone. This
phenomenon, known as genetic drift, can cause the loss of an allele in a population, even if the
allele leads to greater evolutionary fitness. Conversely, genetic drift can cause an allele to
become fixed in a population—that is, the allele can be found in every member of the population,
even if the allele decreases fitness.
Although any population can fall victim to genetic drift, small populations are more vulnerable
than larger populations. Imagine a particular allele is present in 25 percent of a population of
worms. If a flood occurs and randomly eliminates half of the population, the laws of probability
predict that approximately 25 percent of the surviving population will carry the allele. In a
population of 120,000 worms, this means that about 15,000 of the surviving 60,000 worms will
carry the allele. Even if, by chance, the flood claimed the lives of an additional 10 percent of the
carriers, thousands of copies of the allele would still remain in the population. But in a
population of only 12 worms, the laws of probability predict that only 1.5 of the surviving 6
worms would carry the allele. If, by chance, the flood claimed more of the carriers of the allele
than the non-carriers, the allele could be eliminated.
Northern Elephant Seal
Heavy hunting reduced the number of northern elephant seals to as few as 20 individuals in the
19th century. Their numbers have rebounded significantly since a ban on seal hunting became
effective in the early 20th century. However, all living northern elephant seals descended from
the small group that survived the severe hunting of the 19th century. This so-called population
bottleneck drastically reduced the genetic diversity of northern elephant seal populations, leaving
the animals vulnerable to disease and other environmental pressures. The male northern elephant
seal, right, typically weighs more than three times the female seal, left.
ORIGIN OF NEW SPECIES
The forces of natural selection and genetic drift continuously influence and change the
characteristics of a population. However, most often these forces are not sufficient to create an
entirely new species. Different species arise when, for one reason or another, members of a
population cease to interbreed.
A) Allopatric Speciation
When a barrier, such as a stretch of sea or a mountain range, separates different populations of a
particular species, the populations may no longer be capable of crossing the barrier to interbreed.
Speciation caused by geographic isolating mechanisms, or allopatric speciation, is evident in the
many different populations of pupfish that live in the Death Valley region of California
&Nevada. About 50,000 years ago this region had a rainy climate, but rainfall decreased and by
about 4,000 years ago, this region was a desert. The interconnected lakes and streams dried up,
and in their place remained a series of small, isolated stream-fed ponds. Each pond is home to a
different species of pupfish, specially adapted to its pond’s unique temperature and mineral
composition. As the lakes & streams dried up, the dry ground that separated them became a
geographical isolating mechanism that prevented the individual populations from interbreeding.
Consequently, the many pupfish populations evolved independently.
B) Sympatric Speciation
In sympatric speciation, isolating mechanisms may be triggered by differences in habitat, sexual
reproduction, or heredity. Similar plants may fail to breed together because their flowering
seasons are different. Many different types of rain forest orchids, for example, cannot interbreed
because they flower on different days. Some animals mate only if they recognize characteristic
color patterns or scents of their own group. Birds, are stimulated to breed only after witnessing a
song, display, or other courtship ritual that is characteristic in their group. Sometimes two
subpopulations of the same species do not produce offspring with one another, even though they
come into breeding contact. This may be due, for example, to reproductive incongruities between
two subpopulations that cause embryos to die before development and birth.
C) Gradual Change
Speciation may occur even when no isolating mechanism is present. In this case, a new species
may form through the slow modification of a single group of organisms into an entirely new
group. Foraminifera, a tiny species of marine animals that live in the Indian Ocean, demonstrate
this process, known as vertical or phyletic evolution. From about 10 million to 6 million years
ago, the species remained relatively unchanged. These organisms then began a slow and
gradualchange, lasting about 600,000 years, that left them so unlike their ancestors that biologists
consider them an entirely new species.
PATTERNS OF DESCENT
Whatever the cause of their reproductive isolation, independently evolving populations tend to
adhere to general patterns of evolutionary descent. Most often, environmental factors determine
the pattern followed. A gradually cooling climate, for example, may result in a population of
foxes developing progressively thicker coats over successive generations. This pattern of gradual
evolutionary change occurs in a population of interbreeding organisms evolving together. When
two or more populations diverge, they may evolve to be less alike or more alike, depending on
the conditions of their divergence.
A) Divergent Evolution
In the pattern known as divergent evolution, after two segments of a population diverge, each
group follows an independent and gradual process of evolutionary change, leading them to grow
increasingly different from each other over time. Over the course of many generations, the two
segments of the population look less and less like each other and their ancestor species. For
example, when the Colorado River formed the Grand Canyon, a geographic barrier developed
between two populations of antelope-squirrels. The groups diverged, resulting in two distinct
species of antelope squirrel that have different physical characteristics. On the south rim of the
canyon is Harris’s antelope squirrel, while just across the river on the north rim is the smaller,
white-tailed antelope squirrel.
B) Adaptive Radiation
Sometimes divergence occurs simultaneously among a number of populations of a single
species. In this process, known as adaptive radiation, members of the species quickly disperse to
take advantage of the many different types of habitat niches—that is, the different ways of
obtaining food and shelter in their environment. Such specialization ultimately results in a
number of genetically distinct but similar-looking species. This commonly occurs when a species
colonizes a new habitat in which it has little or no competition. For example, a flock of one
species of bird may arrive on some sparsely populated islands. Finding little or no competition,
the birds may evolve rapidly into a number of species, each adapted to one of the available
niches. Charles Darwin noted an instance of adaptive radiation on his visit to the Galápagos
Islands off the coast of South America. He surmised that one species of finch colonized the
islands thousands of years ago and gave rise to the 14 species of finchlike birds that exist there
now. Darwin observed that the greatest differences in their appearance lay in the shapes of the
bills, adapted for their mode of eating. Some species possessed large beaks for cracking seeds.
Others had smaller beaks for eating vegetation, and still others featured long, thin beaks for
eating insects.
Galápagos Finches: The fourteen species of finch that inhabit the Galápagos Islands are
believed to have evolved from a single species resembling the blue-black grassquit,
Volatiniajacarina, abundant in Latin America and the Pacific coast of South America. The
ancestral finch, with its short, stout, conical bill specialized for crushing seeds, probably
migrated from the mainland to the Galápagos Islands. The size and shape of their bills reflect
these specializations, an example of adaptive radiation.
.C)
Co-evolution
Often two or more organisms in an ecosystem fall into evolutionary step with one another, each
adapting to changes in the other, a pattern known as coevolution. Coevolution is often apparent
in flowers and their pollinators. Hummingbirds, for example, have long, narrow beaks and a
relatively poor sense of smell, and they are attracted to the color red. Fuchsias, flowering plants
that rely on hummingbirds for pollination, usually have long, slender flowers in various shades
of red, and they have little or no fragrance.
Many flowers and their pollinators have fallen into evolutionary step with one another in a
process known as coevolution. For example, fuchsia flowers, which rely heavily on
hummingbirds to disperse their pollen, usually have brilliant red or pink flowers with a long,
slender shape and little or no scent. All of these attributes attract hummingbirds, tiny, slender-
beaked birds with a poor sense of smell and eyes that detect colors in the red portion of the
visible spectrum. The hummingbird also benefits from this relationship. In return for dispersing
the fuchsia’s pollen, the hummingbird has almost exclusive access to the sweet, nutritious nectar
buried deep at the base of the fuchsia flower, where shorter-beaked animals cannot reach it.
D) Convergent Evolution
Sometimes distantly related species evolve in ways that make them appear more closely related.
This pattern, known as convergent evolution, occurs when members of distantly related species
occupy similar ecological niches. Natural selection favors similar adaptations in each population.
Convergent Evolution: Although marsupial mammals once populated all land masses, they
remain diversified only on the isolated Australian continent, where they have evolved to fill the
same ecological niches that placental mammals occupy elsewhere. The Tasmanian wolf, for
example, closely resembles the doglike carnivores of other continents. More specialized parallel
adaptations include those of the marsupial and placental anteaters, the marsupial sugar glider and
placental flying squirrels, and the burrowing marsupial wombat and placental ground hog. In this
illustration, placental ground hog. In this illustration, placental mammals are in the top row, and
their marsupial equivalents are in the bottom row.
HOW SCIENTISTS STUDY EVOLUTION
Species do not change overnight, or even in the course of one lifetime. Rather, evolutionary
change usually occurs in tiny, almost imperceptible increments over the course of thousands of
generations—periods that range from decades to millions of years. To study the evolutionary
relationships among organisms, scientists must perform complex detective work, deriving
indirect clues from the fossil record, patterns of animal distribution, comparative anatomy,
molecular biology, and finally, direct observation in laboratories and the natural environment.
A) Fossils
One way biologists learn about the evolutionary relationships between species is by examining
fossils. These ancient remains of living things are created when a dead plant or animal is buried
under layers of mud or sand that gradually turn into stone. Over time, the organism remains
themselves may turn to stone, becoming preserved within the rock layer in which they came to
rest. By measuring radioactivity in the rock in which a fossil is embedded, paleontologists
(scientists who study the fossil record) can determine the age of a fossil.
Fossils present a vivid record of the earliest life on Earth, and of a progression over time from
simple to more-complex life forms. The earliest fossils, for example, are those of primitive
bacteria some 3.5 billion years old. In more recent layers of rock, the first animal fossils
appear—primitive jellyfish that date from 680 million years ago. Still more-complex forms, such
as the first vertebrates (animals with backbones), are documented by fossils some 570 million
years old. Fossils also indicate that the first mammals appeared roughly 200 million years ago.
Although these ancient forms of life have not existed on Earth for millions of years, scientists
have been able, in many instances, to show a clear evolutionary line between extinct animals and
their modern descendants. For example, the extinct, winged creature Archaeopteryx lived about
145 million years ago. Its fossil shows the skeleton of a dinosaur and the feathers of a bird.
Fossils show clear evidence that the earliest human species had many apelike features.These
features included large, strong jaws and teeth; short stature, long, curved fingers; and faces that
protruded outward from the forehead. Later species evolved progressively more humanlike
features.
Archaeopteryx , a birdlike dinosaur. It had teeth and a long, reptilelike tail covered with
feathers. Archaeopteryx lived between 163 million and 144 million years ago during the Jurassic
Period of the Mesozoic Era.
B) Distribution of Species
Scientists also learn about evolution by studying how different species of plants and animals are
geographically distributed in nature, and how they relate to their environment and to each other..
The study of these evolutionary relationships, known as island biogeography, has its roots in
Darwin’s observations of the adaptive radiation of the Galapagos finches. The Hawaiian Islands
provide similar examples, particularly in the species of birds known as honeycreepers. Like the
Galapagos finches, the honeycreepers of Hawaii evolved from a common ancestor and branched
into several species, showing a striking variety of beak shapes adapted for obtaining different
food sources in their various niches.
Eg.Ostriches- The distribution of the world’s flightless birds, called ratites, is indicative of the
shifting of continents in the Earth’s distant past. Ratites evolved on a landmass called
Gondwanaland, which broke apart to give rise to Africa, South America, and Australia.
Following this separation, the ratites of each of these continents gradually evolved into one or
more distinct species—the ostrich of Africa, the rhea of South America, and the cassowary and
emu of Australia.
C) Molecular Similarities
With advances in molecular biology in the last few decades, researchers seek evolutionary clues
at the smallest level: within the molecules of living organisms. Despite the enormous variety of
form and function seen in living things, the underlying genetic code—the molecular building
material of life—displays a striking uniformity. Almost all living organisms have DNA, and in
each case it consists of different pairings of the same building blocks: four nucleotide bases
called adenine, thymine, guanine, and cytosine. Using different combinations of these bases,
DNA directs the assembly of amino acids into functional proteins. The same uniform code
operates within all living things.
These molecules contain more than the master plan for living organisms—each is a record of an
organism's evolutionary history. By examining the makeup of such molecules, scientists gain
insights into how different species are related. For example, scientists compare the protein
cytochrome c from different species. In closely related species, the proteins have amino-acid
sequences that are very similar, perhaps varying by one or a few amino acids. More distantly
related organisms generally have proteins with fewer similarities. The more distant the
relationship, the less alike the proteins.
D) Anatomical Similarities
Detailed study of the internal and external features of different living things, a discipline known
as comparative anatomy, also provides a wealth of information about evolution. The arm of a
human, the flipper of a whale, the foreleg of a horse, and the wing of a bird have different forms
and are adapted to different functions. In the case of the arm, flipper, foreleg, and wing, for
example, each appendage shows a similar bone structure. The study of comparative anatomy has
revealed many instances of correspondence within various groups of organisms and these bodily
structures are said to be homologous.
Analogous and Homologous Structures
Structures that are similar due to evolutionary origin, such as the forearm bones of humans,
birds, porpoises, and elephants, are called homologous. Structures that evolve separately to
perform a similar function are analogous. The wings of birds, bats, and insects, for example,
have different embryological origins but are all designed for flight.
E ) Direct Observation
Information about evolutionary processes is also obtained by direct observation of species that
undergo rapid modification in only a few generations. One of the most powerful tools in the
study of evolutionary mechanisms is also one of the tiniest—the common fruit fly. These insects
have short life spans and, therefore, short generations. This enables researchers to observe and
manipulate its reproduction in the laboratory and learn about evolutionary change in the process.
Scientists also study organisms in their natural environments to learn about evolutionary
processes—for example, how insects develop genetic resistance to human-made pesticides, such
as DDT. While pesticides are often initially effective in killing crop-destroying pests, sometimes
the insect populations bounce back. The pesticide wipes out most of the population, leaving only
the genetically resistant individuals to multiply and flourish. Gradually, resistant individuals
predominate in the population, and the pesticide loses its effectiveness. The same phenomenon
has been observed in strains of disease-causing bacteria that have become resistant to even the
most powerful antibiotics.
DEVELOPMENT OF EVOLUTIONARY THEORY
The origins of life on Earth have been a source of speculation among philosophers, religious
thinkers, and scientists for thousands of years. Many human civilizations used rich and complex
creation stories and myths to explain the presence of living organisms. Ancient Greek
philosophers and scientists were among the earliest to apply the principles of modern science to
the mysterious complexity and variety of life around them. During early Christian times, ancient
Greek ideas gave way to Creationism, the view that a single God created the universe, the world,
and all life on Earth.
A) Ancient Views
The Greek philosopher Anaximander, believed that the Earth first existed in a liquid state.
Further, he believed that humans evolved from fishlike aquatic beings who left the water once
they had developed sufficiently to survive on land. Greek scientist Empedocles speculated in the
400s bc that plant life arose first on Earth, followed by animals. Empedocles proposed that
humans and animals arose not as complete individuals but as various body parts that joined
together randomly to form strange, fantastic creatures..
The Greek philosopher and scientist Aristotle, who lived in the 300s bc, referred to a "ladder of
nature"—a progression of life forms from lower to higher—but his ladder was a static hierarchy
of levels of perfection, not an evolutionary concept. Each rung on this ladder was occupied by
organisms of higher complexity than the rung before it, with humans occupying the top rung.
B) Linnaeus and Scientific Classification
Many centuries later, the idea of a perfect and unchanging natural world—the product of divine
creation—was predominant not only in religion and philosophy, but in science. Gradually,
however, as knowledge accumulated from seemingly disparate areas, the beginnings of modern
evolutionary theory began to take shape. A key figure in this regard was the Swedish naturalist
Carolus Linnaeus, who became known as the father of modern taxonomy, the science of
classifying organisms. In his major work SystemaNaturae(The System of Nature), first published
in 1735, Linnaeus devised a system of classification of organisms that is still in use today. This
system places living things within increasingly specific categories based on common attributes—
from a general grouping (kingdom) down to the specific individual (species). Using this system,
Linnaeus named nearly 10,000 plant and animal species in his lifetime.
C) 19th-Century Foundations
Perhaps the most prominent of those who embraced the idea of progressive change in the living
world was the early 19th-century French biologist Jean-Baptiste Lamarck. Lamarck's theory,
now known as Lamarckism and based in part on his study of the fossils of marine invertebrates,
was that species do change over time. He believed, furthermore, that animals evolve because
unfavorable conditions produce needs that animals try to satisfy. For example, short-necked
ancestors of the modern giraffe voluntarily stretched their necks to reach leaves high in trees
during times when food was scarce. Proponents of Lamarckism thought this voluntary use
slightly changed the hereditary characteristics controlling neck growth; the giraffe then
transmitted these alterations to its offspring as what Lamarck called acquired characteristics.
Modern scientists know that adaptation and natural selection are far more complicated than
Lamarck supposed, having nothing to do with an animal's voluntary efforts..
D) Darwin and Natural Selection
Darwin published a book On the Origin of Species by Means of Natural Selection, it
popularized—evolution through natural selection—set off a storm of controversy. Some of the
protest came from the clergy and other religious thinkers.
Charles Darwin gave the theory of natural selection, which was to become the foundation
concept supporting the theory of evolution. Darwin’s theory holds that environmental effects
lead to varying degrees of reproductive success in individuals and groups of organisms. This
revolutionary theory was published in 1859 in Darwin’s now famous treatise On the Origin of
Species by Means of Natural Selection.
E) Mendel and Early Genetics
Darwin did not know it, but the answer was at hand—although it would not be acknowledged in
his lifetime. In the Augustinian monastery at Brünn (now Brno in the Czech Republic), Austrian
monk Gregor Mendel experimented with the breeding of garden peas, observing how their traits
were passed down through generations. In crossbreeding pea plants to produce different
combinations of traits—color, height, smoothness, and other characteristics—Mendel noted that
although a given trait might not appear in every generation, the trait did not disappear. Mendel
discovered that the expression of traits hinged on whether the traits were dominant or recessive,
and on how these dominant and recessive traits combined. These units are now known as genes.
Mendel performed hundreds of experiments and produced precise statistical models and
principles of heredity, now known as Mendel’s Laws, showing how dominant and recessive
traits are expressed over generations.
F) Population Genetics and the Modern Synthesis
Further investigation into population genetics and such fields as paleontology, taxonomy,
biogeography, and the biochemistry of genes eventually led to what is called the modern
synthesis. This modern view of evolution integrated discoveries and ideas from many different
disciplines. In so doing, this view reconciled the many disparate ideas about evolution into the
all-encompassing evolutionary science studied today. The modern synthesis was advanced in
such books as Genetics and the Origin of Species, published in 1937 by the Russian-born
American geneticist Theodosius Dobzhansky; Evolution: The Modern Synthesis (1942) by
British biologist Sir Julian Huxley; and Systematics and the Origin of Species (1942) by
German-born American evolutionary biologist Ernst Mayr. In 1942, American paleontologist
George Gaylord Simpson demonstrated from the fossil record that rates and modes of evolution
are correlated: New kinds of organisms arise when their ancestors invade a new niche, and
evolve rapidly to best exploit the conditions in the new environment.
G) New Techniques in Molecular Biology
In 1953, American biochemist James Watson and British biophysicist Francis Crick described
the three-dimensional shape of DNA, the molecule that contains hereditary information in nearly
all living organisms. In the following decade, geneticists developed techniques to rapidly
compare DNA and proteins from different organisms. In one such procedure, electrophoresis,
geneticists evaluate different specimens of DNA or proteins by observing how they behave in the
presence of a slight electric charge. Such techniques opened up entirely new ways to study
evolution. For the first time geneticists could quantitatively determine, for example, the genetic
change that occurs during the formation of new species.
H) Punctuated Equilibria
Evolutionary theory has undergone many further refinements in recent years. One such theory
challenges the central idea that evolution proceeds by gradual change. In 1972 the American
paleontologists Stephen Jay Gould and Niles Eldredge proposed the theory of punctuated
equilibria. According to this theory, trends in the fossil record cannot be attributed to gradual
transformation within a lineage, but rather result from quick bursts of rapid evolutionary change.
In Darwinian theory, new species arise by gradual, but not necessarily uniform, accumulation of
many small genetic changes over long periods of geologic time. Gould and Eldredge recognized
that speciation more likely occurs in small, isolated, peripheral populations than in the main
population of the species, and that the unchanging nature of large populations contributes to the
stasis of most fossil species over millions of years. Occasionally, when conditions are right, the
equilibrium state becomes "punctuated" by one or more speciation events. While these events
probably require thousands or tens of thousands of years to establish effective reproductive
isolation and distinctive characteristics, this is but an instant in geologic time compared with an
average life span of more than ten million years for most fossil species.
I) Role of Extinction
Historically, biologists regarded extinction as a natural outcome of competition between newly
evolved, adaptively superior species and their older, more primitive ancestors. Recently,
however, paleontologists have discovered that many different, unrelated species living in large
ecosystems tend to become extinct at nearly the same time. The cause is always some sort of
climate change or catastrophic event that produces conditions too severe for most organisms to
endure. Moreover, new species evolve after the wave of extinction removes many of the species
that previously occupied a region for millions of years. Thus extinction does not result from
evolution, but actually causes it.
Ammonite Fossil
Now extinct, the ammonite mollusk was a shelled cephalopod, usually appearing in a coiled,
spiral shape. The extinction of ammonites coincided with the extinction of dinosaurs, and more
than 75 percent of all the species that lived at the time. By eliminating dominant species and so
opening possibilities for different organisms to thrive, mass extinctions change the make up of
life on Earth.
HUMAN EVOLUTION
Human evolution is the phenotypic history of the genus Homo, including the emergence
of Homosapiens as a distinct species and as a unique category of hominids ("great apes")
and mammals. The study of human evolution uses many scientific disciplines, including physical
anthropology, primatology, archaeology, linguistics and genetics.
Scientists have estimated that humans branched off from their common ancestor
with chimpanzees about 5–7 million years ago.
Several species and subspecies of Homo evolved and are now extinct, introgressed or extant.
Examples
includeHomoerectus(which inhabited Asia,
Africa,
and
Europe)
and
Neanderthals. Archaic Homo sapiens evolved between 400,000 and 250,000 years ago.
All species of organisms originate through the process of biological evolution. In this process,
new species arise from a series of natural changes. In animals that reproduce sexually, including
humans, the term species refers to a group whose adult members regularly interbreed, resulting
in fertile offspring—that is, offspring themselves capable of reproducing. Scientists classify each
species with a unique, two-part scientific name. In this system, modern humans are classified as
Homo sapiens.
The mechanism for evolutionary change resides in genes—the basic units of heredity. Genes
affect how the body and behavior of an organism develop during its life. The information
contained in genes can change—a process known as mutation. The way particular genes are
expressed—how they affect the body or behavior of an organism—can also change. Over time,
genetic change can alter a species’s overall way of life, such as what it eats, how it grows, and
where it can live.
Genetic changes can improve the ability of organisms to survive, reproduce, and, in animals,
raise offspring. This process is called adaptation. Parents pass adaptive genetic changes to their
offspring, and ultimately these changes become common throughout a population—a group of
organisms of the same species that share a particular local habitat. Many factors can favor new
adaptations, but changes in the environment often play a role. Ancestral human species adapted
to new environments as their genes changed, altering their anatomy (physical body structure),
physiology (bodily functions, such as digestion), and behavior. Over long periods, evolution
dramatically transformed humansand their ways of life.
Geneticists estimate that the human line began to diverge from that of the African apes between
8 million and 5 million years ago (paleontologists have dated the earliest human fossils to at least
6 million years ago). This figure comes from comparing differences in the genetic makeup of
humans and apes, and then calculating how long it probably took for those differences to
develop. Using similar techniques and comparing the genetic variations among human
populations around the world, scientists have calculated that all people may share common
genetic ancestors that lived sometime between 290,000 and 130,000 years ago.
Evolution of Human Beings
REFERENCES
1) Wikipedia
2)Microsoft Encarta Reference Library 2004
3) NCERT 12th Biology Book