Chapter 21
Genes within populations
Question?
How
did the diversity of life
originate?
Through the process of
Evolution.
Evolution
The
processes that have
transformed life on earth
from its beginnings to today's
diversity.
Evolution is the most
pervasive principle in biology.
Theory vs theory
Hypothesis
Layperson’s
supported
repeatedly by
data.
Makes testable
predictions
definition.
Confused with
hypothesis in
Science.
Examples of Theory
Cell
Theory
Big Bang Theory
Atomic Theory
Theory of Gravity
Theory of Evolution
Teaching
Philosophy on
Evolution
Evolution
Has
itself "evolved" or changed
over time.
Illustrates “Science as a
Process”.
Students should be able to give
the main points of several views.
Pre-Darwinian Views
1. Greeks
2. Fixed Species
3. Catastophism
4. Hutton and Lyell
5. Lamarck
Greek Philosophers
1. Plato - Organisms are
already perfectly adapted to
their environments.
2. Aristotle - Organisms
arranged on a “scale of life”
from simple to complex.
Result
No
evolution.
Life is already perfect and
doesn’t need to change.
All the rungs on life's "ladder"
are already occupied.
Fixed Species Concept
The
creator had designed
each and every species for a
particular purpose.
Result
No
evolution.
Created the viewpoint that all
species could be identified
and named (Taxonomy).
A major factor in the
Linnaeus classification
system.
Catastrophism
Georges
Cuvier
(1769-1832).
Attempted to
relate fossils to
current life.
Theory
Fossils
were the remains of
species lost due to
catastrophe.
No new species originated;
species could only be lost
over time.
Result - No evolution.
James Hutton
1795
- Gradualism
Profound change is the
cumulative product of slow,
but continuous processes.
Result
Changes
on the earth were
gradual, not catastrophic.
Charles Lyell
1797
- 1875.
Incorporated
Hutton’s
gradualism into a
theory called
Uniformitarianism.
Uniformitarianism
Geological
processes have
operated at the same rate
over the Earth’s history.
Result
The
Earth must be VERY old.
(much older than 6000 years
of the fixed species concept).
Idea that slow and subtle
processes can cause
substantial change.
Jean Baptiste Lamarck
Published
theory in 1809.
Theory - Life
changed from
simple to
complex over
time.
Lamark
Fossils
were the remains of
past life forms.
Evolution did occur.
Mechanisms
1. Use and Disuse Body
parts used to survive
become larger and stronger.
Body parts not used to survive
deteriorate.
Mechanisms
2. Acquired Characteristics
Modifications
acquired by
use/disuse were passed on to
offspring.
Problem
No
knowledge of genetics.
Acquired traits are not
transmitted offspring.
Lamarck’s Credits
Did
suggest correctly the role
of fossils in evolution.
Did suggest that adaptation
to the environment is a
primary product of evolution.
Charles Darwin
Father
of the
modern theory
of evolution.
Theory Descent with
Modification.
Darwin's Background
Trained
as a Naturalist (after
trying religion and medicine).
Voyage of the Beagle
Result
Darwin's
training and travel
opportunities allowed him to
formulate and support his
ideas on Natural Selection.
Galapagos Finches
Alfred Wallace - 1858
Paper
on Natural
Selection
identical to
Darwin's ideas.
Result - July 1, 1858
Dual
presentation of the
Wallace-Darwin ideas to the
Linnaean Society of London.
Darwin - 1859
Publication
of
"The Origin of
Species”
Comment
Darwin
best remembered for
the theory because of his
overwhelming evidence and
because he published.
Darwinian View
History
of life is like a tree
with branches over time from
a common source.
Current diversity of life is
caused by the forks from
common ancestors.
Example
“The Origin of Species”
Documented
the occurrence
of evolution.
Suggested that the
mechanism for evolution was
Natural Selection.
The Facts:
Fact 1 All species
reproduce
themselves
exponentially.
Fact 2 - Most populations are
normally stable in size.
Fact 3 - Natural Resources are
limited (finite).
Inference 1
The
large number of
offspring must compete for
the finite resources.
Result - Most offspring die.
Thomas Malthus
Essay
on human population
growth in 1798.
Disease, famine, homelessness,
and war are inescapable because
human populations grow faster
than food supplies.
Darwin read Malthus.
More Facts
Fact 4 - No two individuals in a
population are exactly alike.
Fact 5 - Variation is inheritable.
Inference 2
Those
individuals whose
inherited characteristics fit
them best to their environment
survive and reproduce.
Inference 3
Offspring
inherit the
favorable characteristics.
Populations shift over time as
the favorable characteristics
accumulate.
Nature
Determines
which
characteristics are favorable.
Determines who survives.
Result - “Natural Selection”
Artificial Selection
When
man determines the
characteristics that survive
and reproduce.
Result - the various breeds of
animals and plants we’ve
developed.
Ex - Mustard Plant
Original
Cultivars
Evolution Success
Measured By
Survival
Reproduction
Whoever
lives long enough
and has kids is the “winner”
in evolution.
Requirements
In
order for Natural Selection
to work, you must have:
Long
periods of time.
Variations within a population.
Subtleties of
Natural Selection
1. Populations are the units of
Evolution.
2. Only inherited characteristics
can evolve.
Comment
Acquired
characteristics may
allow a species to evolve
"outside" of Natural Selection.
Ex: culture, learning
Additional Signs
1. Biogeography
2. Fossils
3. Taxonomy
4. Comparative Anatomy
5. Comparative Embryology
6. Molecular Biology
Biogeography
The
geographical distribution
of species.
Problem:
Species
mixtures on islands
Marsupials in Australia
Evolution Viewpoint
Biogeographical
patterns
reflect descent from the
ancestors that colonized that
area.
Fossils
Relics
or impressions of
organisms from the past.
Problem:
Show
changes over time from
simple to complex.
Many fossils don't have
descendants.
Evolution Viewpoint
Life
has
changed over
time.
Many species
failed to
survive and
became
extinct.
Comments
1. Fossilization is a rare event.
2. Only hard parts fossilize well.
3. Problem in finding fossils.
4. Interpretation.
5. Missing Links.
Taxonomy
Science
of Classification.
Main Categories
Domain
Kingdom
Phylum
Class
Order
Family
Genus
Species
or Division
Problem
- Why can life be
grouped this way?
Evolution Viewpoint Hierarchy reflects the
branching genealogy of the
tree of life.
Question?
Is
the unit of evolution the
individual or the population?
Answer – while evolution
effects individuals, it can only
be tracked through time by
looking at populations.
So what do we study?
We
need to study populations,
not individuals.
We need a method to track the
changes in populations over time.
This is the area of Biology called
population genetics.
Population Genetics
The
study of genetic variation
in populations.
Represents the reconciliation
of Mendelism and Darwinism.
Modern Synthesis
Uses
population genetics as
the means to track and study
evolution.
Looks at the genetic basis of
variation and natural selection.
Population
A
localized group of individuals
of the same species.
Species
A
group of similar organisms.
A group of populations that
could interbreed.
Gene Pool
The
total aggregate of genes
in a population.
If evolution is occurring, then
changes must occur in the
gene pool of the population
over time.
Microevolution
Changes
in the relative
frequencies of alleles in the
gene pool.
Hardy-Weinberg
Theorem
Developed
in 1908.
Mathematical model of gene
pool changes over time.
Basic Equation
p
+q=1
p = % dominant allele
q = % recessive allele
Expanded Equation
p
+q=1
(p + q)2 = (1)2
p2 + 2pq + q2 = 1
Genotypes
p2
= Homozygous Dominants
2pq = Heterozygous
q2 = Homozygous Recessives
Example Calculation
Let’s
look at a population
where:
A = red flowers
 a = white flowers

Starting Population
N
= 500
Red = 480 (320 AA+ 160 Aa)
White = 20
Total Genes = 2 x 500
= 1000
Dominant Allele
A
= (320 x 2) + (160 x 1)
= 800
= 800/1000
A = 80%
Recessive Allele
a
= (160 x 1) + (20 x 2)
= 200/1000
= .20
a = 20%
A and a in HW equation
Cross:
Aa X Aa
Result = AA + 2Aa + aa
Remember: A = p, a = q
Substitute the values
for A and a
p2 +
2pq + q2 = 1
(.8)2 + 2(.8)(.2) + (.2)2 = 1
.64 + .32 + .04 = 1
Dominant Allele
A
= p2 + pq
= .64 + .16
= .80
= 80%
Recessive Allele
a
= pq + q2
= .16 + .04
= .20
= 20%
Result
Gene
pool is in a state of
equilibrium and has not
changed because of sexual
reproduction.
No Evolution has occurred.
Importance of
Hardy-Weinberg
Yardstick
to measure rates of
evolution.
Predicts that gene frequencies
should NOT change over time as
long as the HW assumptions hold
(no evolution should occur).
Way to calculate gene frequencies
through time.
Example
What
is the frequency of the
PKU allele?
PKU is expressed only if the
individual is homozygous
recessive (aa).
PKU Frequency
PKU
is found at the rate of
1/10,000 births.
PKU = aa = q2
q2 = .0001
q = .01
Dominant Allele
p
+q=1
p = 1- q
p = 1- .01
p = .99
Expanded Equation
p2 +
2pq + q2 = 1
(.99)2 + 2(.99x.01) + (.01)2 = 1
.9801 + .0198 + .0001 = 1
Final Results
Normals
(AA) = 98.01%
Carriers (Aa) = 1.98%
PKU (aa) = .01%
AP Problems Using
Hardy-Weinberg
for q2 (% of total).
Solve for q (equation).
Solve for p (1- q).
H-W is always on the national
AP Bio exam (but no
calculators are allowed).
Solve
Hardy-Weinberg
Assumptions
1. Large Population
2. Isolation
3. No Net Mutations
4. Random Mating
5. No Natural Selection
If H-W assumptions
hold true:
The
gene frequencies will not
change over time.
Evolution will not occur.
But, how likely will natural
populations hold to the H-W
assumptions?
Microevolution
Caused
by violations of the
5 H-W assumptions.
Causes of
Microevolution
1. Genetic Drift
2. Gene Flow
3. Mutations
4. Nonrandom Mating
5. Natural Selection
Genetic Drift
Changes
in the gene pool of
a small population by chance.
Types:
1.
Bottleneck Effect
2. Founder's Effect
By Chance
Bottleneck Effect
Loss
of most of the
population by disasters.
Surviving population may
have a different gene pool
than the original population.
Result
Some
alleles lost.
Other alleles are overrepresented.
Genetic variation usually lost.
Importance
Reduction
of population size
may reduce gene pool for
evolution to work with.
Ex: Cheetahs
Founder's Effect
Genetic
drift in a new colony
that separates from a parent
population.
Ex: Old-Order Amish
Result
Genetic
variation reduced.
Some alleles increase in
frequency while others are
lost (as compared to the
parent population).
Importance
Very
common in islands and
other groups that don't
interbreed.
Gene Flow
Movement
of genes in/out of
a population.
Ex:
Immigration
Emigration
Result
Changes
in gene frequencies
within a population.
Immigration often brings new
alleles into populations
increasing genetic diversity.
Mutations
Inherited
changes in a gene.
Result
May
change gene
frequencies (small
population).
Source of new alleles for
selection.
Often lost by genetic drift.
Nonrandom Mating
Failure
to choose mates at
random from the population.
Causes
Inbreeding
within the same
“neighborhood”.
Assortative mating
(like with like).
Result
Increases
the number of
homozygous loci.
Does not in itself alter the
overall gene frequencies in
the population.
Natural Selection
Differential
success in
survival and reproduction.
Result - Shifts in gene
frequencies.
Comment
As
the Environment changes,
so does Natural Selection
and Gene Frequencies.
Result
If
the environment is
"patchy", the population may
have many different local
populations.
Genetic Basis of
Variation
1. Discrete Characters –
Mendelian traits with clear
phenotypes.
2. Quantitative Characters –
Multigene traits with
overlapping phenotypes.
Polymorphism
The
existence of several
contrasting forms of the
species in a population.
Usually inherited as
Discrete Characteristics.
Examples
Garter Snakes
Gaillardia
Human Example
ABO
Blood Groups
Morphs = A, B, AB, O
Other examples
Quantitative Characters
Allow
continuous variation in
the population.
Result –
Geographical
Variation
Clines: a change along a
geographical axis
Yarrow and Altitude
Sources of Genetic
Variation
Mutations.
Recombination
though
sexual reproduction.
Crossing-over
Random
fertilization
Preserving Genetic
Variation
1. Diploidy - preserves
recessives as heterozygotes.
2. Balanced Polymorphisms preservation of diversity by
natural selection.
Example
Heterozygote
Advantage When the heterozygote or
hybrid survives better than
the homozygotes. Also
called Hybrid vigor.
Result
Can't
bred "true“ and the
diversity of the population is
maintained.
Ex – Sickle Cell Anemia
Comment
Population
geneticists believe
that ALL genes that persist in a
population must have had a
selective advantage at one
time.
Ex – Sickle Cell and Malaria,
Tay-Sachs and Tuberculosis
Fitness - Darwinian
The
relative contribution an
individual makes to the gene
pool of the next generation.
Relative Fitness
Contribution
of one genotype
to the next generation
compared to other genotypes.
Rate of Selection
Differs
between dominant
and recessive alleles.
Selection pressure by the
environment.
Modes of Natural
Selection
1. Stabilizing
2. Directional
3. Diversifying
4. Sexual
Stabilizing
Selection
toward the average
and against the extremes.
Ex: birth weight in humans
Directional Selection
Selection
toward one extreme.
Ex: running speeds in race
animals.
Ex. Galapagos Finch beak size
and food source.
Diversifying
Selection
toward both
extremes and against the
norm.
Ex: bill size in birds
Comment
Diversifying
Selection - can
split a species into several
new species if it continues
for a long enough period of
time and the populations
don’t interbreed.
Sexual Mate selection
May
not be adaptive to the
environment, but increases
reproduction success of the
individual.
This is a VERY important
selection type for species.
Result
Sexual
dimorphism.
Secondary sexual features
for attracting mates.
Comments
Females
may drive sexual
selection and dimorphism
since they often "choose" the
mate.
Question
Does
evolution result in
perfect organisms?
Answer - No
1. Historical Constraints
2. Compromises
3. Non-adaptive Evolution
(chance)
4. Available variations – most
come from using a current
gene in a new way.
Summary
Know
the difference between
a species and a population.
Know that the unit of
evolution is the population
and not the individual.
Summary
Know
the H-W equations and
how to use them in
calculations.
Know the H-W assumptions
and what happens if each is
violated.
Summary
Identify
various means to
introduce genetic variation
into populations.
Know the various types of
natural selection.
Summary
Darwin's
ideas now a "Theory”.
Predictions of a Theory are
tested by experiments and
observations.
Be familiar with the pre-Darwin
views of evolution.