Download Chapters 22, 23, and 24 Natural Selection and Mechanisms of

Chapters 22, 23, and 24
Natural Selection and Mechanisms of Speciation
A. Bergeron
AP Biology
PCHS
What do you think these data reveal about the state of science
education in the United States?
Do you think that this is an appropriate
question to ask in a poll?
The Prevailing World View
(Before Darwin and his Contemporaries)
1. The Earth is young (approximately 6,000 years)
! -Promoted by Bishop James Ussher
2. Each type of species was “created”
!
-Species are “fixed”, meaning that they are immutable
!
-The number of species on Earth is not changing
3. Any variations observed in an organism are “imperfections”
!
-Adaptations are designed by the creator
4. Observation alone should support the view of immutable
species.
Evolution Discussion Topics
1. Worldview before Darwin
-Carolus Linneaus, Georges Cuvier, James Hutton,
Charles Lyell
2. Jean-Baptiste Lamarck
!
-Theory of Acquired Characteristics
3. Charles Darwin
!
-Theory of Evolution by Natural Selection
!
-Artificial Selection
!
-Examples of Natural Selection in the “Real World”
4. Evidence for Evolution
!
-Extinction
!
-Comparative anatomy, vestigial organs
!
-Molecular/biochemical evidence
!
-Biogeography
!
-Fossil record - Transitional forms
Georges Cuvier
Promoted “catastrophism” or the idea that
catastrophes (i.e. floods, earthquakes,
volcanic eruptions) altered the distribution
of organisms in a particular geographic area
Rejected the idea of evolutionary change
James Hutton
!
Promoted gradualism or the idea that
!
large changes can take place through
!
the accumulation of small changes over
!
time
Charles Lyell
!
Promoted uniformitarianism or the idea that
!
the geological factors that affect the Earth
!
today are the same that affected the Earth
!
long ago
Progression of Evolutionary Theory through Time
Lamarck - Theory of Acquired Characteristics
One of the first scientists to offer an
explanation for how organisms changed over
time
Promoted the idea of use and disuse
Parts of the body that are used become
larger and stronger; those that are disused
become smaller and weaker
Suggested that organisms pass on acquired
characteristics to their offspring
There is little evidence that acquired
characteristics can be inherited by offspring
Evidence for Lamarckism?
Descent with Modification???
Jean-Baptiste Lamarck proposed the Theory of Acquired
Characteristics
Lamarck proposed that by selective use or disuse of organs,
organisms acquired or lost certain traits during their lifetime.
These traits could then be passed on to their offspring. This
process led to change in a population/species over time.
Larmarck or Darwin?
Given your understanding of both Lamarck’s and Darwin’s concept of
evolution, determine whether the following statements are more Larmarckian
or Darwinian. If the statement is Lamarckian, change it to make it Darwinian.
OR
The widespread use of DDT (an insecticide) in the mid-1900’s
placed a selective pressure on insects to evolve resistance to
DDT. As a result, large populations of insects today are resistant
to DDT.
Larmarck or Darwin?
OR
Many of the bacterial strains that infect humans today are
resistant to a wide range of antibiotics. These resistant
strains were not so numerous or common prior to the use of
antibiotics. These strains must have appeared or evolved in
response to the use of the antibiotics
Larmarck or Darwin?
OR
The children of bodybuilders tend to be much more athletic,
on average, than other children because the characteristics
and abilities gained by their parents during their lives have been
passed on to their children.
Larmarck or Darwin?
OR
According to one theory, the dinosaurs became extinct because
they couldn’t evolve fast enough to deal with climatic change
that affected their food and water supplies.
Larmarck or Darwin?
OR
Life arose in an aquatic environment and later invaded land.
Once animals came on to land, they had to evolve effective
methods of support against gravity and locomotion in order to
survive.
Larmarck or Darwin?
OR
A given phenotypic trait (e.g. height, speed, tooth structure)
and the genes that determine it may have positive survival or
selective value, negative survival or selective value, or neutral
survival or selective value. Which of these it possesses
depends on the environmental conditions the organism
encounters.
Reading Quiz - Chapter 22
1. Charles Lyell promoted a principle of ____________ which
stated that geologic processes have not changed
throughout Earth’s history.
2. A contemporary of Darwin presented research that was quite
similar to Darwin’s theory of natural selection. Who was this
individual?
3. Structures that are similar in structure but may be different in
function are known as _________.
4. According to the principle of biogeography, organisms living
on an island are (more likely/less likely) to resemble
organisms living on the mainland.
5. If Darwin was correct and populations have changed over
time, ______________ should be found as part of the fossil
record.
Do Now - Evolution by Natural Selection
1. How did Darwin define fitness? How did Darwin define
evolution?
2. What is the unit of natural selection (i.e. what is selected for
or against?) What is the unit of evolution?
3. In a population of mice, some individuals have brown fur and
some have black fur. At present, both phenotypes are equally
fit. What could happen to change the relative fitness of the two
phenotypes in the population?
Evolution by Natural Selection
Theory proposed by Charles Darwin in 1859 as
part of his book On the Origin of Species by
Means of Natural Selection
Evolution: A change in a population of organisms
over time
Darwin’s Theory of Evolution by Natural Selection proposed
that individuals better adapted to their environment will:
!
-Be more likely to survive and therefore reproduce
!
-Pass on beneficial adaptations to their
!
offspring (i.e. differential reproductive success)
!
-Generate new populations of organisms that are
!
better adapted to their environment
*Over time, the original population would have changed and
the new population/s could become a new species *
Charles Darwin and the Theory of Evolution
by Natural Selection
Darwin asked, “If evolutionary change has occurred, then HOW
do organisms evolve?”
The Keys to Natural Selection
1. Struggle for Existence
- Nature produces more organisms than can survive
-Litters of cats, dogs, mice, hamsters, etc.
- Organisms must COMPETE with one another for limited resources
2. Organisms in a Population Can Be Different
- Variations in physical traits exist within any population
- Variations are passed on from one generation to the next
3. Organisms that Can Adapt to their Environment Can Survive
- Organisms that have inherited favorable traits which allow them
to cope with their environment and out-compete other organisms
are more likely to SURVIVE and REPRODUCE
Details of Darwin’s Theory of Natural Selection
!
!
!
!
!
!
!
!
!
!
!
!
!
!
!
1. Variations exist within populations of organisms
2. Variations can be passed from parents to offspring
!
-Traits are heritable !
3. Organisms tend to produce lots of offspring
4. Not all of the offspring that are produced will be !
able to survive due to the limited amount of
resources in the environment
5. The organisms that do survive are more likely to
REPRODUCE and pass on beneficial traits to their
offspring making it easier for them to SURVIVE and
reproduce
What Conclusions Can We Draw from the
Theory of Evolution?
Remember, these are conclusions that are drawn from data.
The theory CANNOT be proven (and doesn’t need to be…)
1. Organisms (i.e. living things) have changed over time
-Organisms alive millions of years ago are not alive today
!
-Many of these organisms are extinct but others have
!
“evolved” into the organisms that are alive today
2. All species which exist today
descended from (i.e. originated) a
common ancestor
!
-Modern organisms should have
similar characteristics to those
organisms that were alive long
ago
Descent with Modification
If a Gene Pool Can Change, What is the
Origin of this Variation?
Sources of Genetic Variation
No two organisms are 100% identical at the genetic level
(except for identical twins, of course)
1. Mutation
! -Produces new genes (alleles) not
! previously seen in the gene pool
!
!
!
!
!
-Caused by mistakes made during DNA
replication, chemical agents (i.e. mutagens), or the
environment (e.g. UV radiation)
-The mutation could potentially alter the
! structure of the protein and potentially
! change its function as well
Sources of Genetic Variation
2. Independent assortment of chromosomes during meiosis
Independent assortment produces gametes that (potentially)
have a different distribution of alleles than either parent
Sources of Genetic Variation
3. Genetic recombination - “Crossing over”
Crossing over creates new varieties
of gametes (sex cells) and new
combinations of alleles not previously
seen in the population (in parental
generation)
Discussion Questions
1. How did knowledge of mechanisms of artificial selection (which was used
in developing various strains of domesticated plants and animals) help
Darwin understand how evolution could occur?
2. Complete the table
Observation
All species populations have the reproduction
potential to increase exponentially over time.
The number of individuals in natural
populations tends to remain stable over time.
Environmental resources are limited.
Individuals in a population vary in their
characteristics.
Much of this variation is heritable
How did Darwin make this observation?
What did he read or observe that led to his
understanding?
Discussion Questions
3. Complete the table
Inference
Production of more individuals that the
environment can support leads to a struggle
for existence such that only a fraction of the
offspring survive each generation.
Survival for existence is not random. Those
individuals whose inherited traits best fit them
to the environment are likely to leave more
offspring than less fit individuals.
The unequal ability of individuals to survive
and reproduce leads to a gradual change in
the population, with favorable characteristics
accumulating over many generations.
Observations that led to the inference
The Prevailing World View
(After Darwin and his Contemporaries)
1. The Earth is old
! !
-Current estimates place the age of the Earth at
! !
approximately 4.5 billion years
2. Species are related through common descent
3. Adaptation to the environment is accomplished through the
interplay of random variation and natural selection
4. The process of evolution can be observed and tested in
the laboratory and the field
Fish Species
Shark
Salmon
Trout
Catfish
Crab
Shrimp
Lobster
Clams
Oysters
Octopus
Cod
Swordfish
Snapper
Sea Bass
Grouper
Sole
Smelt
Contributor
Bio-Rad Kaleidoscope Prestained Protein Standard
Myosin
B-galactosidase
Bovine serum albumin
Carbonic anhydrase
Soybean trypsin inhibitor
Lysozyme
Aprotinin
Blue
Magenta
Green
Violet
Orange
Red
Blue
198kD
126kD
85kD
37kD
32kD
17kD
7kD
Distance
Migrated
(mm)
Protein
Molecular
Mass
(from
standard
curve)
Fish
Species
#1
Fish
Species
#2
Fish
Species
#3
Fish
Species
#4
Fish
Species
#5
Count/
Tally of
Proteins
Average Mass of an Amino Acid = 110 daltons
Average Molecular Weight of Protein = # Amino Acids x 110 daltons
#bp that Encode Protein = (3 bp/amino acid) x 110 daltons
Species #1 Species #2 Species #3 Species #4 Species #5
Species #1
Species #2
Species #3
Species #4
Species #5
Reading Quiz - Chapter 23
1. From an evolutionary perspective, do individual organisms or
populations change over time?
2. In H-W equilibrium, p represents the frequency of the
__________ allele while q represents the frequency of the
__________ allele.
3. Name two potential causes of microevolution within a
population.
4. A natural disaster that drastically reduces the proportion of
a particular allele in the gene pool is referred to as a
__________.
5. In ________ selection, the intermediate phenotype is favored
over extreme phenotypes.
Is Natural Selection Occurring within a Population?
1. Is there variation within the population?
2. Are characteristics within the population heritable?
3. Not all members of the population that are
born will survive and reproduce
4. Certain members of the population are more likely to
survive and reproduce than others
Let’s Analyze Several Populations to See if These Four
Tenets of Natural Selection Are Applicable…
Natural Selection - Nylon-eating Bacteria
The nylon polymer (synthetic, human-made) was invented by
DuPont in 1934
Nylon was invented to replace the silk and/or hemp-based
products that were used to make parachutes
Nylon requires a great deal of energy to be broken down.
Incineration produces hydrogen cyanide
Strains of Pseudomonas bacteria grown in an environment that
contains a molecular precursor for nylon synthesis (only source
of carbon and nitrogen in the media) evolve enzymes that break
down nylon
Experiment has been repeated with similar results each time
Natural Selection - Antibiotic Resistance
Imagine a scenario where a population of Mycobacterium
tuberculosis (the cause of TB) infect lung tissue
What selective pressure/s could act on the bacterial
population?
Could this population change over time?
Infection
Mycobacterium
tuberculosis
Selective Pressure = Addition of an Antibiotic
1. Will there be variation within the bacterial population?
2. Are characteristics within the population heritable?
3. Will all members of the population survive and reproduce?
4. Will certain members of the population be more likely to
survive and reproduce than others? If so, which ones?
Drug-resistant Tuberculosis
Approximately 50% of antibiotics target prokaryotic ribosomes,
rendering them unable to synthesize bacterial proteins
Natural selection favors those bacterial variants that possess
ribosomes that are unaffected by antibiotic
Over time bacterial populations have the potential to become
resistant to multiple types of antibiotics (e.g. extensively drug
resistant strains)
The Case of the Alpine Skypilot
What conclusions can we draw regarding the flower size of
alpine skypilots growing in each environment?
Why is this important?
Hint: Is there variation in the population?
Plants growing above
tree line were
propagated and
transplanted to
locations below the
tree line
Flower size of
offspring was
compared to the
flower size of the
original parent (i.e.
the mother)
What does the best
fit line tell us about
the heritability of flower
size?
Is flower size heritable?
Is there limited reproductive success in the population?
(Are more individuals “born” than can survive and reproduce?)
Only certain transplanted individuals originating from the tundra
environment were able to survive in the forest environment
Certain individuals in a population are more likely to survive
and reproduce than others.
Hypothesis: Transplanted flowers
that are pollinated by bumblebees
are more likely to survive and
reproduce than flowers that are
pollinated by flies.
How could this hypothesis be
tested?
What can we conclude regarding a bumblebee’s preference
for flower size in the transplanted skypilot population?
What can we conclude regarding the relative fitness of a
transplated skypilot and the number of bee visits that it received?
Biological fitness refers to an
organism’s ability to
produce fertile (fecund)
offspring
Plants with the largest number
of bee visits (and thus the
largest flowers) are most likely
to survive, reproduce, and
pass on characteristics to the
next generation of plants
Evidence for Evolution
1. Fossil record
•Extinction
•Transitional fossils
2. Comparative anatomy
• Homologous structures
• Embryological origins
• Vestigial organs
3. Molecular and biochemical evidence
4. Biogeography
Fossils Depict Evolutionary Change
Organisms that lived in the
past are very different from
organisms alive today
Transitional Forms
Fossil evidence supports the
view that land-dwelling mammals
are the ancestors of modern-day
whales
All fossils have distinctive types
of ear bones that only organisms
in this lineage share
Fossils were found in rocks that
could only be formed from ocean
deposits
Legs became smaller as the
organism “switched” to a new
habitat
Evolution of the Horse
Evolution is NOT goal-oriented
Examining selected fossils from
the descendants of the modern
horse might lead one to believe
that large, one-toed horses were
the eventual goal
Examination of the entire horse
fossil record reveals multiple
branch points
Although one type of horse
(Equus) was ultimately favored
other others, this result was not
pre-determined
Homologous Structures
Structures in different organisms that are similar due to
common ancestry
Embryological Origins
Strong resemblance of embyros from a variety of species
Suggests descent from a common ancestor
Vestigial Organs
Reduced or incompletely developed structures that have no
(or reduced) function in an organism
Show similarities to functioning organs or structures in other
species - Suggested descent from a common ancestral form
1. Human coccyx
Reduced form of the tailbone found in primates
2. Human appendix
Reduced version of the cecum, aids in the
digestion of cellulose
3. Goosebumps
Original purpose allowed hair shaft to stand erect, signaling
anger or fear
Transitional Forms
Fossil evidence supports the
view that land-dwelling mammals
are the ancestors of modern-day
whales
All fossils have distinctive types
of ear bones that only organisms
in this lineage share
Fossils were found in geological
formations that could only be
formed from ocean deposits
Legs became smaller as the
organism “switched” to a new
habitat
Biochemical Evidence
Comparing nucleotide sequences of genes found in all
organisms suggests common descent
Number of
differences has
allowed for the
formation of
“genetic clocks”
Clocks are
supported by
position of fossils
within the fossil
record
Tracking Evolutionary Divergence
Biochemical evidence has allowed for the construction of
phylogenetic trees which demonstrate the similarities between
organisms at the molecular level
?
Development of Plants & Animals from an Embryo
Genetic Equivalence Hypothesis
Hypothesis of differential gene content not supported
experimentally
M H
K
M H
K
fertilized egg
fertilized egg
M
H
K
M H
K
M H
K
M H
K
All cells have the same genes but different genes are
expressed in different cells
Differential expression of genes leads to the development of
different cell types
Testing Genetic Equivalence
How Is Cell Fate Determined?
Homeobox Genes Suggest that Embryos Have
Similar Developmental Programs
Homeobox genes - Encode proteins that can bind to DNA;
involved in regulating the transcription of genes as part of a
developmental pathway
Hox Genes
Hox genes are involved in
pattern formation during
embryonic development
Hox (short for homeobox)
are master control genes transcripton factor proteins
bind to DNA ➔ regulate
transcription of dev. genes
Hox genes are often found in
clusters on a chromosome Position-specific along body axis
Genes are expressed in
the same general regions
of the developing
embryo suggesting a
structure/function relationship
Hox Genes - Deep Homology
The expression of Hox genes provides the basis for anteriorposterior axis specification throughout the animal kingdom
The expression of human HOXB4 gene can mimic the function
of its Drosophila homologue, deformed (dfd), when introduced
into Dfd-deficient Drosophila embryos (i.e. gene homology)
•Changes in the Hox protein-responsive elements (DNA) of downstream genes
•Changes in Hox gene transcription patterns (mRNA, protein) within a portion
of the body
•Changes in the number of Hox genes (duplications, deletions, etc.)
Homeotic Mutations Affect Embryonic
Development and Formation of Adult Structures
Homeotic mutation - Mutations to either the homeotic gene or
the homeobox target to which the gene product (i.e. protein)
can bind
Homeotic Mutations Affect Embryonic
Development and Formation of Adult Structures
Hox Genes - Evidence for Evolution (Deep Homology)
Hox Genes - Snakes Have Legs! (Or at Least
Primordial Femurs...?)
In most vertebrates, forelimbs
develop anterior to the
expression of HoxC-6
HoxC-6 and HoxC-8 expression
specify the development of
thoracic vertebrae
Effect of Paralogous Hox Gene Mutations on
Vertebral Development
Hox Genes - Deep Homology
Paralogous genes:
Partner genes that
have a similar
function in the
absence of a
partner
How Would You Evaluate These Explanations
of Darwin’s Ideas?
This question was included as part of a Biology I exam at the
University of _________ (wherever you choose to attend).
In two or three sentences describe Darwin’s theory of descent
with modification and the mechanism, natural selection, that he
proposed to explain how this change occurs.
Two students’ answers to the questions are given on the next
slide. Evaluate and grade each response on how well it
represents Darwin’s ideas. Indicate why you would or would not
have awarded full credit.
Answer #1: Darwin saw that populations increased faster than
the ability of the land to support them could increase, so that
individuals must struggle for limited resources. He proposed
that individuals with some inborn advantage over others would
have a better chance of surviving and reproducing offspring and
so be naturally selected. As time passes, these advantageous
characteristics accumulate and change the existing species into
a new species.
Answer #2: Darwin’s theory of evolution explains how new
species arise from already existing ones. In his mechanism of
natural selection, organisms with favorable traits tend to
survive and reproduce more successfully, while those that
lack traits do not. Beneficial traits are passed on to future
generations in this manner, and a new species will be created
in the end!
The allele for black coat is recessive to the allele for white
(pink) coat. Can you determine the number of recessive
alleles in this population of pigs?
What is Microevolution?
Microevolution is a change in the genetic composition of a
population
Populations and Gene Pools
Gene pool: All of the genes (alleles) that
are present in a population of organisms
The allele frequencies in a population
always equal 100%
If the composition of genes (alleles) in the
gene pool changes, there is a good chance
that the population of organisms is changing
or evolving
Microevolution - Online Activity 14.4 - Alter a Gene Pool
Populations and Gene Pools (continued)
Sample Population
48%
heterozygous
black
Frequency of Alleles
allele for
brown fur
allele for
black fur
16%
homozygous
black
36%
homozygous
brown
Suppose two genes (alleles) control fur color in mice.
The black allele is dominant to the brown allele
The collection of both alleles make up the gene pool for this
particular population of mice
Calculating Allele Frequencies
In a population of ten (10) flowers, you
can assume that there are twenty (20)
total alleles (diploid organisms)
If the genotypes of the organisms in
the population are known, you can
calculate the frequency of each allele
in the gene pool
Calculating Allele Frequencies
If a Gene Pool Can Change, What is the
Origin of this Variation?
Sources of Genetic Variation
No two organisms are 100% identical at the genetic level
(except for identical twins, of course)
1. Mutation
! -Produces new genes (alleles) not
! previously seen in the gene pool
!
!
!
!
!
-Caused by mistakes made during DNA
replication, chemical agents (i.e. mutagens), or the
environment (e.g. UV radiation)
-The mutation could potentially alter the
! structure of the protein and potentially
! change its function as well
Sources of Genetic Variation
2. Independent assortment of chromosomes during meiosis
Independent assortment produces gametes that (potentially)
have a different distribution of genes than either parent
Sources of Genetic Variation
3. Genetic recombination - “Crossing over”
Crossing over creates new varieties
of gametes (sex cells) and new
combinations of genes not previously
seen in the population (in parental
generation)
Introduction to Hardy-Weinberg Equilibrium
In order to determine if a population is evolving, we must have
some sort of baseline for comparison
Population: A localized group of individuals belonging to
the same species
Species: A group of populations whose individuals have the
potential to interbreed and produce fertile offspring
A non-evolving population should have a gene pool that is in
equilibrium
Gene pool = All of the alleles present in a population
The relative frequency of dominant and recessive alleles should
remain constant from one generation to the next
Any changes to the allelic frequencies in a gene pool suggest
that the population is changing or evolving
Hardy-Weinberg Equilibrium Practice Problems
1. Which five (5) conditions must be met in order for HardyWeinberg equilibrium to exist within a population?
2. Researchers examining a particular gene in a fruit fly
population discovered that there are two alleles for the gene,
designated A1 and A2. Gene pool analysis revealed that 70%
of the gametes produced in the population contained the A1
allele. If the population is at H-W equilibrium, what proportion
of the flies carries both A1 and A2?
3. In a certain population of 1000 fruit flies,
640 have red eyes while the remainder
have sepia eyes. The sepia eye trait is
recessive to red eyes. How many
individuals would you expect to be homozygous for red eye
color?
Hardy-Weinberg Equilibrium Practice Problems
4. The frequency of a recessive allele in a population of
antelopes is determined to be 0.27. What is the frequency
of the dominant allele?
5. An investigator has determined by inspection that 16% of
a human population has a recessive trait. What are the
genotype and allele frequencies for the population, assuming
that the conditions for H-W equilibrium have been/are met?
6. In a certain population, 21% are homozygous dominant,
49% are heterozygous, and 30% are homozygous recessive.
what percentage of the next generation is predicted to be
homozygous dominant, assuming a H-W equilibrium?
7. The genotype frequencies of a population are determined to
be 0.6 BB, 0.0 Bb, and 0.4 bb. Is it likely that this population
meets all the conditions required for genetic equilibrium?
Hardy-Weinberg Equilibrium Practice Problems
8. After graduation, you and 19 of your closest friends (lets say
10 males and 10 females) charter a plane to go on an
around-the-world tour. Unfortunately, you all crash land
(safely) on a deserted island. No one finds you and you start
a new population totally isolated from the rest of the world.
Two of your friends carry (i.e. are heterozygous for) the
recessive cystic fibrosis allele (c). Assuming that the
frequency of this allele does not change as the population
grows, what will be the incidence of cystic fibrosis on your
island?
9. In a given population, only the "A" and "B" alleles are present
in the ABO system; there are no individuals with type "O"
blood or with O alleles in this particular population. If 200
people have type A blood, 75 have type AB blood, and 25
have type B blood, what are the allelic frequencies of this
population (i.e. What are p and q)?
Hardy-Weinberg Equilibrium Practice Problems
10. The allele for right-pointing mouth in cichlids (A)
is dominant over that for left-pointing mouth (a). Use the
Hardy-Weinberg principle to calculate the allele frequencies
that would correspond to equal frequencies of the two
phenotypes. Which allele has the higher frequency?
How are these scale-eating fish an example of a balanced
polymorphism?
Requirements for Hardy-Weinberg Equilibrium
In an ideal breeding population:
1. Population size is very large
2. Mating is random. Mating pairs show no preference for one
phenotype over another. There is no sexual selection.
3. There is no mutation.
4. Immigration and emigration do NOT occur. No gene flow.
5. There is no natural selection
Hardy Weinberg Equilibrium
Hardy-Weinberg Equilibrium - LabBench
Hardy-Weinberg Equilibrium
Requirements (assumptions) for Hardy -Weinberg equilibrium
to be “true”
1. Population size is very large
2. No mutation is occurring
3. Organisms in the population reproduce sexually and
randomly.
4. Natural selection is not occurring
5. No emigration (i.e. out of) the population or immigration
(i.e. into) the population
If all of these conditions are met, the allele frequencies in the
population are not changing and the population is NOT evolving
Hardy-Weinberg Animation
Hardy-Weinberg Equilibrium
A simple mathematical model of the interplay between alleles
that are present in a particular gene pool
At H-W equilibrium, the frequency of the dominant allele (p) and
the frequency of the recessive allele (q) always equal 1
p+q=1
The frequency of the homozygous dominant genotype in the
population is expressed as p2
The frequency of the homozygous recessive genotype in the
population is expressed as q2
The frequency of the heterozygous genotype in the population
is expressed as 2pq
Thus at H-W equilibrium...
p2 + 2pq + q2 = 1
Hardy-Weinberg Equilibrium
Frequency of dominant allele = p
Frequency of recessive allele = q
p+q=1
p2 + 2pq + q2 = 1
p2 = frequency of homozygous dominant genotype
q2 = frequency of homozygous recessive genotype
2pq = frequency of heterozygous genotype
AP Biology Lab #8 - Population Genetics
Exploring H-W Equilibrium - Activity 1
Dominant: Taster - AA or Aa
Recessive: Non-taster - aa
Tasters
Non-tasters
Tasters +
Non-Tasters
Allele Frequency
A (p)
A (q)
Class Totals
What is the frequency of heterozygous tasters in the class?
Does the class meet any/all of the requirements for H-W
equilibrium?
Would you predict the allelic frequencies for PTC tasting
to remain constant for the next 200 years?
Exploring H-W Equilibrium - Activity 2
Effect of a small population size on allelic frequency in a
gene pool
Initial Class
Frequencies
p= 0.5
q= 0.5
Initial Genotype = A/a
Class Totals
My Genotype
Generation 1
A/A
A/a
a/a
Generation 5
frequencies
Generation 2
Generation 3
p=
Generation 4
q=
Generation 5
Exploring H-W Equilibrium - Activity 3
Applying Selective Pressure
Initial Class
Frequencies
p=
q=
Initial Genotype = A/a
Class Totals
My Genotype
Generation 1
A/A
A/a
Generation 5
frequencies
Generation 2
Generation 3
p=
Generation 4
q=
Generation 5
Do Now - Evolutionary Change?
Genome sequencing projects may dramatically affect how
biologists analyze evolutionary changes in quantitative traits.
For example, suppose that the genomes of many living humans
are sequenced and that genomes could be sequenced from
individuals who lived several millennia in the past (You may
assume preserved tissue is readily available.) If 20 genes have
been shown to influence height, how could you use the
sequence data from these genes to test the hypothesis that
human height has evolved in response to natural selection?
Causes of Evolutionary Change - Genetic Drift
The gradual change in the frequency of alleles within a small
population of organisms
Changes are due to chance
Causes of Evolutionary Change - Genetic Drift
Assume that you have a population of red and green insects
at Hardy-Weinberg equilibrium
Causes of Evolutionary Change - Genetic Drift
After a windstorm, a large proportion of the population blows
away
The distribution of alleles has changed dramatically
If only red alleles were passed
on to the next generation, what
would the population look like
after several generations?
Sources of Genetic Drift
1) Bottleneck Effect
Typically caused by a natural
disaster that eliminates a
substantial portion of a
population and the
corresponding component of
the gene pool
The genetic diversity of the
remaining population is
limited
2) Founder Effect
Caused by a small part of a population emigrating to a new location
Genetic diversity of the new population much smaller than in the
original population
Other Causes of Evolutionary Change in a Population
1. Gene flow
Caused by emigration or immigration of individuals out of
or into the population, respectively
2. Mutation
Directly changes one allele into another (or another form)
3. Natural selection
-Particular individuals are more likely to survive and
reproduce in the population
-These individuals will pass on their genetic information to
their offspring
-Consequently, the population may change (I.e. evolve)
over time
Is Natural Selection Occurring within a Population?
1. Is there variation within the population?
2. Are characteristics within the population heritable?
3. Not all members of the population that are born will survive
and reproduce
4. Certain members of the population are more likely to
survive and reproduce than others
Let’s Look at How Natural Selection Can Alter a
Gene Pool and the Population Represented by the
Gene Pool…
Do Now - Evolution as Genetic Change
The residues ("tailings") of mines often contain such high
concentrations of toxic metals (e.g., copper, lead) that most
plants are unable to grow on or near them. However, some
hardy species (e.g. certain grasses) are able to spread from
the surrounding uncontaminated soil onto such waste heaps.
These plants develop resistance to the toxic metals while their
ability to grow on uncontaminated soil decreases. Because
grasses are wind pollinated, breeding between the resistant
and nonresistant populations goes on.
1. What type of natural selection is affecting the population?
2. Predict what could happen to this population after many
thousands of generations
Disruptive Selection
Individuals at both extremes (in terms of phenotype) have the
highest fitness (most likely to survive and reproduce)
Disruptive Selection
Ducks with long beaks can find food at the bottom of a river or stream
while ducks with short beaks can easily feed at the water’s surface
Ducks with intermediate sized beaks are at a disadvantage
Over time, the extreme phenotypes will be favored over the
intermediate phenotype and the population will change
Stabilizing Selection
Individuals closest to the mean or average phenotype have the
highest fitness within a population of organisms
Stabilizing Selection
Large lizards are more likely to be seen and caught by
predators while small lizards will have difficulty escaping from
predators on the ground
Intermediate-sized lizards in the population are more likely to
survive and reproduce
Effect of Sickle Cell Mutation on Hemoglobin Structure
Normal vs. Sickle-Cell Red Blood Cells
Sickle-Cell Disease and Heterozygote Advantage
Heterozygous individuals do not suffer from sickle cell anemia
and are less susceptible to the parasite that causes malaria,
Plasmodium falciparum
When the parasite enters the RBC it induces hypoxia which leads
to a sickling of RBCs in individuals who are either heterozygous
OR homozygous for the sickle cell allele
Individuals who do not have a sickle cell allele have RBCs that
do not sickle in the presence of the parasite - These individuals
are at the greatest risk of contracting malaria
Spleen eliminates sickle-shaped RBCs (which removes the
parasite from the body) leading to anemia in persons who are
homozygous for the sickle cell allele
Heterozygotes do not suffer from anemia because a sufficient
supply of normal RBCs remains in the body
Directional Selection
Individuals which possess one phenotype (one of the extremes)
have the highest level of biological fitness
Directional Selection
During dry years on the Galapagos, hard seeds were more prevalent
than soft seeds
Birds with deep beaks were better able to crack open hard seeds than
birds with narrow beaks
Over time, the mean beak depth shifted as the finch population
adapted to their environment
Types of Natural Selection - A Summary
How Do New Species Come
to Be?
Monochromatic
P. pundamilia
P. nyererei
Let’s Play Everyone’s Favorite
New Game...
Name that Reproductive Barrier!
Decide whether the following descriptions are
prezygotic or postzygotic barriers to reproduction. In
addition identify the specific type of reproductive
barrier
Different species of bowerbird construct elaborate
bowers and decorate them with different colors in order
to woo females. The MacGregor’s Bowerbird builds a
tall tower of sticks and decorates with bits of charcoal.
Two species of frogs with overlapping ranges breed at
different times of the year. Rana aurora (left) breeds
January - March while Rana boylii - breeds late March May (right)
Imagine a situation in which a population extends over
a broad geographic range, and mating throughout the
population is not random. Individuals in the far west
would have zero chance of mating with individuals in
the far eastern end of the range.
The formation of a river separates the insect population
into two distinct groups; one on either side of the river
The morphology of the damselfly penis (Yes, insects
have sex!) is evidence for a specific form of
reproductive isolation. What is it?
Imagine a species of louse living on a species of gopher.
When the gophers get together to mate, the lice get an
opportunity to switch gophers and perhaps mate with
lice on another gopher. Gopher-switching allows genes
to flow through the louse species.
That’s Bananas!
The modern Cavendish banana has 11 pairs of chromosomes
(2n=22). The Cavendish is an autotriploid species with a
complex evolutionary history.
Illustrate a possible sequence of events that could have led to
the appearance of the Cavendish banana as a result of an
autopolyploid event. (Remember an autopolyploid speciation
event is fundamentally different from an allopolyploid event)
What is a Species?
Biological species concept: A
population or group of
populations whose members
have the potential to interbreed
and produce viable, fertile
offspring (genetic exchange is
possible and that is genetically
isolated from other populations)
General Forms of Speciation
Anagenesis
Accumulation of
heritable changes
resulting in a new
species
(Lamarckian)
Cladogenesis
“Branching evolution;” New species
arises from a population that branches
from an original parent species; Parent
species remains (Darwinian)
Process of Speciation
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
Prezygotic Isolating Mechanisms
Prezygotic barriers: Impede mating between species or
prevent the fertilization of the ova
Geographical Isolation
Population is distributed in a way that prevents or inhibits
efficient reproduction/mating
Barriers that prevent organisms from
encountering one another could lead to
the production of new species
Smaller organisms with limited ranges
are affected greatly by geographic
barriers
Larger organisms that can travel large
distances or plants that use air currents
to distribute pollen may not be affected
Prezygotic Isolating Mechanisms
Prezygotic barriers: Impede mating between species or
prevent the fertilization of the ova
Geographical Isolation
Population is distributed in a way that prevents efficient
reproduction/mating
Behavioral/Sexual Isolation
-Albatross mating ritual
-Blue-footed boobies courtship ritual
Temporal Isolation
Mating/reproductive periods are separated among members
of a population
!
Two fruit fly species have overlapping ranges but Drosophila
!
!
psedoobscura is sexually active in the afternoon while
Drosophila persimilis is active only in the morning
Do Now - Exploring Speciation
Drosophila “Speciation” via Nutrient Modification
Original WT
fly population
Some flies raised
on starch medium
Some flies raised
on maltose medium
Mating experiments
between both fly
populations
After several
generations of living
on each type of
specialized medium,
flies adapted to digest
starch (or maltose)
more efficiently
The populations
had “evolved”
Drosophila “Speciation” via Nutrient Modification
At your tables, predict the results of matings between “starch”
flies and “maltose” flies
At your tables, predict the results of matings between flies taken
from two different “starch” populations or two different
“maltose” populations
Female
9
8
20
Mating frequencies;
Experimental group
Different
Same
Starch
22
Same
18
15
Different
Maltose
Male
Starch
Maltose
Male
Female
12
15
Mating frequencies;
Control group
Assortative (Sexual Selection) Mating in Cichlids
Male and female fish were placed in two aquarium tanks; one
with natural light and one with monochromatic (orange) light.
Under natural light, the colorations of the two different fish
species are readily apparent
However, under monochromatic light, the colorations appear
almost identical
Females mated only with males of their own species under
normal light
Natural Monochromatic
Under monochromatic
light, females
mated indiscriminately
P. pundamilia
P. nyererei
Examples of Postzygotic Isolating Mechanisms
Postzygotic barriers: Fertilization occurs, but the hybrid zygote
does not develop into a viable, fertile adult
Reduced hybrid viability
Hybrid embyro fails to develop normally
Reduced hybrid fertility
Hybrid organism survives but is infertile
Example - Horse ♥ Donkey --> Mule (infertile)
“Backbreeding” of a mule to a horse or donkey fails
Hybrid breakdown
Offspring of hybrid are infertile
Example - Cotton; 2nd generation hybrids are sterile
Allopatric vs. Sympatric Speciation
Allopatric (“Other Country”) Speciation
Production of new species
through the interruption of a
population’s gene flow due
(primarily) to a change in the
local geography
Sympatric (“Same Country”) Speciation
Production of a new species within members of the population
that are not geographically isolated from one another
Reproductively isolated subpopulation in the midst of its
parent population (results in change in genome)
Common in plants but less so in animals
Speciation Practice Problem
In Chapter 24, you read that bread wheat (Triticum aestivum) is an
allohexaploid, containing two sets of chromosomes from each of three
different parent species. Genetic analysis suggests that the three species
pictured below each contributed
chromosome sets to T. aestivum.
(The capital letters shown on the
right represent sets of chromosomes
rather than individual alleles.)
Evidence also indicates that the
first polyploid event was a
spontaneous hybridization of the
early cultivated wheat species
T. monococcum and a wild grass
species. Based on this
information, draw a diagram of one
possible chain of events that could
have produced the allohexaploid
T. aestivum.
How Can Sympatric Speciation Account for New Species
Arising by Genetic Variation?
Polyploidy in Certain Species of Plants
-Autopolyploidy - Multiple sets of chromosomes from the
same species
How Can Sympatric Speciation Account for New Species
Arising by Genetic Variation?
Polyploidy in Certain Species of Plants
-Allopolyploidy - Multiple sets of chromosomes from two
or more different species
Possibility #1
Possibility #2
Processes of Evolutionary Change
Species
that are
Unrelated
form
Related
in
under
under
in
in
Inter-relationships
Similar
environments
Intense
environmental
pressure
Small
populations
Different
environments
can undergo
can undergo
can undergo
can undergo
can undergo
Co-evolution
Convergent
evolution
Extinction
Punctuated
equilibrium
Adaptive
radiation
Do Now - Exploring Speciation in Drosophila
Using the pictures below, write a short story that explains how
the original Drosophila popluation could split into two new
species.
1.
2.
3.
4.
Adaptive Radiation
Some organisms manage to be the first to colonize new islands
The new environment allows for this original population to
undergo evolutionary change
Members of this population may adapt to the environment or
may move to a neighboring island (if it exists)
The gradual accumulations of beneficial adaptations may lead
to the generation of new species from the original population
Adaptive Radiation
At least 13 separate species of
finch inhabit the Galapagos islands
All of these species diverged from
a single ancestral, ground-dwelling,
seed eating finch species
Each species has become
adapted for different diets:
seeds, insects, flowers,
leaves, and the blood of
seabirds
Convergent Evolution
Different organisms living in different places experience similar
environmental influences
Consequently, these organisms may develop analagous
structures, structures similar in structure and function but with
different evolutionary histories (i.e. not derived from common
descent)
While homologous structures indicate common ancestry,
analagous structures do not
Convergent Evolution (cont.)
Some ocean-dwelling fish live in the waters surrounding
Antarctica where the water is very cold
These fish have adapted to their frigid environment by
synthesizing “antifreeze proteins” that prevent their blood from
freezing at low temperatures
Natural selection would favor any fish that had this beneficial
mutation/adaptation
Fish that live in the Arctic also have these proteins but the
gene that encodes the Arctic protein is very different from the
gene that encodes the Antarctic protein
The antifreeze gene evolved at least twice in each of these
separate populations of fish
Coevolution (The Biological Arms Race!)
Different organisms can evolve together if they depend on each
other for survival and/or reproductive success
Red squirrels and crossbilled birds both eat pine (cone) seeds
In some places, there are no squirrels and the birds are free to
consume as many seeds as they can
Unfortunately for the birds, the pines have developed a defense
mechanism which makes it harder for the birds to eat their seeds
Coevolution (cont.)
Pinecones with large, thick scales are harder
for birds to eat
But the birds have developed a counterattack
against the pine trees!
Birds with deeper, shorter, less
curved beaks are better able to eat
seeds from large, thick pine cones
Hypothesis Testing
1. Predict what type of pinecone you would expect to observe in
an area without birds (but with squirrels) or an area without
squirrels (but with birds).
2. Predict the beak phenotype of birds living in an area where
pinecones have thick scales. Do the same for birds living in
an area where pinecones have thin scales.
Punctuated Equilibrium
Punctuated equilibrium refers to the tempo of speciation
Some species appear in “bursts” due to the rapid accumulation
of modifications
Often occurs in small populations that are in a “challenging” new
environment
This rapid speciation may not leave
fossil evidence and the surviving
species may appear to have
been “created” without any
ancestral species
Punctuated equilibrium helped
explain the non-gradual appearance
of some species in the fossil record
Punctuated Equilibrium
Mollusk (i.e. snail) population at
stasis (i.e. genetic equilibrium)
A drop in sea level isolates one
part of the population from the
other
The isolated population
changes in an “attempt”
to adapt to the new
(terrestrial?) environment
Punctuated Equilibrium
1.
3.
2.
4.
5.
Evolution and Speciation Review Questions
1. Natural selection can lead to the evolution of prezygotic
isolating mechanisms but not postzygotic mechanisms.
Explain how this is possible.
2. Directional selection can lead to the fixation of favored alleles.
When this occurs, genetic variation is zero and evolution
stops. Explain why this rarely occurs.
3. Explain why genetic drift is much more important in small
populations than in large populations.
4. Conservation managers frequently use gene flow, in the form
of transporting individuals or releasing captive-bred young,
to counteract the effects of drift on small, endangered
populations. Explain how gene flow can also mitigate the
effects of inbreeding.
Evolution and Speciation Review Questions
5. In 1789 a small band of mutineers led by Fletcher Christian
took over the British warship HMS Bounty. They fled first to
Tahiti and then to tiny Pitcairn Island in the South Pacific,
along with a small population of native Tahitians. Some of
the sailors married Polynesians there and raised families.
All contemporary Pitcairn islanders can trace at least part of
the family tree back to the colonization event. What
evolutionary forces were at work in this small, isolated
population during and after the arrival of the mutineers and
Polynesians?
Instructions for Performing an Entrez/pBLAST alignment
1. Enter www.Pubmed.gov into a web browser
2. Click Protein at top of page
3. Perform a search for common gene sequences
Examples: Elongation factor 1 alpha, alcohol
dehydrogenase, cytochrome C, catalase, carbonic
anhydrase,or you choose one of your own.
4. Locate the amino acid sequence of your selected gene
5. Highlight, copy and paste the sequence to pBLAST
Initial Class
Frequencies
p=
q=
Initial Genotype = A/a
Class Totals
My Genotype
Generation 1
A/A
A/a
Generation 5
frequencies
Generation 2
Generation 3
p=
Generation 4
q=
Generation 5
Generation 6
Generation 7
Generation 8
Generation 9
Generation 10