Transcript homologous structures

5.4.1 Define Evolution
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5.4.1 Define Evolution
 Evolution
 the process of cumulative change in the
heritable characteristics of a population.

‘heritable’ –
 changes must be passed on genetically from one
generation to the next
 Implies that evolution doesn’t happen overnight

‘cumulative’
 one change isn’t enough to have major impact on the
species

‘population’
 changes do not affect just one individual
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5.4.2 Outline Evidence for Evolution
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5.4.2 Outline Evidence for Evolution
Evidence for Evolution provided by:
1. Fossil Record
2. Selective Breeding of Domesticated Animals
3. Homologous Structures
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Fossil Record
 Life 500 million years ago was very different
from life today
 Although planet Earth has had extensive
oceans throughout, fish fossils have only
been found in rocks 500 million years old or
younger

(less than 15% the history of life)
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Fossil Record cont’d
 Although most of the top predators today are
mammals (orca whales, lions, tigers & bears
oh my!) ….
… none of them existed at the time of dinosaurs
or before!
 Most living organisms today have no identical
form in the fossil record.
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Fossil record, conclusion
 1 possible conclusion drawn from observing
fossils is that :
Life on Earth is continually changing.
 Most of the changes have been over huge
timescales (hundreds of thousands or millions
of years)!!!
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Selective Breeding of Domesticated
Animals (a.k.a. Artificial Selection)
 Selective breeding gives a good record of
recent changes in heritable characteristics
 Selective Breeding:

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Choose males and females with most
desirable genetic characteristics
Breed them to get offspring with those specific
characteristics
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Selective Breeding cont’d
 After selective breeding from dozens to hundreds of
generations…
.. certain varieties of animals have unique combinations
of characteristics that didn’t exist before.
 This demonstrates that evolution is happening due to
an accumulation of small changes over time.
 Note: Artificial selection (selective breeding) is NOT
the same as what happens in nature.
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Homologous Structures
 Anatomy = Structure (Form) of the body
 Physiology = Function of the body
 Homologous anatomical structures similar in
form and function, yet found in seemingly
dissimilar species
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Homologous Structures cont’d
 Comparative anatomists noticed that different
species have similar structures used for
different functions

(e.g. the pentadactyl limb of terrestrial
vertebrates).
 These are called homologous structures
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The pentadactyl limb
Frog
Lizard © Chereka Keaton
Bat
© 2008 Paul Billiet ODWS
Human
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Homologous Structures cont’d
 Five-fingered (pentadactyl) limb found in
animals such as humans, whales, and bats.
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‘penta’ = five
‘dactyl’ = fingers
Although the shape and number of bones may
vary, the general format is the same, even
though the functions of the limbs may be very
different
 Homologous structures provide evidence
that such organisms have common ancestors
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5.4.3 Populations tend to produce
more offspring than the
environment can support
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Mechanism for Evolution
 Natural Selection as the mechanism for
evolution
 Natural Selection accomplished by:


Overproduction of offspring
Presence of natural variation
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5.4.3 AHH!! TOO MANY BABIES!!
Not enough well-fare for them all!!!
 Animals & plants produce far more offspring
than could ever survive.


Plants often produce thousands more seeds
than necessary to propagate the species
Fish and turtles lay thousands of eggs but only
a few survive to adulthood
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5.4.4 The consequence of potential
overproduction of offspring is a struggle for
survival
 Too many offspring, not enough resources! 
 Supply and demand

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High demand for water, space, nutrients,
sunlight, but limited supply.
Consequence? Competition for resources to
stay alive.
 “Ah Ah Ah Ah Stayin’ Alive, Stayin’ Alive!”

The STRUGGLE FOR SURVIVAL

Ex: Struggling to survive the IB Programme…
<Sigh.>
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5.4.5 The members of a species show
variation
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Example of No Variation within a
species:
 Organisms like bacteria reproduce by making
a copy of their genetic info and then splitting
into two.
 Result: 2nd generation identical to the 1st
 Future generations identical or show very
little change
 Little chance for DNA modification
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5.4.5 The members of a species show
variation (cont’d)
 Variation is closely related to how successful
an organism is
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Fish with slightly different shaped mouth may
feed better from coral reef than other fish
unable to access
Plants producing different shaped flower may
better attract insects for pollination
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Causes of variety
 Mutations in DNA
 Sexual Reproduction promotes variation in
species
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5.4.6 Explain how sexual reproduction
promotes variation in a species
 2 ways genes are mixed in sexual
reproduction


1st – Meiosis (covered in Topic 4)
2nd – Fertilization (Topic 11)
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5.4.7 Explain how natural selection
leads to evolution
 Greater survival and reproductive success of
individuals with favorable heritable variations
(a.k.a. Natural Selection)
 can LEAD to …
 … change in the characteristics of a
population (a.k.a. Evolution)
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Natural selection  Evolution
 Step 1: Overproduction of offspring

Natural variation of offspring due to genetic
differences (body size, pigmentation, resistance to disease)
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
Useful variations allow individual better chance of
survival (hiding from predators, fleeing danger, finding food)
Harmful variations make survival difficult (wrong
camouflage, heavy bones for birds, having such a large body
size when not enough food to survive)
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Natural selection  Evolution
 Step 2a: Individuals with genetic
characteristics that are POORLY adapted for
environment
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Tend to be LESS successful at accessing
resources
And thus have LESS chance of surviving to
maturity
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Natural selection  Evolution
 Step 2b: Individuals with genetic characteristics that
are WELL-adapted for environment
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Tend to be MORE successful at accessing resources
And thus have BETTER chance of surviving to
maturity
Since they survive to adulthood, these successful
organisms have a better chance to reproduce and
pass on their genetic characteristics to the next
generation.
Over many generations, accumulation of changes in
the heritable characteristics of a population results in
evolution. THE GENE POOL HAS CHANGED!
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5.4.8 Explain 2 examples of evolution
in response to environmental change
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One example MUST be Antibiotic Resistance
to Bacteria
Other examples could include:
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The changes in size and shape of the beaks of
Galapagos finches
Pesticide resistance
Industrial melanism
Heavy-metal tolerance in plants
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Antibiotic Resistance in Bacteria
 Biotic = Living
 Antibiotics: medications that kill or inhibit the
growth of bacteria

Given to patients suffering from bacterial
infections
 Overuse of antibiotics can lead to the
production of resistant strains of bacteria
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 Disclaimer: The following scenario is
fictitious. Any similarity in names or the
scenario to real events is purely coincidental.
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Antibiotics Resistance scenario
1.
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5.
Yo’nique gets sick from bacterial infection
(tuberculosis).
Dr. Wahoo gives her antibiotic to kill bacteria
Yo’nique gets “better” because most of the bacteria
are destroyed. Awww yee-uah!
But WAIT!!! By a modification of its genetic makeup,
one little bacterium is resistant to the antibiotic! OH
NO!!!
The bacterium is not killed by the antibiotic! It later
multiplies in the Yo’nique’s body to make her icky
sicky again!! 
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Antibiotics Resistance scenario (cont’d)
Yo’nique goes back to the hospital, and the same incompetent
Dr. Wahoo gives her the SAME antibiotic!
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Well, because the new strain of bacteria is resistant to that
same antibiotic, she doesn’t get any better. 
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So Yo’nique has to go back to the hospital, and Dr. Wahoo
realizes his mistake. Hoping Yo’nique won’t realize it too and
sue, he prescribes a different antibiotic.
9.
Yo’nique doesn’t get better and unfortunately dies of a upper
respiratory infection.  Sniff.
10. Dr. Wahoo is sued for malpractice by Yo’nique’s family; he loses
his license, and is now asking if you want to supersize your fries
and coke with your Big Mac meal. Such a shame.
6.
Rest In Peace Yo’nique. Yo’nique, you one in a million.You mah gurl!
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Pesticide Resistance Scenario
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Pesticides: chemicals that kill animals that are regarded as
pests.
Pesticides applied to the field and kills the majority of the mice
that were eating Farmer Billy Joe Bob’s crops.
Due to natural variations, a few mice are slightly different and
unaffected by the poison.
Resistant mice survive and reproduce, making a new
population of “mighty mice” – some or all of the members
possess the genetic resistance.
Seeing more mice eating his crops, Billy Joe Bob puts out
more poison. This time fewer mice die.
To kill the resistant “mighty mice”, a new pesticide must be
used. But Farmer Billy Joe Bob doesn’t realize this, and
instead the mighty mice become superheroes and fight crime.
Mighty Mice chillax with Hamtaro, Teenage Mutant Ninja
Turtles, Thundercats, and Picachu.
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Question 1:
(a) Explain briefly Darwin’s theory of evolution.
(4 marks)
(b) Outline two modern examples where
evolution can be observed.
(2 marks)
(c) State two ways in which the remains of past
living organisms have been preserved. (2 marks)
(Total 8 marks)
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(a) Explain briefly Darwin’s theory of
evolution.
 parents produce more offspring than survive;
 there is competition among members of a species for
survival /
struggle for existence;
 species show variation;
certain variations will give a selective advantage /
survival of fittest;
 depending on the environment;
these variations will be passed on to the next
generation;
 leading to change in allele frequency;
4 max
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(b) Outline two modern examples
where evolution can be observed.
 change of beak shape in Galapagos finches;
 resistance to pesticides / antibiotics;
 bird predation on moths;
 (allow, though some evidence refutes this)
heavy metal tolerance in plants;
 melanism in ladybirds (ladybugs)
2 max
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(c) State two ways in which the remains of
past living organisms have been preserved
 fossils;
 prints / moulds;
 preserved in amber / tar / peat / petrification;
 frozen in ice
2 max
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Question 2:
Explain the evidence for evolution provided by
the pentadactyl limb. (2 marks)
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Q2: Explain the evidence for evolution
provided by the pentadactyl limb.
 vertebrates (nearly all) have pentadactyl
limbs / homologous structures;
 the structure in all is very similar (in spite of
different uses);
 likely to have evolved from a common
ancestor;
2 max
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