Download variation and selection

VARIATION AND SELECTION
variation
OBJECTIVES
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Define variation
Distinguish between phenotypic variation and genetic
variation
Continuous variation e.g. height in humans
Discontinuous variation e.g. tongue rolling
Record and present the results of investigations into
continuous and discontinuous variation
Define mutation
Mutagenic factors
Define adaptive feature as an inherited feature that
helps an organism to survive and reproduce in its
environment
Interpret images or other information about a species
to describe its adaptive features
VARIATION
Individuals in a population are usually similar to
each other, but not identical. Some of this
variation within a species is genetic, some is
environmental, and some is a combination of
both.
 So variation is the differences between individuals
of the same species
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GENETIC VARIATION
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Children usually look a little like both their mother
and their father, but they will not be identical to
either one of them. This is because they get half of
their inherited features from each parent.
Each sperm cell and each egg cell contains half of the
genetic information needed for an individual (each
one is haploid - it has half the normal number of
chromosomes). When these join at fertilisation, a new
cell is formed. This zygote has all the genetic
information needed for an individual (it is diploid - it
has the normal number of chromosomes).
Examples of genetic variation in humans include
blood group, skin colour and eye colour.
GENETIC VARIATION
PHENOTYPIC VARIATION
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Characteristics of species can be affected by
factors such as climate, diet, accidents, culture
and lifestyle. For example, if you eat too much
you will become heavier, and if you eat too little
you will become lighter. A plant in the shade of a
big tree will grow taller to reach more light
CONTINUOUS AND DISCONTINUOUS
VARIATION
Continuous variation
 For any species, a characteristic that changes
gradually over a range of values shows
continuous variation. Examples of such
characteristics are:
 height
 weight
 hand span
 Height ranges from that of the shortest person in
the world to that of the tallest person. Any height
is possible between these two extremes. So it is
continuous variation.
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CONTINUOUS AND DISCONTINUOUS
VARIATION
Discontinuous variation
 A characteristic of any species with only a limited
number of possible values shows discontinuous
variation. For example:
 gender (male or female)
 eye colour
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There are four blood groups (A, B, AB or O).
There are no values in between (intermediate
values), so this shows discontinuous variation.
MUTATION
Mutation is a change in a gene or chromosome.
It is a rare, random change in the genetic
material and it can be inherited.
 Causes of mutation
 Mutation can be spontaneous (it just happens). It
can also happen because of:
1. ionising radiation
2. chemical mutagens - such as tar from cigarette
smoke
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READ ONLY
DOWN’S SYNDROME
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Down’s syndrome is caused by the presence of
three copies of chromosome 21 (or an extra bit of
a chromosome 21), rather than just the usual two
copies. It happens when a gamete (sperm cell or
egg cell) forms abnormally during meiosis, and
has an extra copy of chromosome 21.
ADAPTIVE FEATURE
an inherited feature that helps an organism to
survive and reproduce in its environment
 What are their adaptive feature?
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VARIATION AND SELECTION
Objectives:
 Describe natural selection
 Describe selective breeding with reference to:
– selection by humans of individuals with desirable
features
– crossing these individuals to produce the next
generation
– selection of offspring showing the desirable
features
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NATURAL SELECTION
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Individuals in a species show a wide range of
variation and this variation is because of differences
in their genes.
Because of the competition for resources Individuals
with characteristics most suited to their environment
are more likely to survive and reproduce.
The genes that allow these individuals to be
successful are passed to their offspring.
Those that are poorly adapted to their environment
are less likely to survive and reproduce. This means
that their genes are less likely to be passed on to the
next generation.
Given time, a species will gradually evolve. Both
genes and the environment can cause variation, but
only genetic variation can be passed on to the next
generation
NATURAL SELECTION
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Who will win?
NATURAL SELECTION
SELECTIVE BREEDING
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Selective breeding is the traditional method for improving
crops and livestock, such as increasing disease resistance or
milk yield.
Natural selection and selective breeding can both cause
changes in animals and plants. The difference between the
two is that natural selection happens naturally, but selective
breeding only occurs when humans intervene. For this reason
selective breeding is sometimes called artificial selection.
Different varieties of plants and animals with desired
characteristics can be developed by selective breeding. For
example:
cows that produce lots of milk
chickens that produce large eggs
wheat plants that produce lots of grain
The new varieties may be economically important. For
example, they may provide more or better quality food to feed
people.
STEPS OF SELECTIVE BREEDING
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selection by humans of individuals with
desirable features
crossing these individuals to produce the next
generation
selection of offspring showing the desirable
features