Download 16. Human Evolution

Biology
Teach Yourself Series
Topic 16: Human evolution
A: Level 14, 474 Flinders Street Melbourne VIC 3000
T: 1300 134 518 W: tssm.com.au E: [email protected]
© TSSM 2011
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Contents
Human evolution ................................................................................................................................................ 4
Initial terminology .......................................................................................................................................... 4
As it appears in Unit 4................................................................................................................................ 4
Primates .......................................................................................................................................................... 4
As it appears in Unit 4................................................................................................................................ 4
Features of primates ....................................................................................................................................... 5
As it appears in Unit 4................................................................................................................................ 5
Review Question .................................................................................................................................... 5
Divergence of primates .................................................................................................................................. 6
As it appears in Unit 4................................................................................................................................ 6
Review Question .................................................................................................................................... 6
Modern Humans ............................................................................................................................................. 7
As it appears in Unit 4................................................................................................................................ 7
Overview of human features .......................................................................................................................... 7
As it appears in Unit 4................................................................................................................................ 7
Theories of human evolution .......................................................................................................................... 8
As it appears in Unit 4................................................................................................................................ 8
Review Questions .................................................................................................................................. 9
Evolution of modern humans ....................................................................................................................... 10
As it appears in Unit 4.............................................................................................................................. 10
Review Questions ................................................................................................................................ 12
Review Questions ................................................................................................................................ 14
Bipedalism .................................................................................................................................................... 16
As it appears in Unit 4.............................................................................................................................. 16
Review Questions ................................................................................................................................ 17
Skull shape ................................................................................................................................................... 18
As it appears in Unit 4.............................................................................................................................. 18
Review Questions ................................................................................................................................ 18
Cultural evolution ......................................................................................................................................... 19
As it appears in Unit 4.............................................................................................................................. 19
Review Questions ................................................................................................................................ 19
Mitochondrial DNA ..................................................................................................................................... 21
As it appears in Unit 4.............................................................................................................................. 21
Human intervention in evolution .................................................................................................................. 21
As it appears in Unit 4.............................................................................................................................. 21
Artificial selection ........................................................................................................................................ 21
As it appears in Unit 4.............................................................................................................................. 21
Review Question .................................................................................................................................. 22
Reproductive technologies ........................................................................................................................... 22
As it appears in Unit 4.............................................................................................................................. 22
Gene screening ............................................................................................................................................. 22
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As it appears in Unit 4.............................................................................................................................. 22
Advancement in medical treatments ............................................................................................................ 23
As it appears in Unit 4.............................................................................................................................. 23
Cloning ......................................................................................................................................................... 23
As it appears in Unit 4.............................................................................................................................. 23
Production of transgenic organisms ............................................................................................................. 24
As it appears in Unit 4.............................................................................................................................. 24
Gene therapy................................................................................................................................................. 24
As it appears in Unit 4.............................................................................................................................. 24
Review Question .................................................................................................................................. 24
Solutions to Review Questions ........................................................................................................ 25
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Human evolution
Human evolution generally refers to the evolution of modern humans. It also, however, incorporates many
other species in the human evolutionary tree, including human species other than ourselves. Our knowledge
of human evolution is continually improving as new discoveries are made.
Initial terminology
As it appears in Unit 4
Some text books use the term hominin and others use hominid. Others use both almost interchangeably. The
definition of these terms has changed over time. Current definitions are shown below:
Hominid: all modern and extinct humans and great apes.
Hominin: all modern humans and extinct humans. This group includes the genera Homo, Australopithecus,
Paranthropus and Ardipithecus.
Robust: sturdy strong or thick bones.
Gracile: finer bones.
Foramen magnum: The hole in the base of the skull which the spinal cord passes through.
Prehensile tails: tails that can be used as an extra limb.
Pentadactyl: having five fingers.
Brachiation: swinging by the arms.
Sexual dimorphism: males and females have phenotypic differences.
Megadont: Large teeth
Primates
As it appears in Unit 4
Primates are a group of mammals that consist of 5 main groups:
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Hominins: modern humans and ancestral humans.
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Apes: chimpanzees, gorillas, orang utans and gibbons.
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New world monkeys: monkeys from South America. These monkeys have prehensile tails as they are
strictly arboreal e.g. spider monkeys.
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Old world monkeys: monkeys that are from Africa and Europe. They have tails that are not
prehensile and many are ground dwellers e.g. baboons and macaques.
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Prosimians: lemurs and slow loris.
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Features of primates
As it appears in Unit 4
All primates share many common structural, functional and behavioural characteristics. These include:
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Rounded faces with reduced snouts.
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Eyes protected by bony ridges.
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Relatively large brains, with a small area devoted to smell and a larger area devoted to vision.
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Large eyes that point forward, giving binocular 3D vision. Colour vision present.
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Generalised tooth pattern (variation in tooth size and shape). Primates have canines as well as molars.
A varied, omnivorous, diet is possible.
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Collarbones present, enabling brachiation (swinging by arms).
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Mobile limb joints, giving a wide range of limb movement.
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Five digits on hands and feet (pentadactyl).
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Nails, not claws as climbing is accomplished by grasping not clawing
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Opposable thumbs to enable grasping.
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Many are quadrupedal, but most sit upright. Some are bipedal, freeing hands for other activities.
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Internal fertilisation, long gestation.
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Long period of parental care.
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Social groups usually well developed.
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Hierarchies common.
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Communication evident.
Review Question
1. Explain the advantage of having a generalized tooth pattern.
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Divergence of primates
As it appears in Unit 4
The diagram below shows a cladogram indicating the genetic relatedness of the primates. It is important to
remember that there is still some uncertainty regarding the extent of relatedness between the different
species.
Humans
Bonobo
Chimpanzee
Western gorilla
Eastern lowland
gorilla
Eastern mountain
gorilla
Sumatran orangutan
Bornean orangutan
Gibbon
Old world monkeys
New world
monkeys
Review Question
2. Based on the information supplied identify the species that is most closely related to modern humans.
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Modern Humans
As it appears in Unit 4
Human evolution is believed to be an example of divergent evolution from a common ancestor. Somewhere
in evolutionary history the first primate ancestor appeared. Then, over time, mutations occurred in some
populations of that animal, and natural selection acted to favour the best adapted individuals for particular
environments. Over time this process resulted in the evolution of new species. New 'branches' of the primate
tree grew. Some branches flourished, some died. Today we are left a variety of similar but significantly
different species. All are primates, and all came from a common ancestor, but they are each the products of
the different environments and selection pressures that shaped their present forms.
The ancestral species of a number of descendant species is called the common ancestor. In the primate
group the times of divergence of different species from the common ancestor (and from each other) have
been estimated in a number of ways. These include the comparison of phenotypic structures, proteins and
DNA sequences.
Modern humans (Homo sapiens) are believed to be the result of four major evolutionary changes:
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Bipedalism
Smaller jaws with reduced teeth
Large brain compared to body size
Cultural and technological evolution
Fossil evidence indicates that humanoid ancestors initially became bipedal, which was followed by changes
to the teeth and jaws. This was later followed by an increase in brain size and an increase in the development
of technology.
Overview of human features
As it appears in Unit 4
Modern humans have specific features which confer a selective advantage. The table below lists many of the
features of humans and possible advantages of having these features.
TRAIT
Has a larger cranial capacity and brain
Has a flatter face and lack the thick supraorbital
ridges of apes
More gracile skeletal structure
Reduced amount of body hair
Bipedalism
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ADVANTAGE
Ability to think and cooperate
Fewer parasites.
Enables increased heat loss by radiation.
Ability to see over obstacles.
Ability to carry things such as offspring, tools and
food.
Ability to hold tools and weapons.
Energy efficient way of locomotion that favoured
long distance travel.
Thermoregulation – less surface area presented to the
sun, exposure to cooler air, greater amount of airflow
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Foramen magnum is further forward
Spine is curved rather than straight
Thigh bone is angled outwards rather than straight
Feet shape
Reduction in jaw size
around the body.
Skull is centrally balanced on the spine enabling
upright posture.
The S shaped spine acts as a spring to assist walking
upright.
This creates an increased carrying angle and ensures
that the knee is brought under the body when
walking.
The human big toe points straight ahead while that of
apes points off to the side. Humans have a better
transmission of weight in the forward direction
assisting humans to be bipedal.
Dietary changes meant that food needed to be
chewed less which lead to a decrease in the size of
the chewing muscles attached to the jaw and the jaw
itself.
Reduction in the size of molar teeth
The food eaten by humans is less tough and fibrous
than that eaten by other hominids.
Legs are longer than arms
Enabling bipedal locomotion to be more efficient.
Theories of human evolution
As it appears in Unit 4
There are two main theories that you need to be aware of.
The out of Africa (replacement or Eve) theory: this theory states that all modern humans originated in
Africa and then spread out in successive waves to the rest of the world. They completely replaced all other
species and populations without any interbreeding occurring.
The multiregional theory: this theory states that modern humans evolved independently in many regions all
over the world. The different populations remained similar enough to successfully interbreed with each
other.
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Review Questions
Identify which of the theories the following information supports and explain why you believe this.
3. Transitional forms are found only in Africa.
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4. Traits found in groups of modern humans native to a specific area can only be traced to ancestral
humans from the same area.
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5. There is little genetic diversity amongst all modern humans.
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6. There is the greatest amount of genetic diversity in African populations.
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7. Traits specific to modern humans arose simultaneously all over the world.
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Evolution of modern humans
As it appears in Unit 4
DNA hybridisation has been used to show that humans and pygmy chimpanzees have approximately 98.4%
of their DNA in common. The evolutionary paths taken by chimpanzees and humans diverged about 5
million years ago but the fossil record relating to this is very fragmentary. There are, however, a variety of
fossils younger than this which show the evolutionary pathway of a group of mammals called hominids.
In 1974 a partial skeleton that was given the name “Lucy” was found. Lucy showed bipedalism, had a light
(gracile) build, apelike features and teeth similar to those of modern humans. This species existed up until
about 3 million years ago.
The next section of the fossil record shows a divergence in the hominid line. One line developed a thick
(robust) skeletal structure and lead to the development of Australopithecus aethiopicus. This species seemed
to disappear about 1.5 million years ago. It is thought this line was an evolutionary dead end.
The other line appeared about 2 million years ago and lead to the Homo genus. One species in this line is
named Homo habilis.
Just under 2 million years ago Homo erectus developed in Africa and this species had spread to Asia and
Europe by 1 million years ago.
Modern H. sapiens first appeared about 100,000 years ago in Africa and began expanding outwards,
replacing H. erectus and Homo neanderthalensis. H. neanderthalensis disappeared from the fossil record
about 30,000 years ago.
At least 4 million years ago the first hominid (human like) ancestor appears in the fossil record in Africa. As
the hominids evolved, many phenotypic changes were observed:
•
Body size varied, with a tendency to an increase over time.
•
Some species had thick, robust skeletons, others were gracile, or fine boned.
•
Skull capacity, and presumably, brain volume increased.
•
Skull bones and teeth had proportions more like apes than like modern humans.
•
Pelvis and thigh bones indicated that they walked on two feet (bipedal gait).
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Increasing brain size and the ability to make and use tools slowly developed.
There is a great deal of evidence that shows that many groups of early H. sapiens migrated out of Africa
about 200,000 years ago. The pressures of selection and adaptation years and produced regional variations
that we see in human populations today.
It is useful to know all of this background information, however, examination questions generally ask you to
interpret information supplied such as in the chart on the following page rather than remembering
information about specific hominins.
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The chart below shows one possible overview of the evolution of humans.
H = Homo
Au = Australopithecus
P = Paranthropus
Ar = Ardipithecus
O = Orrorin
S = Sahelanthropus
The bars on this chart indicate the times when specific hominins were present but does not indicate the
degree of relatedness.
This chart can be used to establish which hominins were contemporaries and to eliminate which organisms
were direct ancestors.
For example H. habilis and H. ergaster were contemporaries, with H. habilis becoming extinct about half a
million years before H. ergaster.
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Review Questions
8. Identify the other hominins that were contemporaries of H. habilis
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9. Is it possible for H. erectus to be the direct ancestor of modern humans (H. sapiens)? Use the
provided data to support your answer.
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This flow chart below shows another frequently used representation of the evolution of modern humans.
This chart indicates relatedness but does not include a time frame.
Homo sapiens
Homo neanderthalensis
Homo heidelbergensis
Homo erectus
Homo habilis
Homo ergaster
Paranthropus
robustus
Paranthropus
boisei
Australopithecus africanus
Paranthropus aethiopicus
Australopithecus afarensis
Hominid fossils from areas all over the world have been used to piece together a linear relationship between
different species of hominids. However, there are many gaps or “missing links” in the fossil record. When
new discoveries are made these are either incorporated into the existing view or used to change it. A few
examples of additional hominid species discovered in the last decade include: Homo floresiensis, Homo
antecessor, Australopithecus anamensis and Ardipthecus ramidus.
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Review Questions
10. Using the information from the flow chart; what conclusions could be made regarding H. sapiens and
H. neanderthalensis?
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11. Which of the theories of human evolution is supported by this flow chart?
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12. Explain why both the chart on page 10 and the flow chart on page 11 may be altered in the future.
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In October 2009 information was released about an organism referred to as the latest member of the
ancestral humanoid family, Ardipithecus ramidus, nicknamed “Ardi”.
The first A. ramidus fossils were located in the Afar rift, Ethiopia, in 1992 but they were so fragile that it
took a group of scientists 17 years to reassemble the pieces and extrapolate information from them.
Ardi had a projecting muzzle that did not thrust forward as much as those of modern apes and the
supraorbital ridge was different to that of modern humans and other primates. However, examination of the
base of the skull indicates that Ardi's brain and spinal cord were positioned in a way similar to that of
modern humans.
13. What conclusions can be made about Ardi’s method of locomotion? Provide a reason to justify your
answer.
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14. What term is used to describe fossils such as Ardi which have features of both modern and ancestral
forms?
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15. Scientists found that Ardi had many traits not found in modern day apes. Explain the significance of
these findings.
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Bipedalism
As it appears in Unit 4
Bipedalism is the ability to walk upright on two legs. It is thought a major adaptation that lead to the
evolution of modern humans. It is necessary to be aware of the advantages of bipedalism as well as to
identify the adaptations that allow organisms to be bipedal.
It is possible that bipedalism was an adaptation that enabled hominids to adapt to changing environments.
Bipedalism confers the following advantages:
1) Since arms are not used for locomotion a bipedal organism has their hands free to carry objects such as
food, offspring, weapons and tools.
2) The upright stance aids thermoregulation as there is a greater amount of air flow around the body aiding
cooling and also there is less surface area exposed to the sun during the hottest parts of the day.
3) An increase in height would enable bipedal organisms to see over tall grass.
There are a variety of anatomical adaptations in humans which allow for an efficient bipedal gait, including:
Foramen magnum position: The foramen magnum is a hole in the base of the skull. It allows the spinal
cord to pass through into the cranium. In humans the foramen magnum is located in the middle of the base of
the skull so that the skull is centrally balanced on the spine enabling upright posture. In other primates the
foramen magnum is further towards the back of the skull, forcing the head forwards promoting either
knuckle walking or a quadrupedal gait.
Spine shape: The human spine has a natural S shape (called kyphosis) which acts as a spring.
Carrying angle: In humans the thigh bone is angled outwards rather than being straight. This creates an
increased carrying angle (vagus) and ensures that the knee is brought underneath the body when walking
which lets us put one foot in front of the other.
Feet shape: The human big toe points straight ahead while that of the other primates points off to the side.
This allows humans to have a better transmission of weight in the forward direction.
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Review Questions
In the diagram below the oval represents an organisms head and the smaller square represents the position of
the foramen magnum.
Organism 1
Organism 2
Organism 3
16. The diagrams represent a human, a lizard and a gorilla. Identify which organism is which and explain
how you can tell.
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Skull shape
As it appears in Unit 4
Skull shape is another feature that has changed over time. Modern skulls have the following features:
•
Decreased or absent brow ridges.
•
Face is flatter
•
Jaw is decreased in size
•
Presence of a definite chin
•
Canines are reduced in size
Review Questions
The diagram below shows the side view of the upper and lower jaws of 3 hominid species.
Species 1
Species 2
Species 3
17. Which species is the most modern? Provide 1 reason to support your answer.
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18. An archeologist found some fossilised ancestral human remains, but the only bone intact was the
skull. Explain how the skull could be used to determine whether the human was bipedal or not.
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Cultural evolution
As it appears in Unit 4
Early Hominids developed a unique phenotype. The combination of structural features and enlarged brain
enabled Hominids to manipulate their environment in a way no other animal has been able to do.
Over hundreds of thousands of years, Hominids, and later members of the Genus Homo gradually learned
how to live and survive in a variety of environments. Although these adaptations were passed on to offspring
the method of doing so was cultural rather than biological. This is called cultural evolution.
Examples of cultural evolution include:
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Making and using fire
Making complex tools
Building shelters
Making clothes
Domesticating animals and establishing agriculture
Developing spoken and written forms of communication
Developing art
Religion and ceremonies
Examining areas where prehistoric humans lived provides clues about their daily lives. For example, the
presence of charcoal indicates they had fire, marks on animal bones indicate what kind of weapon they were
killed with or how they were cut up, the presence of different grains indicates the type of agriculture
practices and paintings or burials provide some indication of beliefs.
Generally, locations that are considered desirable by one group are also considered desirable by others.
There are many examples where one group has inhabited an area and later left (or died out) and then another
group has moved in. This may occur many times in one location. Each group leaves traces of their
occupation behind. Each time this happens a layer called an occupation horizon is left behind. Generally the
more recent groups leave more complex traces behind them.
Review Questions
A cave which had been blocked up was excavated and there was found to be several occupation horizons
(layers indicating occupation by different groups of people). Deposits of ash and charcoal were found along
with fossilised bones, broken pottery vessels, flint tools and animal bones. Several individuals were also
found to be buried along with flint knives, leaf shaped arrowheads and stone axes. A few personal items such
as bone pins and shell necklaces were found mixed with the material used to fill in the burial sites.
19. What conclusions can be made about these people?
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20. Excavations from one area found two different classes of artefacts. One class included bones with cut
marks and crude stone tools. The other class included finely crafted flint tools, cosmetic pigments
and ornaments.
When dating was performed it was found that these objects were approximately the same age but that
there was an abrupt change in the techniques of decoration, tool usage and construction from the first
class to the second class. Identify a plausible conclusion that can be drawn from this information.
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21. Explain why a higher occupation horizon would be expected to contain more complex artefacts.
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Mitochondrial DNA
As it appears in Unit 4
Mitochondrial DNA (mtDNA) is inherited through the maternal line because the mitochondria from the
sperm are located in the tail which is not combined with the ova when fertilisation occurs.
Human mitochondrial DNA has been traced back to one female, called mitochondrial Eve, who lived in
Africa between 200 and 300 million years ago. This does not mean that she was the only woman of her time
who was ancestral to modern humans. Sources of mitochondrial DNA are lost if a female does not have any
offspring or has only male offspring.
Mitochondrial DNA is used to establish the degree of relatedness between individuals as well as species as it
mutates at a known rate. In the case of humans mitochondrial DNA has slowly been diverging from that of
mitochondrial Eve and the amount of mutations that accumulate are roughly proportional to the amount of
time that has passed. This means that similarity of mtDNA for any two humans provides a rough estimate of
how closely they are related through their maternal ancestors. If they have identical mtDNA, they are fairly
closely related and if they have very different mtDNA, it means their last common maternal ancestor lived
long ago.
Human intervention in evolution
As it appears in Unit 4
Topic 14 covered information about the link between natural selection and evolution.
Humans are the only species currently known to be able to have a impact on both by both natural and
artifical means. Humans have influenced their own gene pool as well as that of many other species.
Artificial selection
As it appears in Unit 4
Artificial selection refers to humans choosing which organisms are allowed to reproduce based upon their
phenotype. Only those individuals with traits that we see as desirable are chosen as parents of the next
generation. Humans are influencing the frequency of alleles in subsequent generations instead of the impact
of selection pressures.
This intervenes with the process of evolution in the following ways:
1) Since humans are choosing the organisms that reproduce natural selection does not occur; survival of is
not being determined by environmental factors.
2) Genetic variability is often greatly reduced when compared to natural populations as the selection
pressure imposed by humans can be particularly strong.
3) Artificial selection can result in a rapid change in the gene pool as a few individuals with desirable
characteristics are allowed to breed whilst others are not. For example, one or two bulls of high quality may
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be used to father the offspring of all of the cows in a herd of cattle. Since some organisms make a greater
contribution than others genetic variation in the population is reduced. Natural selection and therefore
evolution is dependant upon variation, so this may result in a population of animals that are not able to adapt
and evolve in the future.
Review Question
It has been found that domesticated wheat began to be planted approximately 10,000 years ago. Wild wheat
plants have few small seeds that are easily removed from the plant when they are ripe. Domesticated wheat
plants have many more seeds that are larger in size and that are so tightly attached to the plant that they have
to be removed by mechanical means such as threshing. Archaeobotanists have examined wheat spikes
unearthed during archaeological digs and have found the phenotypic changes to wheat took approximately
300 years to occur.
22. Explain why scientists believe that the changes observed in the domesticated wheat are the result of
human intervention rather than the result of natural selection.
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Reproductive technologies
As it appears in Unit 4
These have 2 main effects:
1) They enable organisms to reproduce whereas previously they would not have been able to do so. This
alters allele frequencies in future generations.
2) When breeding animals a specific stud male is able to produce many more offspring than normal,
increasing the frequency of his alleles in future generations.
Gene screening
As it appears in Unit 4
Techniques such as karyotyping can be used to identify chromosomal abnormalities. DNA sequencing or the
use of restriction enzymes can be used to determine if an unborn child has a faulty allele. Parents can then
become informed about the likelihood of their child having a genetic condition and then make the choice as
to whether to have that child or not, based on the severity of the condition and the quality of life the child
could expect to experience.
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Advancement in medical treatments
As it appears in Unit 4
There are many medical conditions (for example haemophilia and cystic fibrosis) which previously would
have caused death in infancy. Although treatment is still not perfect there is still a chance that people with
these conditions will survive long enough to have offspring, passing on the allele/s responsible for the
condition and therefore increasing the incidence of these genetic diseases.
Cloning
As it appears in Unit 4
Artificial cloning is where selected genes/cells/organisms are manipulated to produce genetically identical
copies.
Somatic nuclear transfer cloning involves inserting the nucleus from an adult somatic cell into a donor egg.
The process which resulted in the production of dolly the sheep is summarised in the diagram below.
A donor egg is removed from sheep 1
A skin cell is removed from sheep 2
The nucleus is removed from the egg
cell from sheep 1.
The nucleus from the skin cell of sheep
2 is inserted into the enucleated egg cell
of sheep 1.
The resultant embryo is implanted into
sheep 3.
The embryo eventually matures into a lamb that will be genetically identical to sheep 2.
This type of technique is used to produce identical copies of organisms with desirable traits. A population of
clones has near zero genetic variability in its nuclear genome (mutations can still occur). This impacts on
natural selection and evolution in these populations.
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Production of transgenic organisms
As it appears in Unit 4
Transgenic organisms contain genes from other species; since the traits being provided are seen as desirable
to humans, organisms with these traits may be selected for at the expense of unmodified organisms.
Gene therapy
As it appears in Unit 4
Gene therapy is the treatment of a genetic disorder by introducing an engineered or a non-faulty copy of a
gene into the cells of a patient. This can be done in two ways - inserting the gene directly into the patients
tissues, or by taking a tissue sample from the patient, inserting the gene into them and then inserting the
treated cells back into the patient.
Disorders caused by a single faulty gene are easier to treat than those disorders that are polygenic.
For gene therapy to be successful the following conditions have to be met:
1) It must be known what tissues or organs the corrected gene needs to be inserted into, and these tissues or
organs must be accessible.
2) The sequence of the correct gene sequence must be known.
3) The correct gene must be able to be isolated or synthesised and be capable of being inserted into a vector.
Review Question
23. Briefly summarise the impact of human intervention in evolution.
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