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Evolution
Natural Selection and selective Breeding
EVOLUTION AND NATURAL SELECTION
The basic idea behind the theory of evolution is that different species have developed over time from other life
forms
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5 billion years old and there is scientific evidence that life on Earth began more than
three billion years ago
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The key points are
that:
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

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Individuals in a species show a wide range of variation and this variation is because of differences in their
genes
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The genes that allow these individuals to be successful are passed to their offspring
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This means that
their genes are less likely to be passed on to the next generation
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Both genes and the environment can cause variation, but only genetic variation can be passed on to the next
generation
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This is an example of natural
selection
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This has
led to more infections that are difficult to control, particularly in hospitals
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The difference between the
two is that natural selection happens naturally, but selective breeding only occurs when humans intervene
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Different varieties of plants and animals with desired characteristics can be developed by selective breeding
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These are the main steps involved:
1
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3
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1

decide which characteristics are important enough to select
choose parents that show these characteristics
choose the best offspring from parents to produce the next generation
repeat the process continuously

Genetic Modification And Cloning
Genetic modification (GM, also called genetic engineering) involves taking a gene from one species and putting it
into another species
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2
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4
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For example, a plant may be
produced with improved resistance to pests
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There are concerns about the possible health risks
of genetically modified food
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Others think it is morally wrong to create new life forms, or to move genes between different species
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This means that less herbicide
needs to be used
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For example:
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the potential development of herbicide-resistant weeds
loss of biodiversity because fewer weeds survive - resulting in reduced food and shelter for animals

CLONING MAMMALS
The stages of cloning a mammal include:
1
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3
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removal of diploid nucleus from a body cell
enucleation - removing the nucleus from an egg cell
insertion of the diploid nucleus into the enucleated egg cell
stimulation of the diploid nucleus to divide by mitosis

The new animal is genetically identical to the animal that donated the nucleus from one of its body cells
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Some of this variation within a species
is genetic, some is environmental, and some is a combination of both
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When these join at fertilisation, a new cell is formed
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Environmental causes; Characteristics of animal and plant species can be affected by factors such as climate, diet,
accidents, culture and lifestyle
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A plant in the shade of a big tree will grow taller to reach more light
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Continuous; For any species, a characteristic that changes gradually over a range of values shows continuous
variation
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Any height is possible
between these two extremes
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Discontinuous variation; A characteristic of any species with only a limited number of possible values shows
discontinuous variation
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There are no values in between (intermediate values), so this shows
discontinuous variation
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It is a rare, random change in the genetic material and it can be
inherited
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It can also happen because of:

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ionising radiation
chemical mutagens - such as tar from cigarette smoke

Ionising radiation includes gamma rays, X-rays and ultraviolet rays
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A mutation may be neutral and have no effect
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A mutation may sometimes be beneficial
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