Natural Selection and
Evolution
Spring 2007
Dobson High School
Hope Finzer
Earth’s History
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Earth Origins:
4.6 billion years old – originally a ball
of hot molten rock – bombarded by
meteorites and extreme volcanic
activity – the atmosphere was
inhospitable but volcanic gases paved
the way for our current atmosphere
Earth’s History
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Earth’s Oceans:
Formed about 3.9 billion years ago
after the Earth began to cool and
allow the water vapor to condense into
water forming the oceans – its is in
these oceans that the first living
organisms appeared 3.5 billion years
ago
History in the Rocks
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Fossils:
Any evidence of an organism
that lived long ago – classified
by how the fossil is formed
Types of fossils:
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CastsA mold of an organism
filled by minerals in the
surrounding rock – produces a
replica
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Trace FossilsMarkings or evidence of
animal activities – footprints,
trails, and burrows
History in the Rocks
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Types of fossils:
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ImprintsFossils that form before
sediment hardens into rock –
leaves or feathers that fall
into mud and leave imprints
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MoldsWhen an organism is
buried, it decays leaving an
empty space that has the
exact shape of the organism
History in the Rocks
Petrified Fossils-
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The hard parts of an organism are sometimes
penetrated and replaced by minerals atom for
atom, when minerals harden an exact stone copy
of the original organism is produced
Amber-Preserved & Frozen Fossils
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Entire, intact organism is caught in ice or tree
sap (amber) – very rare – preserves the internal
parts of organisms important to scientific study
of extinct species
What is Paleontology?
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Paleontology- study of geological
periods and fossil remains
Why are sedimentary rocks
important for Earth’s history?
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Sedimentary rocks contain important
information about the history of the Earth.
They contain fossils, the preserved remains
of ancient plants and animals.
Differences between successive layers
indicate changes to the environment which
have occurred over time.
Sedimentary rocks can contain fossils
because, unlike most igneous and metamorphic
rocks, they form at temperatures and
pressures that do not destroy fossil
remnants.
Sedimentary Rock formation
Formation of Fossils in
Sedimentary Rock
a. organism dies and falls into a sandy or muddy
bottom of a body of water or is carried there
by floods
b. overtime sediment is deposited on top of the
dead organism until it is completely covered
c. mud and sand begin to compress around the
organism – forming a sequence of sedimentary
rocks – usually only the hardest parts of the
organism remain
Formation of Fossils in
Sedimentary Rock
d. fossil becomes imbedded in the rock,
then geological events, such as Earth
movements or erosion, the fossil can
come to the surface
e. scientists discover the fossil and
extract it from the surrounding rock –
then the fossil is studied
How do you tell the age of a
fossil?
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Relative Dating- the layers of
sedimentary rock are layered with its
newest layer on top and the oldest on
the bottom – fossils are determined
the same way - deeper fossils are
older then shallower fossils
How do you tell the age of a
fossil?
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Radiometric Dating- radioactive
isotopes are used – the fossil emits a
decay rate and scientists using
radioactive isotopes whose decay rate
is known and if the known decay rate
matches the fossil the age can be
determined – radioactive decay
Earth’s Geologic Time Table
Earth’s Geologic Time Table
Precambrian Era-
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Life begins – 3.5 billion years ago – spherical and
filamentous organisms that resemble
photosynthetic bacteria cyanobacteria and domeshaped stromatolites – Precambrian accounts for
87% of Earth’s history – prokaryotes dominated
the early Precambrian era
Eukaryotes appeared about 1.5 billion years ago
By the time the Precambrian era ended about
544 million years ago the oceans were filled with
unicellular and multi-cellular organisms such as
algae, sponges, and jellyfish
Earth’s Geologic Time Table
Paleozoic Era-
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From 544 t 245 million years ago –
characterized by the appearance of
many plants and animals –
Scientists call this era the explosion of
lifeSeas were filled with worms,
echinoderms, and primitive arthropods
(Examples: insects, spiders, lobsters,
and crabs)
1st half – fishes and earliest
vertebrates appeared, some evidence
of plant life on land
Middle – amphibians appeared
2nd half – reptiles appeared
Earth’s Geologic Time Table
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Mesozoic Era
245 to 66 million years ago – many geological and
life changes during this era
Divided into periods:
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Triassic – mammals 1st appeared – the 1st were
small mouse-like and 1st dinosaurs appeared
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Jurassic – began 208 million years ago – Age of
Dinosaurs – figure 17.9 on page 407
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Cretaceous – began 144 million years ago – the
spread of the mammals and the evolution of
flowering plants such as oak, fig, and elm trees
Geological changes – the hypothesis of continental
drift – how the continents move or plate tectonics
Earth’s Geologic Time Table
Cenozoic Era-
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“The Age of
Mammals”
66 million years ago –
we live in this era
mammals flourish,
evolution of mammals
into modern
groupings, primates
spread across the
planet about 60
million years ago–
humans 1st appeared
200,000 years ago
Extinction’s
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Billions of species have become extinct in the 3.5
billion years that life has existed on Earth
The fossil record indicates several mass extinctions
The 1st was the extinction of the dinosaurs during
the Cretaceous period about 66 million years ago –
many scientists believe this extinction was the result
of a crash of a massive asteroid on the Earth
changing the atmosphere by massive amounts of dust
in the air
Estimates of approximately 2/3rds of all species died
during this extinction
Extinction’s
The Origins of Life
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Spontaneous GenerationThe idea that life was produced from
nonliving matter
The Origins of Life Notes
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Francesco Redi’s Experiments- disproves
spontaneous generation Redi’s hypothesis: Only flies can produce more flies
Redi’s steps:
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Rotten meat is placed in 2 experimental jars and
1 control jar
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Cloth was placed over 1 experimental jar
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A cork was placed over 1 experimental jar
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The control jar, with no covering, filled with fly
maggots because flies landed on the meat and
laid their eggs
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The cloth covered jar had maggots on the meat
as the eggs were laid on the cloth and fell
through
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The corked experimental jar had no fly maggots
Redi’s experiment
The Origins of Life Notes
Louis Pasteur and Biogenesis-
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Pasteur setup an experiment in which only air was
exposed to a nutrient broth – no microorganisms
were allowed to get to the broth – his
experiment showed that spontaneous generation
does not happen because no organisms grew in
the nutrient broth
Pasteur came up with the concept of:
Biogenesis: living organisms come only from other
living organisms – the cornerstone of Biology
Louis Pasteur and Biogenesis
Origins: The Modern Ideas
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Simple organic molecules formed from the
primitive atmosphere: prokaryotes
The primitive atmosphere was composed of
water vapor, hydrogen, methane, and
ammonia and a 1930’s Russian scientist
Alexander Oparin proposed that life began
in the early oceans when the suns energy
and light energy (lightening) hit the
atmospheres chemical components reactions
occurred producing simple organic
compounds
Origins: The Modern Ideas
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Oparin imagined the reactions occurred in the
atmosphere and then rain poured down
bringing the organisms with it – forming a
primordial soup
In 1953 American scientists Stanley Miller
and Harold Urey tested Oparins experiment
by simulating the primordial scenario Water vapor circulated with ammonia,
hydrogen, and methane that was subjected to
electrical sparks of “lightening” the mixture
was repeatedly heated and cooled, simulating
night and day
Origins: The Modern Ideas
After 1 week they found that the mixture
develop amino acids, sugars, and other
organic compounds
The formation of complex organic
compounds and pre-cells: eukaryotes-
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Miller and Urey’s experiments showed that heat
and amino acids will link together to form small
proteins – leading scientists to speculate that
life began in small pools of water where amino
acids were concentrated
Miller-Urey experiment
Origins: The Modern Ideas
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Scientist Sydney Fox showed how
heating amino acids could produce cells –
a protocell – a large ordered structure
that carries out the activities
associated with life: growth, division,
and metabolism
How Did Life Begin?
Beliefs and Theories:
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Divine Origins
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Life was created by a supreme being –
this is a belief not a scientific theory
Extraterrestrial Beginnings
Life was brought to Earth by meteorites –
due to some organic material found in
meteorites
How Did Life Begin?
Beliefs and Theories:
Primordial Soup-
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Ancient oceans filled with organic molecules and
atmospheric elements, and heat from planet
reactions – caused reactions among amino acids
forming the 1st primitive life forms to evolve
Bubble Theory-
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Primordial reactions occurred in bubbles of lipids
(fats) where methane and ammonia could have
been present – molecules were kept close
together and more chemical reactions took place
– developing complex molecules that we call cell –
the building block of life
The Evolution of Cells
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Heterotrophic prokaryotes
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Were the 1st true cells
Scientists speculate 1st forms of life
were prokaryotes that were anaerobic
organisms that required no oxygen for
existence, especially since the
atmosphere had little to no oxygen
These prokaryotes were heterotrophs
because they obtained their food from
their surroundings
The Evolution of Cells
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Heterotrophs evolved into autotrophs –
autotrophs can make their own food –
these organisms were similar to
archaebacteria – they live with little
sunlight and oxygen – found in sulfur
springs and deep sea events
The Evolution of Cells
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Photosynthesizing prokaryotes
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Autotrophs evolved into
photosynthesizing prokaryotes (who
produce oxygen as a by-product of
respiration)
These new photosynthetic prokaryotes
began putting large amounts of oxygen
into the atmosphere
The event began the Oxygen revolution
about 2.8 billion years ago which is seen in
the fossil record
The Evolution of Cells
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Ancient storms with lightening caused
the oxygen to be converted into ozone –
resulting in the protective ozone
layer
– preventing harmful UV rays from
destroying the newly formed life
– thus paving the way for more complex
organisms to evolve
Charles Darwin and Natural
Selection
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Fossils interested scientists in evolution
The fossil record has helped form the basis
of early evolutionary concepts
These fossils convinced scientists that life
slowly changed over time or evolved
18th century scientists proposed many ideas
but only the theory proposed by Charles
Darwin has become accepted
Charles Darwin is considered to be the
founder of modern evolutionary theory
Charles Darwin and Natural
Selection
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Darwin studied the natural world during the
voyage of the Beagle
In 1831, at 21 years old, Darwin began his 5year journey as a naturalist – his job was to
collect, study, and store biological
specimens discovered on the journey
He traveled from England to South America,
around Capehorn and north to the Galapagos
Islands and west through the South Pacific
to Australia
Charles Darwin and Natural
Selection
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Darwin’s observation in the Galapagos
Darwin noted in his observations of
Galapagos animal inhabitants that they were
unique to the islands yet similar to species
seen in other parts of the world
By the end of his trip Darwin was convinced
evolution occurs – that species change over
time
Yet he still wanted to test his ideas before
he could explain how such changes occur
Charles Darwin and Natural Selection
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Darwin completed his studies in
England
Upon Darwin’s return to England in
1836, he started a 22 year study
of his collections and began
conducting experiments
Darwin was also interested in an
essay written stating that the
human population was growing
faster than the food supply
Charles Darwin and Natural Selection
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Applying this to his studies – he knew
many organisms reproduced many
offspring but large numbers of species
did not cover the Earth, so Darwin
concluded that there must be a struggle
for existence among individuals
Competition for food and space, escape
from predators, and the need to find
shelter
Only some individuals survived long
enough to reproduce – but which ones?
Charles Darwin and Natural Selection
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Darwin began experiments where he
would select certain variations in his
pigeons and breed the pigeon’s for
the desired trait
These breeding experiments are
called – artificial selection
Charles Darwin and Natural
Selection
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Darwin’s explanation for evolution
Darwin wanted to know if there was a
force in nature similar to artificial
selection
Darwin examined his data and began to
form his idea of evolution by natural
selection
Charles Darwin and Natural
Selection
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Natural selection is a mechanism for change in
populations that occurs when organisms with
favorable variations for a particular
environment survive, reproduce, and pass
these variations or traits on to the next
generation
Organisms with less favorable variations are
less likely to survive to pass on their traits
Each new generation will be made up of
organisms that have the favorable trait
Darwin published his findings in 1859 in his
book “On the Origin of Species by Natural
Selection”
How natural selection works
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Over population of a species. Example:
1000’s of fish eggs laid at once
Within any population individuals will show
slight variations. Examples: in fishes: color,
fin and tail size, and speed
Individuals with favorable traits in their
environment are most likely to survive than
individuals with less favorable variations.
Example: fishes whose skin color blends in
with its surroundings will less likely fall prey
to predators
Surviving individuals will breed and produce
offspring with the variations that allowed
them to survive predator attacks
Components of Natural
Selection
Evolution: Areas of Disagreement
Gaps in the fossil record: fossil history does not
give a complete history of life on Earth
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Many gaps are present
Transitional fossils are absent
Limits of accuracy in radiometric dating: not all
samples produce consistent results
Inconclusive origins of life: we absolutely do not
know how life began
Similarities among embryos: many structures are
shared by different organisms during early fetal
development but are differentiated later – Some
people interpret this data to indicate that all
organisms are related – others do not
Natural Selection and Adaptations
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Structural adaptations arise over many
generations
The mole rat adapting larger teeth and
claws to be able to dig deeper holes
and avoid predators
Structural adaptations that change
the structure of body parts:
Natural Selection and
Adaptations
Example: can develop quickly in a geological
perspective of a minimum of 100 years
Mimicry: structural adaptation that provides
protection for an organism by enabling it to
copy the appearance of another species
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Example: the coral snake and milk snake
Camouflage: a structural adaptation that
enables an organism to blend in with its
surroundings
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Example: the insect – the walking stick – looks
like a branch or stick on a tree
Structural Adaptations
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Mimicry
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Camouflage
Natural Selection and
Adaptations
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Physiological adaptations can
develop rapidly
These are changes in
organism metabolic
processes:
Example: bacteria becoming
resistant to penicillin
Evidence for
Evolution
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Fossils
show
changes
overtime
Evidence for Evolution
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Anatomical studies indicate
evolutionary relationships
The organisms look different
from the outside and vary in
function, yet the details of their
skeletons are similar
Scientists use these similarities
as evidence of evolution from a
common ancestor – as the
ancestors moved to new
environments they adapted to the
new surroundings
This accounts for the differences
that are seen
Evidence for Evolution
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Homologous structures: a
modified structure that
is seen among different
groups of descendents –
having a common
evolutionary origin
Example: forelimbs of
bats, crocodile, and
humans
Evidence for Evolution
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Analogous
structures:
structure
without a
common
evolutionary
origin that is
similar in
function but
not in structure
Example: wings
of birds and
insects
Evidence for Evolution
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Functionless structures
indicate evolutionary
pathways
Structures that once had
a function but are no
longer used but continue
to be passed from
generation to generation:
Example: the human
appendixes, the eyes of
the mole rat – these
structures are called
vestigial structures
Evidence for Evolution
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Embryological development shows
evolution from a common ancestor
In early embryological studies it is found
that fish, reptiles, birds, and mammals
are all similar in structure – each have a
tail and gill slits – some of which do not
develop the tail and gills, while others do
Evidence for Evolution
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Genetic comparisons may reveal hidden
relationships
Noting similarities in nucleotide
sequences of DNA can help determine
relationships between organisms
Example: Human DNA is 20% identical
to mouse DNA and 98% identical to
chimpanzee DNA
Mechanisms of Evolution
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Population Genetics
and Evolution
An organism cannot
adapt during its
lifetime
Natural selection
operates only on
populations over
many generations
Populations evolve –
individuals DO NOT
Mechanisms of Evolution
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Genes change overtime when there is high
allelic frequency in the gene pool – an
entire collection of genes among a
population
Allelic frequency – the percent of a
particular trait in the gene pool
When the frequency of alleles does not
change from generation to generation
there is no genetic frequency
Mechanisms of Evolution
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Changes in genetic equilibrium lead to
evolution
Populations at genetic equilibrium are
not evolving
Evolution can only occur when genetic
equilibrium is disrupted
What factors cause changes to
genetic equilibrium?
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Mutations: spontaneous, environmental
causes such as radiation and chemicals
Mutations are important – they cause
genetic changes in a gene pool
Some mutations are harmful but once in
a while these mutations are favorable
for evolution
What factors cause changes to
genetic equilibrium?
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Genetic drift: alteration of
allelic frequencies by chance
processes – common among
small populations
Example: Amish – small
population causing extra toes
gene to express it self in 1 and
14 people rather than 1 in 1000
people
This can allow recessive genes
to be expressed more readily –
recessive genes that carry
genetic disorders – Tay-Sachs
What factors cause changes to
genetic equilibrium?
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Movement of Individuals: genetic
equilibrium can also be affected by
immigration of individuals into and out
of a population
The factor that causes the greatest
change in gene pools is natural selection
Natural Selection acts upon the
variation in populations
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3 types of natural selection
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stabilizing selection
directional selection
disruptive selection
3 types of natural selection
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Stabilizing selection:
Favors average individuals
Example: butterflies
Dark colored butterflies cannot avoid
predators – stand out against sky
Light colored butterflies cannot avoid
predators – stand out against plants
Sky colored butterflies can avoid predators
because they match the skies color
3 types of natural selection
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Directional selection:
When one of the extreme forms of a
trait is favored by natural selection
Example: insects who burrow deep
into trees will only be able to be
caught by long beaked woodpeckers
who can use their beaks to reach
them deep in the tree
The long beaked woodpeckers will
have more food then shorter beck
woodpeckers and the short beck
woodpeckers will die out and then the
long beaked woodpeckers will become
the norm
3 types of natural selection
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Disruptive selection:
Individuals with either of the 2
extreme forms of a trait are at a
selective advantage
Example: marine limpets vary in color
from white to dark brown – white
limpets on a white rock avoid the sight
of predators – dark brown limpets on
dark rock avoid the sight of predators
but tan colored limpets are easily seen
on either rock and predators are able
to prey on these limpets
Disruptive selection eliminates the
middle or intermediate forms of an
organism
The Evolution of Species
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Physical barriers can prevent interbreeding
Geographic isolation – occurs if a physical
barrier separates a population into groups
This is one way in which new species form
Example: frogs separated by deforestation
eventually the frogs genes will not resemble
the old ones and through adaptation and natural
selection the groups of frogs become so far
apart in their genes that they are considered a
new species
The Evolution of Species
The Evolution of Species
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Geographic isolation can lead to differences
in mating behaviors
Gene pools become closed and no new genes
are being introduced – eventually
reproductive isolation occurs – when
formerly interbreeding organisms are
prevented from producing fertile offspring
Example: frogs – if one group mates in the
fall and another in the summer – they fail to
be able to mate together because they
cannot breed at the wrong time of the year
The Evolution of Species
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Speciation can
occur when
chromosome
numbers change
Polyploid – any
species with any
multiple of the
normal set of
chromosomes
caused by
mistakes in
meiosis
The Evolution of Species
Speciation can occur quickly or slowly
Quick: example: polyploids only take one
generation
Gradualism: that species originate through a
gradual build up of new adaptations
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Supported by fossil record
Idea proposed by Charles Darwin
Punctuated equilibrium: speciation occurs
quickly in rapid bursts with long periods of
stability in between
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Also supported by the fossil record
Patterns of Evolution
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Species diversify when introduced to
new environments
Adaptive radiation: the process of
evolution of an ancestral species into
an array of species that occupy
different niches or places in their
environment
Adaptive radiation is an example of
divergent evolution
Adaptive Radiation
Patterns of Evolution
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Divergent evolution: the pattern of
evolution in which species that once
were all similar to the ancestral species
become more and more distinct
Example: finches in Hawaii
All finches came from the same
ancestor but each has different
plumage and beak size
Divergent Evolution
Patterns of Evolution
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Distantly related species can evolve similar
features
Convergent evolution: the pattern of
evolution in which distantly related
organisms evolve similar traits
Example: dolphins and fishes having similar
body shapes
These species occupy similar environments
and face similar selection pressures
Convergent Evolution
Convergent Evolution