Download AS 2.3.3 Evolution - Mrs Miller`s Blog

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(a) define the term variation;
(b) discuss the fact that variation occurs within as well as between species;
(c) describe the differences between continuous and discontinuous variation, using
examples of a range of characteristics found in plants, animals and microorganisms;
(d) explain both genetic and environmental causes of variation;
(e) outline the behavioural, physiological and anatomical (structural) adaptations of
organisms to their environments;
(f) explain the consequences of the four observations made by Darwin in proposing
his theory of natural selection; (HSW1)
(g) define the term speciation;
(h) discuss the evidence supporting the theory of evolution, with reference to fossil,
DNA and molecular evidence (HSW1, 4, 7a, 7b);
(i) outline how variation, adaptation and selection are major components of evolution;
(j) discuss why the evolution of pesticide resistance in insects and drug resistance in
microorganisms has implications for humans (HSW6a, 7c).
• Variation is the differences
between individuals- no two
individuals are exactly alike
even if they look similar
• Identical twins start as one cell
that divides and then
separates into two cells,
although they started with the
same original DNA, the
subsequent divisions have
introduced changes, also
environmental changes may
have taken place in the womb
or after birth.
Variation within
• Members of
the same
species can
show variation
• Examples are:
eye, hair and
skin colour,
nose shape,
eye shape etc
Variation between
• Variation between
species is more
obvious- it is what
we use to separate
one species and
Continuous Variation
• There are usually 2
extremes and a full
range of intermediate
values between the
extremes- most
individuals are close
to the mean value and
the number at the
extremes is low
• Examples are: human
height, length of
leaves on an oak tree,
length of stalk of a
Discontinuous Variation
• There are two or more
distinct categories, with no
intermediate values
• There may be more than
one type than the other or
they may be distributed
• Examples are: sex- male
or female in plants and
• Some bacteria have
flagella, some do not
• Human blood groups- A,
B, AB or O
Inherited and Genetic Variation
• Genes define our
• The combination of
alleles that we inherit is
not the same as any
other living thing
• Human cells have approx
25000 genes, many with
different alleles, so the
chance of having the
same ones is extremely
Environmental Variation
• Characteristics affected by the
• Examples: skin tone can
become darker in the sun,
hawthorn trees may grow
sideways in areas with strong
gales, affecting the direction of
growth, lack of water can affect
the height of growth
• Environmental and genetic
variation can be linked e.g.
humans now have a better
diet, and so can grow taller,
however your genes also limit
final height
• Adaptations can be
physiological, or
• Adaptations help the
organism to cope with
environmental stresses
and obtain the things they
need e.g. to find water,
gather nutrients, respond
to changes, defend from
Behavioural Adaptations
• An aspect of the
behaviour of an organism
that helps it to survive the
conditions it lives in
• Example: an earthworm
has no eyes, so cannot
tell if a bird is about to eat
it, however on touch,
earthworms rapidly
contract and recoil into
their burrows- this
adaptation allows them to
avoid being eaten
Physiological Adaptations
• A physiological or
biochemical adaptation
ensures the correct
functioning of cell
• Example: yeast
(Saccharomyces) can
respire sugar either
aerobically or
anaerobically depending
on oxygen availabilitythis also involves
producing the correct
enzymes for the type of
respiration required
Anatomical Adaptations
• Anatomical means
• Any structure that
enhances the survival
of an organism is an
• Example: bacteria like
Legionella have
flagella that help them
move independently
Case Study: Plants
Xerophytes grow in areas with a low
water supply e.g. cacti
Behavioural adaptations: open and
close stomata, open stomata at night
only, fold and roll leaves when lack of
water, open stomata when short of
water to wilt leaves and reduce
surface area
Physiological adaptations: stem with
accordion-fold structure that folds
more in dry periods, when water is
plentiful the cells expand and the stem
enlarges and becomes more rounded
Anatomical: shallow roots spread out
over a large area, long roots e.g.
camel thorn tree (40m long), fleshy
stem and leaves, waxy leaves, curled
and folded leaves with sunken stomata
Charles Darwin
• The theory of evolution was
not his idea
• He proposed a mechanism
for this process
• He called it natural selection
• Darwin was interested in the
variation between members
of the same species and this
underpinned his work
• He studied in the Galapagos
Islands and worked on a ship
called the HMS Beagle
The Theory of Natural Selection
Darwin made 4 important observations:
1. Offspring generally appear similar to their
2. No two individuals are identical
3. Organisms have the ability to produce
large numbers of offspring
4. Populations in nature tend to remain
fairly stable in size
Developing the Theory
• Darwin read an essay by Thomas
Malthus who said that the human
population would outstrip its food
supply- Darwin disagreed and
said that competition for food and
resources would keep the
population in check
• Darwin saw that too many young
are produced and that this leads
to competition for food and
• As all offspring are different, the
better adapted ones will obtain all
the food and survive long enough
to reproduce and pass this onto
their offspring
• The less well adapted ones are
likely to die before they
reproduce, so the population does
not grow indefinitely
Link to Evolution
• Over a long period of time, small variations
arise and if beneficial, will be passed on to the
next generation
• Over many generations a species has many
variations and may even change into another
species that has so many changes it could
never reproduce with its ancestors as the DNA
has changed so much
Darwin’s Conclusions
• There is a struggle to
• Better adapted
individuals survive
and pass on their
• Over time, a number
of random changes
may give rise to a
new species
Selection Pressures
If you can survive after undergoing these selection
pressures, you are more likely to pass on your
random adaptations
• Availability of suitable food; adapted to eat the
available food
• Predators; adaptations to avoid being seen and
eaten, or to escape
• Diseases; survival of diseases
• Physical and chemical factors; living in cold
places, shade, hot summers
• Speciation means the
formation of a new
species from a preexisting one
• Does not occur suddenly
• Long, slow accumulation
of changes meaning the
species cannot interbreed
freely to produce fertile
(viable) offspring
• Takes many generations
• Faster in bacteria as
reproduction happens
every 20 minutes
How does Speciation Occur?
• Some reproductive barrier occurs
• Some organisms unable to breed in the
• Variations providing a benefit will be
passed on
• A collection of small changes means the
individuals become so different they
cannot breed with the rest of the group
Reproductive barriers
• Geographical separation: different members of
the same species living on different islands will
not breed freely, speciation is likely to occur e.g.
galapagos islands- this is known as allopatric
• Biochemical change preventing fertilisation e.g.
behavioural dance not recognised or sexual
organs change. This is known as sympatric