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EVOLUTION/POPULATION
GENETICS
CH. 15-16
SOME INFLUENCES ON
DARWIN’S THOUGHT
•
•
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•
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LAMARCK
MALTHUS
LYELL
HUTTON
WALLACE
:
Contributor’s to Darwin’s
thinking included:
• Charles Lyell –
uniformatarianism
• Georges Cuvier – species
extinction (Catastrophism)
• Thomas Malthus – struggle for
existence (resources)
:
Contributor’s to Darwin’s
thinking included:
• James Hutton - Gradualism
• John Baptiste Lamarck –
Inheritance of acquired
Characteristics and Law of Use
and Disuse
• Alfred Russel Wallace –
organisms evolved from common
ancestors
Lamarck’s Theory of
Evolution
• Jean-Baptiste Lamarck,
1809
• One Of First Scientists To
Understand That Change
Occurs Over Time
• Stated that Changes Are
Adaptations To
Environment acquired in
an organism’s lifetime
• Said acquired changes
were passed to offspring
Lamarck’s Theory of
Evolution
• Idea called Law
of Use and
Disuse
• If a body part
were used, it got
stronger
• If body part NOT
used, it
deteriorated
Lamarck’s Theory of
Evolution
• Inheritance Of Acquired Traits
– Traits Acquired During Ones Lifetime
Would Be Passed To Offspring
Clipped ears of dogs could be passed to offspring!
Lamarck’s Mistakes
• Lamarck Did NOT Know how traits
were inherited (Traits are passed
through genes)
• Genes Are NOT Changed By
Activities In Life
• Change Through Mutation Occurs
Before An Organism Is Born
Population Growth
• Thomas Malthus, 1798
• Economist
• Observed Babies Being Born
Faster Than People Were Dying
• Population size limited by
resources such as the Food
Supply
The Struggle for Existence
• Malthus’ Influence:
– High Birth Rates & Limited Resources
Would Force Life & Death Competition
• Each Species Struggles For:
– Food
– Living Space
– Mates
Population Growth
• Malthus Reasoned That
If The Human Population
Continued To Grow
Unchecked, Sooner or
Later There Would Be
Insufficient Living Space
& Food For Everyone
• Death Rate Will Increase
To Balance Population
size & Food Supply
Population Growth
• Darwin Realized
Malthus’s Principles
Were Visible In Nature
• Plants & Animals
Produce Far More
Offspring Than Can Be
Supported
– Most Die
– If They Didn’t – Earth
Would Be Overrun
Charles Lyell
• Proposed theory of
Uniformitarianism
• Geological processes
at uniform rates
building & wearing
down Earth’s crust
• Proposed that the
Earth was millions of
years instead of a
few thousand years
old
Principles of Geology
• Published by Lyell Just Before The
Beagle Set Sail & read by Darwin
• Explained Geological Processes That
Shaped The Earth
• Helped Darwin Understand Sea Shells
In The Andes Mountains At 12,000+
Feet
– Expanded Earth’s Age
Hutton’s Theory of Geological
Change
•James Hutton, 1795, Scottish
geologist
•Studied invertebrate fossils in Paris
Museum
•Described The Geological Forces
That Have Changed Life on Earth
Over Millions of Years (erosion,
earthquakes, volcanoes…)
Hutton’s Theory of Geological
Change
• Changes in Earth’s
crust due to slow
continuous
processes
• Idea Known as
Gradualism
Wallace’s Contribution
• Alfred Russel Wallace
Independently came to
same Conclusion as
Darwin that species
changed over time
because of their struggle
for existence
• When Darwin read
Wallace’s essay, he knew
he had to publish his
findings
Darwin's Theory
1. Individual Organisms In Nature Differ
From One Another. Some Of This
Variation Is Inherited
2. Organisms In Nature Produce More
Offspring Than Can Survive, And
Many Of These Offspring Do No
Reproduce
Darwin's Theory
3. Because More Organisms Are
Produced Than Can Survive, Members
Of Each Species Must Compete For
Limited Resources
4. Because Each Organism Is Unique,
Each Has Different Advantages &
Disadvantages In The Struggle For
Existence
Darwin's Theory
5. Individuals Best Suited To Their
Environment Survive & Reproduce
Successfully – Passing Their Traits To
Their Offspring.
6. Species Change Over Time. Over Long
Periods, Natural Selection Causes Changes
That May Eventually Lead To New Species
Darwin's Theory
7. Species Alive Today Have Descended
With Modifications From Species That
Lived In The Past
8. All Organisms On Earth Are United
Into A Single Tree Of Life By Common
Descent
Section 15-3
Concept Map
Evidence of
Evolution
includes
The fossil record
Geographic
distribution of
living species
Homologous
body structures
Similarities
in early
development
which is composed of
which indicates
which implies
which implies
Physical
remains of
organisms
Common
ancestral
species
Similar genes
Similar genes
Homologous Structures
Section 15-3
Turtle
HOMOLOGOUS
STRUCTURES
Alligator
Ancient lobe-finned fish
Bird
Mammal
Homologous Body Structures
• Not All Serve Important Functions
– Vestigial Organs
• Appendix In Man
• Legs On Skinks
Evidence for Evolution - Comparative Embryology
Similarities In Embryonic Development
Fossil Record
• Earth is Billions of Years Old
• Fossils In Different Layers of Rock
(sedimentary Rock Strata) Showed
Evidence Of Gradual Change Over Time
Geographic Distribution of
Living Species
Section 15-3
Beaver
Beaver
NORTH
AMERICA
Muskrat
Muskrat
Beaver and
Muskrat
Coypu
Capybara
Capybara
SOUTH
AMERICA
Coypu
Coypu and
Capybara
Similarities in DNA Sequence
Evolutionary
Time Scales
Macroevolution:
Long time scale
events that
create and
destroy species.
Evolutionary
Time Scales
Microevolution:
Short time scale
events (generationto-generation) that
change the
genotypes and
phenotypes of
populations
VARIATION & GENE POOLS: GENE POOL,
Section 16-1
RELATIVE FREQUENCY OF GENES/ALLELES
Sample Population
48%
heterozygous
black
Frequency of Alleles
allele for
brown fur
allele for
black fur
16%
homozygous
black
36%
homozygous
brown
Relative frequency # of times an allele occurs in a gene pool,
compared w/ the # of times other alleles for the same gene occur
MUTATION
• THE ULTIMATE SOURCE OF GENETIC
VARIATION!!!!!!
Phenotypes for Single-Gene Trait: The # of phenotypes
a given
Section 16-1
trait has is determined by how many genes
control the trait
Frequency of Phenotype
(%)
100
Controlled by a single gene
that has 2 alleles leads to 2
distinct phenotypes
80
60
40
20
0
Widow’s peak
Phenotype
No widow’s peak
Controlled
by 2/ more
genes (2/
more
alleles
Frequency of Phenotype
Generic Bell Curve for Polygenic Trait: many
Section 16-1
possible genotypes and phenotypes
Phenotype (height)
NATURAL SELECTION ON POLYGENIC
TRAITS: 3 MODELS
Section 16-2
Key
Directional Selection
Low mortality,
high fitness
Food becomes scarce.
High mortality,
low fitness
Favors traits at 1 extreme of a range of traits
Graph of Stabilizing Selection
Section 16-2
Stabilizing Selection
Individuals
with the most
common trait
are most
adapted, while
individuals
who differ
from the norm
are poorly
adapted.
Key
Low mortality,
high fitness
High mortality,
low fitness
Birth Weight
Selection
against both
extremes keep
curve narrow
and in same
place.
Graph of Disruptive Selection
Section 16-2
Disruptive Selection
Low mortality,
high fitness
High mortality,
low fitness
Population splits
into two subgroups
specializing in
different seeds.
Beak Size
Number of Birds
in Population
Key
Number of Birds
in Population
Largest and smallest seeds become more common.
Beak Size
When both extreme phenotypes are favored by natural selection
Balancing Selection
Natural selection acts in opposite directions
*** “Heterozygote Advantage”- ind who is heterozygous for a particular
gene has a greater fitness than a homozygous ind
EX: Distribution of sickle-cell allele coincides with the occurrence of
malaria
SS Normal hemoglobin
ss Sicke-cell disease
Ss- codominance (protects against malaria)
GENTIC DRIFT: A RANDOM CHANGE IN
ALLELE FREQUENCY
• IN SMALL POPULATIONS, INDIVIDUALS
THAT CARRY A PARTICULAR ALLELE MAY
LEAVE MORE DESCENDANTS THAN OTHER
INDIVIDUALS, JUST BY CHANCE. OVER
TIME, A SERIES OF CHANCE
OCCURRENCES OF THIS TYPE CAN CAUSE
AN ALLELE TO BECOME COMMON IN A
POPULATION.
• FOUNDER EFFECT: A SITUATION IN WHICH
ALLELE FREQUENCIES CHANGE AS A
RESULT OF THE MIGRATION OF A SMALL
SUBGROUP OF A POPULATION.
Section 16-2
Genetic Drift
Sample of
Original Population
In small populations,
Descendants
Founding an
Population
A can become
allele
more or less common
by chance (explain
how allele frequencies
can fluctuate
Founding Population B
unpredictably from 1
gen to the next)
Section 16-2
Genetic Drift
Sample of
Original Population
Founding Population A
Founding Population B
Descendants
FOUNDER
EFFECT:
2 small groups from a
large, diverse
population could
produce new
populations that differ
from the original
population
Section 16-2
Genetic Drift
Sample of
Original Population
Descendants
Founding Population A
Founding Population B
EVOLUTION VERSUS GENETIC
EQUILIBRIUM
HARDY-WEINBERG PRINCIPLE: ALLELE
FREQUENCIES IN A POPULATION WILL
REMAIN CONSTANT AS LONG AS FIVE
CONDITIONS (FACTORS) REMAIN
CONSTANT. GENETIC EQUILIBRIUM IS
REACHED. (IS THE POPULATION
EVOLVING?)
5 CONDITIONS REQUIRED TO
MAINTAIN GENETIC EQUIIBRIUM
1.
2.
3.
4.
5.
RANDOM MATING
- Select mates w/o bias
LARGE POPULATION
- Genetic drift does not affect large pops
NO MOVEMENT INTO OR OUT OF THE
POPULATION
- no intro of new allele
NO MUTATION
- no new allele introduced
NO NATURAL SELECTION
- No phenotype can have selective advantage
DOES THIS EVER HAPPEN?????
• In 1908, Hardy and Weinberg
independently demonstrated that
DOMINANT ALLELES DO NOT
REPLACE RECESSIVE ALLELES IN A
POPULATION!
HARDY-WEINBERG EQUATION
You have sampled a population in which you know that the percentage of the homozygous
recessive genotype (aa) is 36%. Using that 36%, calculate the following:
The frequency of the "aa" genotype.
The frequency of the "a" allele.
The frequency of the "A" allele.
The frequencies of the genotypes "AA" and "Aa."
The frequencies of the two possible phenotypes if "A" is completely dominant over "a."
• FREQUENCY OF aa GENOTYPE: 36%
(GIVEN)
• FREQUENCY OF a ALLELE:The frequency of
aa is 36%, which means that q2 = 0.36, by
definition. If q2 = 0.36, then q = 0.6, again by
definition. Since q equals the frequency of the a
allele, then the frequency is 60%.
• The frequency of the "A" allele. Answer: Since q
= 0.6, and p + q = 1, then p = 0.4; the frequency
of A is by definition equal to p, so the answer is
40%.
• The frequencies of the genotypes "AA" and "Aa."
Answer: The frequency of AA is equal to p2, and the
frequency of Aa is equal to 2pq. So, using the
information above, the frequency of AA is 16% (i.e. p2
is 0.4 x 0.4 = 0.16) and Aa is 48% (2pq = 2 x 0.4 x 0.6
= 0.48).
• The frequencies of the two possible phenotypes if "A"
is completely dominant over "a." Answers: Because
"A" is totally dominate over "a", the dominant
phenotype will show if either the homozygous "AA" or
heterozygous "Aa" genotypes occur. The recessive
phenotype is controlled by the homozygous aa
genotype. Therefore, the frequency of the dominant
phenotype equals the sum of the frequencies of AA
and Aa, and the recessive phenotype is simply the
frequency of aa. Therefore, the dominant frequency is
64% and, in the first part of this question above, you
have already shown that the recessive frequency is
36%.
AS NEW SPECIES EVOLVE (SPECIATION), POPULATIONS
BECOME REPRODUCTIVELY ISOLATED FROM EACH OTHER
Section 16-3
Reproductive Isolation
results from
Isolating mechanisms
which include
Behavioral isolation
Geographic isolation
Temporal isolation
produced by
produced by
produced by
Behavioral differences
Physical separation
Different mating times
which result in
Independently
evolving populations
which result in
Formation of
new species
.
Flowchart
Section 17-4
Species
that are
Unrelated
form
Related
in
under
under
in
in
Interrelationshiops
Similar
environments
Intense
environmental
pressure
Small
populations
Different
environments
can undergo
can undergo
can undergo
can undergo
can undergo
Coevolution
Convergent
evolution
Extinction
Punctuated
equilibrium
Adaptive
radiation
a. Divergent evolution when 2/
more species originate from a
common ancestor
Ex: flipper of whale and limb of
human
b. Convergent evolution process
by which unrelated organisms
come to resemble each other
Ex: torpedo shape of shark and
penguin/ wing of insect and bat
c. Coevolution process by which 2
species evolve in response to
changes in each other over time