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
AP 
Biology
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
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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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Hardy Weinberg: Population Genetics
Using mathematical approaches
to calculate changes in allele
frequencies…this is evidence of
evolution.
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Hardy-Weinberg equilibrium
 Hypothetical, non-evolving population

preserves allele frequencies
 natural populations rarely in H-W
equilibrium

useful model to measure if forces are acting on
a population
 measuring evolutionary change
G.H. Hardy
AP mathematician
Biology
W. Weinberg
physician
Evolution of populations
 Evolution = change in allele frequencies
in a population

hypothetical: what conditions would
cause allele frequencies to not change?
1. very large population size (no genetic drift)
2. no migration (no gene flow in or out)
3. no mutation (no genetic change)
4. random mating (no sexual selection)
5. no natural selection (everyone is equally fit)
H-W occurs ONLY in non-evolving
populations!
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Populations & gene pools
 Concepts
a population is a localized group of
interbreeding individuals
 gene pool is collection of alleles in the
population

 remember difference between alleles & genes!

allele frequency is how common is that
allele in the population
 how many A vs. a in whole population
AP Biology
H-W formulas
 Alleles:
p+q=1
B
 Individuals:
p2 + 2pq + q2 = 1
BB
BB
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b
Bb
Bb
bb
bb
Hardy-Weinberg theorem
 Counting Alleles
Frequencies are
usually written as
decimals!
assume 2 alleles = B, b
 frequency of dominant allele (B) = p
 frequency of recessive allele (b) = q

 frequencies must add to 1 (100%), so:
p+q=1
BB
AP Biology
Bb
bb
Hardy-Weinberg theorem
 Counting Individuals



frequency of homozygous dominant: p x p = p2
frequency of homozygous recessive: q x q = q2
frequency of heterozygotes: (p x q) + (q x p) = 2pq
 frequencies of all individuals must add to 1 (100%), so:
p2 + 2pq + q2 = 1
BB
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Bb
bb
Using Hardy-Weinberg equation
population:
100 cats
84 black, 16 white
How many of each
genotype?
p2=.36
BB
q2 (bb): 16/100 = .16
q (b): √.16 = 0.4
p (B): 1 - 0.4 = 0.6
2pq=.48
Bb
q2=.16
bb
MustWhat
assume
are population
the genotype
is in
frequencies?
H-W equilibrium!
AP Biology
Using Hardy-Weinberg equation
p2=.36
Assuming
H-W equilibrium:
Expected data
Observed data
How do you
explain
the data?
AP
Biology
2pq=.48
q2=.16
BB
Bb
bb
p2=.20
=.74
BB
2pq=.64
2pq=.10
Bb
q2=.16
bb
Origin of the Equation
 Assuming that a trait is
recessive or dominant

Allele pairs AA, Aa, aa
would exist in a population
 p+q=1
 The probability that an
individual would
contribute an A is called p
 The probability that an
individual would
contribute an a is called q
 Because only A and a are
present in the population
the probability that an
individual would donate
one or the other is 100%
 p2 + 2pq + q2
AP Biology
Male
Gametes
A(p)
Male
Gametes
a(q)
Female
gametes
A(p)
AA
p2
Aa
pq
Female
Gametes
a(q)
Aa
pq
aa
q2
Example of an evolving population:
 Peppered moth



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Variation of colors in the population existed (Black,
Peppered, White)
As environmental conditions changed the frequency of
the recessive allele increased.
This was seen as an adaptation to the environment that
allowed the species to continue to live.
The Origin of Species
Mom, Dad…
There’s something
you need to know…
I’m a MAMMAL!
AP Biology
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.
AP Biology radiation
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”
AP Biology
Allopatric Speciation
 Physical/geographical
separation of two populations
 Allele frequencies diverge
 After a length of time the two
populations 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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Origin of the Earth
What must Earth have been like
before living things took over?
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The Primitive Earth
 Atmosphere:
All chemicals/compounds necessary are thought
to have originated on earth
 Inorganic precursors:
 Water vapor
 Nitrogen
 Carbon dioxide
 Small amounts of hydrogen and carbon
monoxide
 These were the monomers for forming more
complex molecules.
 Experiments have shown that it is possible to form
organic from inorganic.

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Electrodes discharge
sparks
(lightning simulation)
Origin of Organic Molecules
 Abiotic synthesis
1920 - Oparin
first molecules
formed by strong
energy sources
 1953 - Miller & Urey
test hypothesis

Water vapor
CH4
NH3
Mixture of gases
("primitive
atmosphere")
H2
Condenser
Water
 formed organic
compounds
 amino acids
 adenine
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Heated water
("ocean")
Condensed
liquid with
complex,
organic
molecules
Key Events in Origin of Life
 Origin of Cells (Protobionts)

lipid bubbles  separate inside from outside
 metabolism & reproduction
 Origin of Genetics


RNA is likely first genetic material
multiple functions: encodes information (selfreplicating), enzyme, regulatory molecule,
transport molecule (tRNA, mRNA)
 makes inheritance possible
 makes natural selection & evolution possible
 Origin of Eukaryotes

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endosymbiosis
Timeline
 Key events in
evolutionary
history of life on
Earth
3.5–4.0 bya:
life originated
 2.7 bya:
free O2 =
photosynthetic
bacteria
 2 bya:
first eukaryotes

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~2 bya
First Eukaryotes
 Development of internal membranes


create internal micro-environments
advantage: specialization = increase efficiency
 natural selection!
infolding of the
plasma membrane
plasma
membrane
endoplasmic
reticulum (ER)
nuclear envelope
nucleus
DNA
cell wall
Prokaryotic
cell
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Prokaryotic
ancestor of
eukaryotic
cells
plasma
membrane
Eukaryotic
cell
1st Endosymbiosis
 Evolution of eukaryotes



origin of mitochondria
engulfed aerobic bacteria, but
did not digest them
mutually beneficial relationship
 natural selection!
internal membrane
system
aerobic bacterium
mitochondrion
Endosymbiosis
Ancestral
AP
Biology
eukaryotic
cell
Eukaryotic cell
with mitochondrion
2nd Endosymbiosis
 Evolution of eukaryotes



Eukaryotic
cell with
mitochondrion
origin of chloroplasts
engulfed photosynthetic bacteria,
but did not digest them
mutually beneficial relationship
 natural selection!
photosynthetic
bacterium
chloroplast
Endosymbiosis
Eukaryotic cell with
AP Biology
chloroplast & mitochondrion
mitochondrion
Theory of Endosymbiosis
 Evidence

structural
 mitochondria & chloroplasts
resemble bacterial structure

genetic
Lynn Margulis
 mitochondria & chloroplasts
have their own circular DNA, like bacteria

functional
 mitochondria & chloroplasts
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move freely within the cell
 mitochondria & chloroplasts
reproduce independently
from the cell
Cambrian explosion
 Diversification of Animals

543 mya
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within 10–20 million years most of the major
phyla of animals appear in fossil record