Evolution
Unit
 Chapters 22, 23,
24, 25 and 26
AP Biology
What is Evolution?
• Change in the genetic makeup of a population
over time.
• Fitness – those with favorable variations for
survival and reproduction.
– Populations can evolve, not individuals.
• Diverse gene pool good for long-term survival
of a species. Genetic variations are important!
• How do genetic variations occur?
Where does Variation come from?
 Mutation

random changes to DNA
 errors in mitosis & meiosis
 environmental damage
 Sexual reproduction

mixing of alleles
 genetic recombination
 new arrangements of alleles in every offspring
 new combinations = new phenotypes
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Genetic variation in a population
Essence of Darwin’s ideas
 Natural selection
heritable variation exists in populations
 over-production of offspring

 more offspring than the environment can support

competition
 for food, mates, nesting sites, escape predators

differential survival
 successful traits = adaptations

differential reproduction
 adaptations become more
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common in population
Lamarckian vs. Darwinian view
 LaMarck

in reaching higher
vegetation giraffes
stretch their necks &
transmits the acquired
longer neck to offspring

 Darwin

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giraffes born with longer
necks survive better &
leave more offspring who
inherit their long necks
Natural Selection
•
•
•
•
Major mechanism of evolution
Environment is always changing
Acts upon the phenotype of the population
Based on Darwin’s idea that resources are limited
and that there is competition for those resources.
• Adaptation = a genetic variation favored by
natural selection.
• When allele frequencies shift, speciation occurs
– Thus, the frequency change is NOT RANDOM
Effects of Selection
 Changes in the average trait of a population
DIRECTIONAL
SELECTION
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giraffe neck
horse size
STABILIZING
SELECTION
DISRUPTIVE
SELECTION
human birth weight
rock pocket mice
Natural selection
in action
Resistance…
NOT immunity!
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MRSA
Heterozygote Advantage
 Keeps the recessive
allele in the population
 Ex: Sickle Cell Anemia



aa – dies of sickle cell
anemia
Aa – some side affects
BUT resistant to malaria!
AA – no disease present
BUT prone to malaria
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Hidden variations can be exposed through selection!
Terminal
bud
Lateral
buds
Cabbage
Artificial selection
Brussels
sprouts
Leaves
Flower cluster
Kale
Cauliflower
Stem
Flower
and
stems
Broccoli
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Wild mustard
Kohlrabi
In addition to natural
selection, evolutionary
change is also driven
by random processes…
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Genetic Drift
 Chance events changing frequency of
traits in a population

not adaptation to environmental conditions
 not selection

founder effect
 small group splinters off & starts a new colony
 it’s random who joins the group

bottleneck
 a disaster reduces population to
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small number & then population
recovers & expands again but
from a limited gene pool
 who survives disaster may be random
Ex: Cheetahs
 All cheetahs share a small number of alleles

less than 1% diversity
 2 bottlenecks

10,000 years ago
 Ice Age

last 100 years
 poaching & loss of habitat
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Conservation issues
 Bottlenecking is an important
Peregrine Falcon
concept in conservation
biology of endangered
species
loss of alleles from gene pool
 reduces variation
 reduces adaptability

Breeding programs must
consciously
outcross
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Golden Lion
Tamarin
Human Impact on variation
 How do we affect variation in other
populations?

Artificial selection/Inbreeding
 Animal breeds

Loss of genetic diversity
 Insecticide usage

Overuse of antibiotics
 resistant bacterial strains
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2006 Fossil Discovery of Early Tetrapod
“Tiktaalik”
“missing link” from sea to land animals
Evidence Supporting Evolution
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Evidence for Evolution
• Paleontology – fossils show change in a species
•
•
over time
Biogeography – Similar species are found in
similar ecosystems around the world
Morphology – Comparing structures
–
–
Homologous structures – body parts with
similar structure but possible different function.
Shows common ancestry
Analogous structures – similar structure
develops in organisms that share a common
ecosystem but not a common ancestry
• Biochemical or Molecular AP Biology
–
Similarities in gene sequences, proteins, DNA
Fossils
 Preserved remains of living things
 Paleontology is the study of the fossil

record
Most organisms do not leave a fossil after
death

Explains the “missing links”
 Sedimentation Fossils

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As the organism decomposes the
spaces will be filled with the minerals
from the silt
The Archaeopteryx Fossil
Reptilian Features
Forelimb has three
functional fingers
with grasping
claws.
Lacks the
reductions and
fusions present in
other birds.
Breastbone is small
and lacks a keel.
True teeth set in
sockets in the jaws.
The hind-limb girdle
is typical of
dinosaurs, although
modified.
Long, bony tail.
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Avian Features
Vertebrae are
almost flatfaced.
Impressions of
feathers attached
to the forelimb.
Belly ribs.
Incomplete fusion
of the lower leg
bones.
Impressions of
feathers attached
to the tail.
LEFT: Archaeopteryx lithographica
Found in 1877 near Blumenberg, Germany
How old is that fossil?
 Relative Dating

Age of fossils based according to their location
in strata
 Absolute Dating

Age of fossils determined by analyzing the
content of radioactive isotopes found in the
fossil.
Half-life: The length of time required for half of the
radioactive elements to change into another stable
element.
Unaffected by temperature, light, pressure, etc.
All radioactive isotopes have a dependable half life.
Ex: C14 decays into N14
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Relative Dating
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Absolute Dating
How radioactive
“naturally
occurring”
elements get inside
an organism:
A.K.A –
Radiometric dating
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Homologous Structures
Anatomical evidence
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Analogous structures
Convergent
Don’t
be fooled
by evolution
their looks!
Those
tails
Does fins
this &
mean
& sleek
they bodies
have a are
recent
common
ancestor?
analogous
structures!
Solving a similar problem with a similar solution
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Molecular Homology
Human
Macaque
Dog Bird
Frog
Lamprey
The sequence in
DNA
proteins
Why &compare
is a &molecular
DNA
proteins
record
of evolutionary
across
species?
relationships.
Comparative hemoglobin structure
8
0
32
45
67
125
10 20 30 40 50 60 70 80 90 100 110 120
Number of amino acid differences between
hemoglobin (146 aa) of vertebrate species and that of humans
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Vestigial organs
Why would whales
have pelvis & leg bones
if they were always
sea creatures?
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These are
remnants of
structures that were
functional in
ancestral species
Evolution evidence at the cellular level
 Domains: Archaea, Bacteria and
Eukarya

Elements conserved through all: DNA,
RNA and many metabolic pathways.
 Eukaryotes – core features:
Cytoskeleton
 Nucleus
 Membrane-bound organelles
 Linear chromosomes
 Endomembrane system

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The Origin of Species
Mom, Dad…
There’s something
you need to know…
I’m a MAMMAL!
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2010-2011
Speciation
• Changes in allele frequency are so great
that a new species is formed
• Can be slow and gradual or in “bursts”
• Extinction rates can be rapid and then
adaptive radiation follows when new
habitats are available
Correlation of speciation to food sources
Seed
eaters
Flower
eaters
Insect
eaters
Rapid speciation:
new species filling niches,
because they inherited
successful adaptations.
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Adaptive
So…what is a species?
• Population whose members can
interbreed & produce viable, fertile
offspring
• Reproductively compatible
Distinct species:
songs & behaviors are different
enough to prevent interbreeding
Eastern Meadowlark Western Meadowlark
How do new species originate?
 When two populations become
reproductively isolated from each other.
 Speciation Modes:

allopatric
 geographic separation
 “other country”

sympatric
 still live in same area
 “same country”
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Allopatric Speciation
 Physical/geographical
separation of two populations
 Allele frequencies diverge
 After a length of time the two
population are so different
that they are considered
different species
 If the barrier is removed
interbreeding will still not
occur due to pre/post zygotic
isolation
Sympatric Speciation
Formation of a new species without geographic isolation.
Causes:
– Pre-zygotic barriers exist to mating
– Polyploidy (only organism with an even number of
chromosomes are fertile…speciation occurs quickly)
– Hybridization: two different forms of a species mate
in common ground (hybrid zone) and produce
offspring with greater genetic diversity than the
parents….eventually the hybrid diverges from both
sets of parents
Sympatric Speciation
Gene flow has been reduced between flies that feed on
different food varieties, even though they both live in the
same geographic area.
Pre-zygotic Isolation
Sperm never gets a chance to meet egg
•Geographic isolation: barriers prevent mating
•Ecological isolation: different habitats in same
region
•Temporal isolation: different populations are
fertile at different times
•Behavior Isolation: they don’t recognize each
other or the mating rituals
•Mechanical isolation: morphological differences
•Gamete Isolation: Sperm and egg do not
recognize each other
PRE-Zygotic barriers
 Obstacle to mating or to fertilization if
mating occurs
geographic isolation
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behavioral isolation
ecological isolation
temporal isolation
mechanical isolation
gametic isolation
Post Zygotic Isolation
• Hybrid Inviability – the
embryo cannot develop
inside the mothers womb
• Hybrid Sterility – Adult
individuals can be
produced BUT they are
not fertile
• Hybrid Breakdown – each
successive generation has
less fertility than the
parental generation
Evolutionary Time Scale
• Microevolution – changing
of allele frequencies in a
population over time.
• Macroevolution – patterns
of change over geologic
time. Determines
phylogeny
– Gradualism – species are
always slowly evolving
– Punctuated equilibrium –
periods of massive
evolution followed by
periods with little to no
evolution
Patterns of Evolution
• Divergent Evolution (adaptive radiation)
• Convergent Evolution
–
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Two or more species that share a
common environment but not a common
ancestor evolve to be similar
Is it a shark or a
dolphin??
Coevolution
 Two or more species reciprocally
affect each other’s evolution

predator-prey
 disease & host
competitive species
 mutualism

 pollinators & flowers
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Mass Extinctions
• At least 5 mass extinctions have occurred throughout
history.
• Possible causes: dramatic climate changes occurring
after meteorite collisions and/or continents drift into new
and different configurations.
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