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UNIT 5 PART 2: THE MODERN
THEORY OF EVOLUTION
• The evidence
shows that
evolution occurred
but not how or why.
• There have been
different theories.
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Lamarck’s Theory of Evolution
• He believed that species were not
constant but that they changed, and that
new species evolved from preexisting
species.
• He thought these changes were caused by
their need to adapt to changes in the
environment.
• His theory had two principles:
– Use and disuse
– Inheritance of acquired characteristics
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Use and Disuse
• The more a body part is used the stronger
and more developed it becomes. Parts
that are not used become weaker and less
developed.
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Inheritance of Acquired
Characteristics
• Characteristics developed
by use could be passed
on to offspring.
• Lamarck said this was
how giraffes got their long
necks: by using them to
stretch for food their necks
became longer and that
trait was passed on to
offspring. Over time
giraffe’s necks became
longer and longer.
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Weismann disproves Lamarck
• Weismann cut the
tails off mice and bred
them for over 20
generations. Each
time the mice were
born with tails.
• We now know that
traits are passed on
by genes which are
not affected by life
experiences.
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Charles Darwin
• Sailed on the HMS
Beagle as ship’s
naturalist
• Collected and
observed specimens
from South America
and the Galapagos
Islands
• He thought that
species changed
slowly over time.
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Darwin’s Finches
• Darwin believed that
the species on the
Galapagos Islands
evolved from a species
that came from the
mainland.
• Each evolved to be
different due to
different environmental
conditions on each
island.
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Darwin now had to figure out how
evolution occurred
• He reasoned that because there is a
struggle to survive, those who are better
adapted to the environment (more fit) will
survive to reproduce and those not as well
adapted (less fit) won’t.
• This he called natural selection because
it is nature that is selecting who survives.
• Wallace had the same idea and asked
Darwin to look at his work. Their work was
then published separately in 1859.
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The Main Points of The DarwinWallace Theory of Evolution:
• Overproduction – produce more offspring
than needed because only a few will survive.
• Competition – offspring must compete for
food, water, shelter so only a few survive.
• Variation – differences in individuals. Some
may affect the ability to compete.
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ADAPTATIONS – because of variations some
individuals will be better adapted to survive
and reproduce than others. These individuals
are the most fit. An adaptation is any kind
of inherited trait that improves an organism’s
chances of survival.
These beaks are
adaptations for
eating different
types of food.
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• The more variations that exist in a species
the better that species chances of survival
are if the environment changes.
• A change in the environment does not cause
the variations – the variations must be
present before the environment changes in
order to be selected for.
• A selecting agent determines who survives.
• Natural selection – the environment selects
those best adapted to survive and
reproduce: survival of the fittest. These
favorable traits will be passed on.
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Types of Adaptations
•
•
•
•
•
•
Structural – involve a part of the body
Physiological – involve the metabolism
Behavioral – involve actions
Warning coloration – warns predators
Camouflage – to blend in with environment
Mimicry – copies something else
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SPECIATION – over
time favorable
adaptations
accumulate in
species and
unfavorable traits
disappear.
Eventually
changes result
in new species.
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Speciation – the formation of new
species
• There is a new species once a population has
lost the ability to interbreed with its neighbors.
• Isolation is needed for speciation:
Geographic – a population is divided by a natural
barrier such as a mountain or river. Over time
each group adapts to its environment and its
gene pool changes.
Reproductive – the loss of the ability to interbreed
by two isolated groups. Can also be caused by
changes in behavior, mating times, and sterility.
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Populations Evolve Not Individuals
• Over time the allele frequencies change
because of natural selection. Useful
variations will increase; bad ones will
decrease.
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Two models of the Rate of Evolution
• Gradualism – evolution
occurs slowly and
continuously over long
periods of time.
• Transitional forms are
missing from the fossil
record because they
were less common.
• Punctuated Equilibrium
– species stay the
same for long periods
(equilibrium) then have
a short rapid evolution.
• Transitional forms are
missing because there
was not enough time.
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New traits can arise by:
• Mutation – If it is good, nature will select for
it, otherwise it will go away.
• Genetic Recombination – during sexual
reproduction
• Migration – individuals moving into or out of
a population take their genes with them. 26
Adaptive Radiation
(Divergent Evolution)
• The process by
which a species
evolves into a
number of
different species
each in a new
environment.
• Example:
Darwin’s finches:
adapted to trees
or ground; seeds
or insects
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Convergent Evolution
• Natural selection
causes unrelated
species to
resemble one
another.
• This produces
analogous
structures.
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Coevolution
• Two or more species
evolve in response to
each other.
• Example: Flowers
and their pollinators
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Industrial Melanism – The
Peppered Moth
• The peppered moth in
England had been
well studied for a long
time.
• The moth is mostly
white, but “peppered”
with dark specks.
• In 1845 a dark one
was seen for the first
time.
• By 1900, most of the
moths around the cities
were dark, while those
in the country were
mostly white. Why?
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Case Study: Antibiotic Resistance
• When penicillin is used on bacteria for the first time
99.99% of the bacteria die.
• The .01% of the bacteria that survive have an
adaptive variation that allowed them to survive
penicillin.
• These bacteria multiply and their offspring will now be
resistant to penicillin.
• Each time penicillin is used on the bacteria, more and
more of them survive, until it is no longer effective.
• It is important to understand that the bacteria had the
variation before they were exposed to penicillin, and
not as a result of it.
• Penicillin is acting as the selecting agent.
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• https://www.youtube.c
om/watch?v=mZt1Gn
0R22Q
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