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AREA OF STUDY 2 – CHANGE OVER TIME
The development of evolutionary theory
The theory of evolution formally began with Jean-Baptiste Lemarck who suggested a theory of
acquired characteristics in 1809. According to Lemarck an individual organism could change
itself to suit an environmental condition and then pass this on to their offspring. For example, he
reasoned that Giraffe’s could see leaves out of reach in the tops of trees so they stretched their
necks to reach them and then passed this on to their offspring. Hence, Giraffe’s changed from
being browsers of grass and shrubs to browsers of tree tops, with much less competition. We
now know that this is not possible, but Lemarck predates the genetic work of Mendel.
Alfred Wallace and Charles Darwin, working separately, came up with a theory of natural
selection first published in 1858. They observed that:
- reproduction rates were so high that unless death rates were also very high populations
would be very large.
- Each group of organisms showed differences between individuals (variation)
- Offspring are similar to their parents.
They each independently concluded that the differences between individuals determine how
well they survive and the traits that increase the possibility of survival are passed on to their
offspring.
Natural Selection can be summarised as
1. There is variation in the population
2. offspring resemble their parents
3. more offspring are born than can possibly survive
4. there is a struggle to survive, some individuals are better suited than others (more fit)
and they survive and pass on their characteristics to their offspring.
Evolution occurs when the gene pool of a species permanently changes and a new species arises
from the existing species. The gene was the missing mechanism to explain how this could occur,
yet Wallace and Darwin had access to Mendel’s work and never read it.
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Natural selection as a mechanism of evolution
A selection pressure may be placed on individuals in a population and cause some to survive and
some to die. Charles Darwin’s theory of natural selection assumes that in any population:
 there is variation with respect to a particular trait/characteristic
 some traits are better suited to survival than others (a selecting agent determines which
traits are better suited to the environment)
 there is a struggle for survival, not all individuals will survive and reproduce successfully
 those individuals that have suitable traits will survive and pass on the desirable traits to
their offspring
 traits that are favourable will slowly increase in the population over time.
One example is the Peppered moths which vary in their colour. In dark environments (cities) the
darker ones are most camouflaged and survive better than those with a lighter colour. The
darker moths are said to have a high genetic fitness in this environment and will be able to
reproduce, with their favourable traits being passed onto their offspring, while the white moths
are eaten and their numbers slowly decline (lower genetic fitness). Thus over time the allele
frequencies will change in the population.
Geological time
The earth is believed to be 4500 million years old. Geologists divide time into three main groups:
 Era – large periods of time divided by major catastrophic extinctions
 Periods – divides era into smaller time periods usually defined by a major event
 Epochs – a distinctive period of time within a period.
During the history of the earth there have been a number of significant events. The most
significant may have been the mass extinctions of all dinosaur species some 65 mya. Also we
have been able to map the movement of the tectonic plates underlying the continents.
The Earth’s crust floats on a layer of molten rock. As there are convection currents at work
within this crust, these plates move – very slowly. It is believed that at one time all the land
surface existed as one giant continent called Pangaea – about 200 mya. This broke up into a
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northern hemisphere continent called Laurasia and a southern hemisphere continent called
Gondwana. Over the last 100 my the continents of south America, Africa, Antarctica, and
Australia (with some of the associated islands of Madagascar, New Zealand and Papua New
Guinea) have drifted apart. We continue to drift, with Australia moving north toward Asia at
about 6cm a year
Evidence of evolution
We once believed that each species was the result of an act of creation - extinctions were
possible, but all species that existed had been there since God had created them. Evolution
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suggests that species can change over time in response to changes in their environment and
new species are therefore able to arise and extinctions will occur.
There are six main sources of evidence for the theory of evolution.
FOSSIL RECORD
Types of fossils:
Trace fossils – footprints or other animal marks, which are buried and then harden.
Amber fossils – fossils become trapped in plant sap, which hardens and forms amber.
Cast fossils – an organism is buried in rapidly hardening mud, decays completely and then the
space it occupied - the mould – is filled with another kind of material. This results in the
formation of a cast.
Preserved body parts – teeth, bones and shells either whole or fragments
The process of fossilisation is:
 Organism dies and is buried quickly in an area that is still or undisturbed. Can be
disturbed by predation or decay.
 Oxygen is excluded very quickly after death so that maximum preservation is possible.
 Hard parts of the organisms leach out, leaving a cast or space in the sediment, which is
then filled by minerals and hardened.
 The resulting filled space is a mould of the original organism in the sediment. The
sediment is buried by repeated layers and appropriate pressure and temperature are
exerted to form the rock in which the fossil is preserved.
What increases the chances of fossilisation?:
 Hard parts present, such as shells, bones or teeth
 The dead organism must be buried quickly, such as by mud or volcanic ash, and then not
disturbed by scavengers or natural climatic effects.
 Anaerobic conditions which prevent further decay
 Low acidity stops them from being eaten away.
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Fossils can be aged:
 Relative age – older layers of rock form before younger ones and so the deeper layers
are older than the more recent layers, so if we date the age of the rocks found with the
fossil we have a relative age of the fossil.
 Absolute age – using radioactive isotopes we are able to date the fossil itself. As all
matter decays at a known rate (called the half life) the amount of atoms of an element
present in relation to the amount of atoms of the element that it decays to gives a
reasonably accurate estimate of age. The most commonly used are:
o Carbon – 14 (half life of 5730 years) decays to nitrogen – used to date fossils
under 40 000 years old
o Potassium (half life 1.2 billion years) decays to argon – use to date fossils older
than 40 000 yrs (K/Ar)
 Electron Spin resonance – many common minerals collect electrons in their crystal
lattice at a steady rate. This method dates objects by measuring the number of
electrons present. Useful for dating corals, mollusk shells, and tooth enamel.
 Luminescence techniques – minerals emit light when heated and when exposed to light.
The amount of light emitted can be used to date substances containing minerals.
BIOGEOGRAPHY
Many species which have a common origin are spread all over the world. One example is the
many species from the Southern Hemisphere that trace their origins back to the supercontinent
of Gondwana eg flightless birds like the emu, ostrich, rhea, cassowary and kiwi, and plant
species like the southern beech (Nothafagus). By discovering their fossil remains and the fossil
remains of their ancestors we are able to suggest evidence for Gondwana existing in the past.
Dating those remains allows us to recreate the time frame of the break up of Gondwana.
COMPARATIVE ANATOMY
Evolutionary relationships can be established on the basis of structural similarities and
differences. If separated organisms have a common ancestor, they would have similar basic
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structural features. How similar an organism is to another should give some indication of how
related they are in terms of evolution. For example homologous structures are structures that
have the same origin and are similar in structure but may have completely different functions.
Humans, pigs, bats, lizards, frogs, whales and cats all have limbs with five digits, suggesting they
all have a common origin (common ancestor) and have all inherited similar genes.
Vestigial structures are structures that have been lost or there has been a reduction of a
structure during evolution. Human embryos have gill slits and tails during the early stages of
development in utero.
COMPARATIVE EMBRYOLOGY
Embryos from mammal species can be compared to embryos from other mammal species, and
the species of other phyla and across millions of years ago, indicating a common set of ancestral
instructions.
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COMPARATIVE BIOCHEMISTRY – DNA
a) Molecular (or DNA-DNA) hybridisation:
Two single stranded pieces of DNA from two different organisms are collected. The samples are
heated to cause separation of the double strands and restriction enzymes are used to cut the
DNA sequences into shorter pieces. As the solution cools the double strands reform due to
complementary base pairs bonding together.
The more similar the sequences the more bonding of base pairs you will get.
These hybrid double strands are then heated and the higher the temperature that can be
achieved before the hybrids break again will depend on how closely related the species are.
b) DNA sequencing:
The more similar two species are the more similar their DNA sequences will be. When the
sequences of nucleotides are known it is possible to compare two or more species.
When 7100 bases were sequenced from the following species, the differences in sequences
were as follows:
human – chimpanzee – 1.6%
human – gorilla – 1.7 %
human – orang-utan 3.4%
This indicates that these four species are closely related, probably arising from a common
ancestor.
COMPARATIVE BIOCHEMISTRY – PROTEINS
Many organisms produce the same enzymes, therefore must have the same or similar genes.
Many organisms have similar amino acid sequences. Cytochrome C is a molecule common to
many organisms, used in respiration. This suggests that these organisms share a common
ancestor Insulin derived from pigs can be used by humans suggesting a common ancestor
between pigs and humans (we are both placental mammals of course, but not all placentals
share the same insulin protein structure).
Patterns of evolution
Divergent Evolution describes an evolutionary pattern of change from a common ancestor. It
results in a process called adaptive radiation whereby populations of one species move into new
environments and then over time become separate species as they adapt to the unexploited
habitats. Evidence for Divergent Evolution includes homologous structures such as the limbs of
mammals. If they are related by evolution from a common ancestor, it would be expected that
they would show similarities in structures and this is seen in the forearms/legs/fins of bats,
whales, cats, horses, humans etc.
Convergent Evolution describes an evolutionary pattern of change in which separate species
adapt in similar ways in responses to their environments putting comparable body parts to
similar uses and in time resemble one another in structure and function (analogous structure).
Evidence for Convergent Evolution includes the wings of flys, bats and birds. They all perform
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the same function, but the organisms have evolved different structures to achieve the outcome
of flight.
Change in populations
The study of allele frequencies in a population and the agents that change these frequencies is
known as population genetics.
Gene pool is the term used to describe all the genetic information within a population. Each
gene in the gene pool usually exists in two or more slightly different forms called alleles. As the
gene pool of a population changes the individuals may change in appearance and the species
may evolve.
Individuals inherit different combinations of alleles, this comes from:
 the independent assortment of chromosomes during metaphase 1 of meiosis
 crossing over of chromatids in prophase 1
 random nature of fertilisation where any sperm is equally likely to fertilise any ovum
 random mating of individuals
This leads to variation in genotypes and therefore phenotypes eg, hair colour, depends on the
alleles you inherit from your parents and the effect of the environment
Allele frequency is the term used to describe the abundance of any given allele in a population,
relative to other alleles at the same locus. Eg, the allele for blond hair in Icelandic populations
compared with the allele for other hair colours.
Under certain conditions allele frequencies may remain stable for periods of time.
The allele frequencies will stay the same over generations if
 there is no mutation, so fewer new alleles are likely to arise
 the population is infinitely large, small changes are hard to distinguish
 the population is isolated so there is no migration, no alleles enter from elsewhere
 mating is random
 all individuals survive and reproduce successfully and at the same rate.
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The population is therefore not evolving with respect to that gene pool.
A number of factors may lead to changes in allele frequencies.
Factors affecting population equilibrium
The following factors all have a greater impact on a small population as the gene pool is already
small, so any change can be easily seen and affect a high percentage of individuals.
Founder Effect: If a few individuals leave a population and establish a new population
somewhere else, usually geographically isolated, this is known as the founder effect and those
individuals are the founder population. The allele frequencies of the individuals in the new
populations may be quite different to those of the original population and selection pressures in
the new habitat may favour different phenotypes. This can have a profound effect on an
isolated island, possibly evolving into a new species.
Genetic drift: chance events that increase or decrease variations in alleles over time. An allele
not associated with a decrease or increase in mortality, emigration or reproduction may become
more common in one generation to the next by chance. For example, a female may be
homozygous for a colour blind allele and have 10 children, that allele would increase in
frequency slightly in the next generation. Genetic drift becomes very important when
populations are very small. However, if the parent population is very large then the gene pool
will probably be very similar to the parent population. Non-random mating between closely
related individuals (inbreeding), who may have many alleles in common, can lead to many
homozygous conditions becoming more common.
Bottleneck: an extreme case of genetic drift whereby a catastrophic decline in population size
leads to a random shift in the allele frequencies among survivors: because the population must
rebuild from so few individuals, severely limited genetic variation may be an outcome. This is
believed to have occurred in the cheetah population, with all extant members related to one
female. The Helmeted Honeyeater is a Victorian endangered species which suffers from a
bottleneck – genetic diversity is critically low for this species and they are only one catastrophic
event from extinction.
Examples of a severe decline in numbers in a population may come from:
 contagious disease
 loss of habitat
 hunting
 massive volcanic explosions
Individuals may leave (emigrate) or come to a population (immigrate) and cause the alleles they
are carrying to decrease or increase in the population accordingly. Individuals may also migrate
due the presence of a particular allele this is called allele specific migration.
Immigration is usually balanced by emigration for the species in terms of numbers; however, it
may affect the frequency of an allele, due to the increased movement of some alleles in or out
of populations. This is known as gene flow.
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The rate of gene mutation (the introduction of a new allele) is usually very low and has little
impact on allele frequencies in populations, because most mutations lead to changes in
structure, function, or behaviour and decrease an individuals chance of surviving and
reproducing. However, when a mutation is beneficial, or not harmful, to an individuals chances
of survival, then a new allele may be added to the gene pool.
Variation amongst individuals leads to some being more suited to their environment. Whether
this variation is their ability to feed, by having suitable feeding apparatuses (structural variation)
or their colour differences, which may or may not camouflage them in their environment
(biochemical variation). Variation results from genetics and the organism’s environment. The
factors that change allele frequencies will therefore change phenotypic frequencies.
A species is a group of organisms that are actually or potentially able to interbreed and produce
viable offspring. When populations of a species become geographically isolated change can
occur over time in response to different environmental pressures and new species arise. This is
described as Allopatric speciation. There are a number of ways that populations of a species
may become isolated.
Pre-reproductive isolating mechanisms – stops populations from physically mating
- geographic mechanisms such as mountains, rivers, seas and lakes separate populations
(this now includes habitat fragmentation due to human impacts such as roads, trains,
cities, farms and logging).
- time mechanisms causing populations to breed at different times so individuals do not
meet at the right time to successfully breed
- behavioural mechanisms such as different courtship behaviours in individuals from
different populations of the same species
- morphological mechanisms – mating is made physiologically impossible because
genitalia are different in size, shape or location.
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Post-reproductive isolating mechanisms – stops the offspring from potential mating from
developing into adults
- high gamete mortality
- high zygote mortality
- hybrid sterility – such as the mule which results from breeding between horses and
donkeys
Extinction is also a natural phenomenon. If a species cannot change, or cannot change fast
enough, in response to changed environmental conditions then that species may cease to exist,
that is, become extinct.
Evolutionary relationships
Evolutionary relationships are mapped using phylogenetic trees. Phylogenetic trees start with a
common ancestral species and then show the points at which new species have branched off.
The information that is required to construct these trees is available from:
- structural data – teeth, bones of specimens or fossils
- biochemical data – the use of DNA and proteins to indicate relationships between
species.
Conservation of Genes
To compare DNA depends on the degree of difference in nucleotide sequences. Some sections
of genes remain unchanged for many generations (that is conserved) while other parts change
rapidly. Interestingly, the introns (non-coding sections) tend to change reasonably readily while
the exons (coding regions) are conserved. This allows us to use various biochemical mechanisms
to determine the degree of relatedness between species.
 DNA sequencing – that is if the sequence of bases in strands of DNA from
individuals from different species are similar this can be used as evidence for
the two species having arisen from a common ancestor. The more similar the
DNA sequences the closer the in time the ancestor.
 DNA-DNA hybridisation – in this technique DNA is extracted from the cells of
two organisms under investigation, heated to separate the double strands and
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then cut with restriction enzymes. The two samples are mixed and as they cool
the strands realign and form the double stranded DNA. The greater the number
of bonds the stronger the bonds between strands. The strands are then heated
again. The more heat required, the more bonding that has occurred and
therefore the closer the two species are related. It has been measured that
Humans and Chimpanzees share 97.6 % of their DNA.
Mitochondrial DNA – consists of a single circle of DNA. It is almost entirely from
the maternal line as ova contain mitochondria. The DNA codes for about 13
genes, 22 transfer RNA’s and two ribosomal RNA’s. mtDNA mutates at a steady
rate and can therefore be used as molecular clock. The greater the amount of
difference in mtDNA the longer the clock has been running and the further apart
the two species are. mtDNA also supports the belief that Humans and
Chimpanzees are the most closely related of the primates supporting the DNADNA hybridisation data.
Chloroplast DNA – plants have DNA in their chloroplasts. It is a circular, double
strand containing about 100 genes. This DNA has changed very little over
evolutionary time and can be used in a similar way to mtDNA to test for the
evolutionary relationship of plant species.
Proteins – as protein structure is determined by the sequence of amino acids,
which is determined by the sequence of bases in the coding DNA, the number of
differences in the sequences of same or similar proteins between species can
indicate how closely related two species are. For example, when comparing
Human and Chimpanzee haemoglobin 97% reactivity is found suggesting a close
relationship.
Hominid evolution: patterns, origins
Primates – a group of mammals that have binocular vision, flat nails, exposed, sensitive finger
pads, and large brain relative to body size.
Hominins – a sub-group of primates that walk upright and have relatively large brains.
Hominids – a sub-group of hominins containing all the human and human like species.
Humans are primates - a group of organisms including lemurs, tarsiers, new world monkeys, old
world monkeys and Hominins (the great apes - chimps and gorillas and humans). We share a
number of characteristics which confer adaptive advantages:
- relatively large brain for body size
- 5 digits on hands and feet able to grasp objects
- opposable thumbs
- large forward facing faces
- 4 upper and 4 lower incisors – omnivorous diet
- flexible skeletons – movement through trees
- generally small litters – intensive and extended parental care
The primates split into two main branches: the prosimians (lemurs and lorises) and the
anthropoids (new and old world monkeys) very early. Primates have evolved from a common
ancestor by divergence over a period of 65 my
- common ancestor probably a small tree dwelling hominin in Africa
- hominids first appeared about 6 mya
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human divergence - 2.5 mya,
Humans are considered Hominids because we have skeletal changes that allow us to walk
upright. This occurred about 3.5 mya and appears to predate the appearance of the first stone
tools. There are five key human evolutionary trends:
 There was less reliance on the sense of smell and more on daytime vision.
 Skeletal changes led to upright walking, which freed hands for novel tasks.
 Changes to bones and muscles led to refined hand movement.
 Teeth became less specialised.
 Elaboration of the brain and changes in the skull led to speech.
Humans have not evolved from chimpanzees and apes, but rather, humans, chimpanzees, and
apes have evolved from a common ancestor several million years ago, which in turn had shared
a common ancestor with the orang-utans, which shared a common ancestor with the gibbons.
humans
chimps other apes
common ancestor
There are many versions of the evolution of Homo sapiens from its early ancestors, the most
common would put the fossil remains in the following order with variations in who is an
ancestor of whom. Maeve Leakey is now claiming another species Kenyanthropus platyops,
which she feels was present at the same time as A. afarensis and A. africanus, but different to
them.
1.
Ardipithecus ramidus
2.
Australopithecus anamensis
3.
A. afarensis
4.
A. africanus
5.
Homo habilis
6.
A. boisei
7.
H. erectus
8.
H. heidelbergensis
9.
H. neanderthalensis
10.
H. sapiens
Recent work suggests that erectus and habilis coexisted. The discovery of Kenyapothus may also
cause revision of this sequence.
Australopithecus formed two groups:
Graciles - lightly built skulls ancestral to the Homo
Robust - heavy skulls, big teeth - disappeared before Homo
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Homo
- much variation - but all belong to a single species
- may have been as many as 7 species of Homo over time, new one found recently in
Indonesia – “The Hobbit” or Homo floresiensis.
- H. habilis - first to make tools
- H. erectus - use of tools and fire - great traveller - found in Africa, Europe, Asia and Java
- H. heidelbergensis - intermediate from H. erectus to sapiens
- H. neanderthalensis - overlapped with H. sapiens, but not our ancestors - sapiens may have
out competed H. neanderthalensis
- Cro Magnon Man - first of the great artists and tool makers
- H. sapiens
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A) Pan troglodytes, chimpanzee, modern
(B) Australopithecus africanus, STS 5, 2.6 My
(C) Australopithecus africanus, STS 71, 2.5 My
(D) Homo habilis, KNM-ER 1813, 1.9 My
(E) Homo habilis, OH24, 1.8 My
(F) Homo rudolfensis, KNM-ER 1470, 1.8 My
(G) Homo erectus, Dmanisi cranium D2700, 1.75 My
(H) Homo ergaster (early H. erectus), KNM-ER 3733, 1.75 My
(I) Homo heidelbergensis, "Rhodesia man," 300,000 - 125,000 y
(J) Homo sapiens neanderthalensis, La Ferrassie 1, 70,000 y
(K) Homo sapiens neanderthalensis, La Chappelle-aux-Saints, 60,000 y
(L) Homo sapiens neanderthalensis, Le Moustier, 45,000 y
(M) Homo sapiens sapiens, Cro-Magnon I, 30,000 y
(N) Homo sapiens sapiens, modern
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Two theories attempt to explain how Homo’s left Africa and conquered the world.
 The Out of Africa theory has the most support amongst scientists and has been the
dominant theory for a period of time. It states that modern humans originated in Africa
100 000 years ago and migrated from Africa replacing local ancient populations of
Homo’s.
 Regional continuity theory suggests that modern humans evolved independently in
many regions of the world from populations of Homo erectus that had migrated from
Africa 2 mya.
Both theories use biochemical data and the fossil record as evidence for their case.
Interrelationships between biological, cultural and technological evolution
Biological
Evolution
Cultural
Evolution
Technological
Evolution
Cultural evolution is the development of complex behaviours to do with burial rights, religious
rituals, child raising, hunting and gathering.
These are clearly linked with technological evolution, as the cultural behaviours become more
complex, then tools are needed and technology starts to evolve from simple clubs and sharp
rocks, to arrow heads, stone axes, digging tools and spears.
But both rely on biological evolution, the development of the brain to develop, learn and
transmit these complex behaviours and the muscle and skeletal changes required to support the
use of tools.
Homo sapiens, with their large brain capacity in relation to their body size, have been able to
evolve biologically, culturally and technologically more than any other single species.
Human intervention in evolutionary processes
Humans intervene in evolution by changing the environment. This causes extinctions through
the clearing of habitats, hunting, over exploitation of a species and the use of pesticides and
antibiotics. We have interfered with the genetic basis of life by genetic manipulation such as:
 Selective breeding – has been used in agriculture and with companion animals for many
thousands of years. For example, all dog breeds belong to one species that evolved from
the wolf – Canis familiaris. Each breed has different characteristics which are used by
humans to hunt, protect, round up or just be a companion. In the same way the wild
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mustard, Brassica oleracea, has been breed to produce cabbage, broccoli, cauliflower
and kohlrabi – important food plants.
 Domestication – many animals, apart from dogs, have been domesticated from wild
populations for our needs – horses, donkeys, goat, sheep, cattle, chickens, geese, ducks,
pigeons to name a few.
 Artificial cloning of organisms – we are now cloning organisms of several species for the
same reasons – namely to breed a species that meets our needs, only cloning is quicker.
Think of Dolly the sheep, the Blue Rose, there are now many other examples of Horses,
Pigs, Cattle etc.
 Genetic engineering – transformations (inserting flounder genes for anti-freeze into
tomatoes so that they can survive frost, or resistance to pesticides into commercial
plant crops), stem cells differentiation (with the desire to custom build organs for sick
and dying humans), genetic screening (to test for the presence of genetic disorders with
the potential to terminate pregnancies and remove certain alleles from the population),
and gene therapy (using bacteria and viruses to either grow human hormones such as
insulin, or deliver genetic information to reduce the effects of a disease such as Cystic
Fibrosis).
Clearly there are many questions raised by this activity and people argue the ethical, moral and
medical rights and wrongs of these activities.
SUGGESTED ACTIVITIES
1.
Rewrite all the headings and sub-headings from this section of the course.
2.
Now add the most important concept below each of the headings and sub-headings
3.
Prepare a glossary of the main concepts in this section of the unit.
4.
Write 10 short answer questions to test this section of the unit.
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