Download Chapter 23 Presentation-The Evolution of Populations

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 Natural Selection is a theory. How
much evidence is required to support a
theory and what sort of counter
evidence is required to refute it?
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Chapter 23
The Evolution of
Populations
“Evolution is indeed a matter of chance, but in the random
lottery of mutations, some numbers and combinations
better meet the imperatives of ecological necessity, and
they arise and are selected for repeatedly.”
--Sean B. Carroll
Essential Idea
 The diversity of life has evolved and
continues to evolve by natural selection.
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A Common Misconception
 Many people think individuals evolve.
 This is not true.
 Populations evolve as a result of natural
selection acting on each individual
within a given population.
 Those individuals better fit to survive
are more likely to reproduce and pass
on genes that will benefit future
generations.
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Microevolution
 Evolution on a small scale.
 The change in the genetic make up of a
population from generation to
generation.
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Darwin’s “Problem”
 Darwin’s problem was that there were many
limitations of science in his time.
 He did not have a good explanation for how
such heritable variations that were required
for natural selection appeared in a
population.
 Nor did he have an explanation for how they
were transmitted from organisms to their
offspring.
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Recall the Blending Hypothesis:
 At the time, the blending hypothesis was
what people used to explain why offspring
look like both parents.
 Darwin and others realized this was wrong
because it would eliminate variation within a
population.
 Ironically, shortly after Darwin published
Origin, Gregor Mendel published his paper
on genetics.
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Mendel’s Pea Experiments
 Mendel’s paper went unnoticed for
nearly 50 years.
 In the early 20th century, as scientists
uncovered the work of Mendel, it
became apparent that its implications
and relatedness to Darwin’s idea were
profound.
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Population Genetics
 Scientists began drawing parallels
between Darwin and Mendel, and
melded them into what is known as
population genetics--the study of how
populations change over time.
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The Modern Synthesis
 As more was learned about Darwin and
Mendel, scientists developed the
Modern Synthesis--a comprehensive
theory of evolution that incorporates
many fields of science.
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Populations
 Populations are groups of individuals
that can breed with one another and are
localized in certain regions.
 Some populations are isolated from
others.
 Still others can easily mix with like
members of a population.
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Populations
 Within a population, all of the genes are
called the gene pool and it consists of
all alleles at a given locus.
 If only one allele exists in a population,
it is said to be fixed and all individuals
are homozygous.
 If more than one allele exists, then
individuals are either homozygous or
heterozygous.
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Allele Frequency
 Consider a
population of 500
with 2 alleles, CR
and CW
 CRCR gives Red
 CWCW gives White
 CRCW gives Pink
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Allele Frequency
 Our Population
Breakdown:
 320 red, CRCR
 160 Pink, CRCW
 20 White, CWCW
 These numbers
suggest a blending
hypothesis
 Why can’t we use
blending?
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The Hardy-Weinberg Theorem
 This theorem is a way to examine how
allele frequencies change over time when
only segregation and independent
assortment are working on the alleles.
 The properties of a non-evolving gene
pool--in the absence of natural selection.
 The theorem states that the frequencies of
the alleles will remain constant in a
population when it is not evolving. 15
The Hardy-Weinberg Theorem
 In other words, sexual reproduction
alone will not change the frequency of
the alleles in a non-evolving population.
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The Hardy-Weinberg Theorem
 The theorem describes Mendelian
inheritance in non-evolving populations.
 It also helps us to understand long-term
evolutionary change--that is, the
preservation of genetic variation gives
the opportunity for natural selection to
occur.
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Hardy-Weinberg Frequency
 In our population, there are 1000 copies
of genes (500 individuals, 2 copies).
 800 of them are CR
 200 of them are CW
 When we have 2 alleles, by convention
we represent them as p and q.
 p = CR = 800/1000 = 0.8 or 80%
 q = CW = 200/1000 = 0.2 or 20%
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The Hardy-Weinberg Theorem
 To determine the probabilities in
our wild flower example:
 The chance of CRCR is:
 p • p = p2 = 0.8 • 0.8 = 0.64
64%
 The chance of CRCW is:
 p • q = 2pq = 0.8 • 0.2 = 0.32
32%
 The chance of CWCW is:
 q • q = q2 = 0.2 • 0.2 = 0.04 4%
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The Hardy-Weinberg Equation
 The Hardy-Weinberg Equation
becomes: p2 + 2pq +q2 = 1
 Again, with a non-evolving gene pool,
the frequencies of alleles will remain
constant if mating is random. You can
think of it like a deck of cards, no matter
how many times you shuffle them, the
types of cards and their frequencies
remain the same.
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5 Reasons Hardy-Weinberg
Doesn’t Hold True:

Departure from these 5 conditions
results in evolution.
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1.
2.
3.
4.
5.
Extremely large population size.
No gene flow.
No mutations.
Random mating.
No natural selection.
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Mutation and Sexual
Recombination
 These provide variety within gene pools.
 Mutations are changes in the nucleotide
sequences that give rise to new genes and
new alleles. Sometimes they’re good,
usually they are not.
 Most mutations occur in somatic cells and
are never passed on.
 Only a small percentage of gametes ever get
into the populations, so any mutation
occurring in the gametes likely won’t get
passed on.
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Mutation and Sexual
Recombination
 Mutation rates in general are low. The
larger the organism the less likely a
mutation will occur and vice-versa.
 For example: Plants and animals with long
generation times are relatively large and
have a much lower frequency of mutations
than do microorganism and viruses.
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Mutation and Sexual
Recombination

Sexual recombination is the best way
to produce variation within a
population on a generation to
generation time scale.
Movie
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Mutation and Sexual
Recombination
 There are 3 main understandings which
play a major role in evolution by altering
allele frequencies:
1.Natural selection
2.Genetic drift
3.Gene flow
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1. Natural Selection
 There are a lot of things that need to be
understood about natural selection and
the role it plays in evolution.
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1. Natural Selection
 As you know, when organisms have
traits/adaptations that make them better
suited for survival, they are said to be
more fit to survive.
 Adaptations are characteristics that
make an individual suited to its
environment and way of life.
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1. Natural Selection
 This survival often changes the allele
frequency within a population.
 Natural selection can only occur if there
is variation among members of the
same species.
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1. Natural Selection
 Species tend to produce more offspring
than the environment can support.
 Those individuals that are better
adapted tend to survive and produce
more offspring while the less well
adapted tend to die and produce fewer
offspring over time.
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1. Natural Selection
 Those individuals that have
reproductive success pass the
characteristics on to their offspring.
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1. Natural Selection
 Thus natural selection will increase the
frequency of those characteristics that
make individuals better adapted and
decrease the frequency of other
characteristics leading to changes
within the species.
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2. Genetic Drift
 Genetic drift is an unexpected fluctuation in allele
frequency from one generation to the next. This is
often due to a chance event where a large proportion
of the population is wiped out.
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2. Genetic Drift
 There are two situations which increase
the likelihood of genetic drift that have a
large impact on a population:
 A. The bottle neck effect
 B. The founder effect
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A. The Bottle Neck Effect
 A sudden change in the environment which
drastically changes a population can have a
profound impact on the genetic makeup of the
population.
 It may change the population in such a way that the
survivors no longer represent the original population.
 The survivors are said to have gone through a
“bottleneck.”
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B. The Founder Effect
 When a few organisms become isolated
from a large population and establish a
new population whose gene pool is not
reflective of the new population, we say
the “founder effect” has occurred.
 These founders pass through an
isolation bottleneck and represent a
gene pool with altered allele
frequencies.
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3. Gene Flow
 Gene flow occurs when populations
gain or lose alleles as organisms come
and go within a population. Gene flow
tends to reduce differences between
populations.
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Variation
 Variations are heritable differences
within a population and comprise the
raw material for diversity and natural
selection.
 Only the genetic component of variation
can have evolutionary consequences
as a result of natural selection.
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Variation
 Variation within a population comes
from either discrete characters or
quantitative characters:
 Discrete-an either or basis determined
from a single locus.
 Quantitative-comes from 2 or more loci
that determine the phenotype.
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Variation
 Variation can arise from mutation,
meiosis, and the process of sexual
reproduction between individuals in a
species.
 This variation is what natural selection
operates on.
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Fitness
 The adaptive advantage of an organism
which allows it to make a genetic
contribution to the gene pool of the next
generation.
 In other words, the reproductive
success of an organism.
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Modes of Selection
 Natural selection alters the frequency
distribution of heritable traits in three
ways:
 1. Directional selection.
 2. Disruptive selection.
 3. Stabilizing selection.
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Directional Selection
 This is most common
when a population’s
environment changes
or when members of a
population migrate to a
new habitat with
different environmental
conditions.
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Disruptive Selection
 This occurs when
conditions favor
individuals in both
extremes over those
of normal average
phenotypes. It can be
important in the early
stages of speciation.
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Stabilizing Selection
 Acts against extreme
phenotypes, it favors
the intermediates. It
reduces variation and
maintains the status
quo of a given
phenotype.
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Selection, In General
 Regardless of the mode of selection,
selection works to favor certain
heritable traits through differential
success.
 Disruptive and stabilizing selection tend
to reduce variation, but there are
methods nature uses to preserve it.
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Methods Nature Uses:
 1. Diploidy
 2. Balancing Selection
 A. Heterozygous advantage
 B. Frequency dependent selection
 3. Neutral Variation
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Diploidy
 Many eukaryotes are diploid and this
hides a lot of variation from selection.
 Recessiveness can be transferred from
generation to generation even if they
are harmful because they only cause
harm when inherited from both parents
when the zygote is formed.
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Balancing Selection
 Occurs when natural selection
maintains stable frequencies of two or
more phenotypic forms in a population.
 Heterozygous advantage--acts in a way
that is favored by natural selection over
either homozygous form.
 Frequency dependent selection--the
fitness of any one morph declines if it
becomes too common in a population.
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Malarial Parasite, Human
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Malarial Parasite, Mosquito
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Plasmodium falciparum
http://www.nature.com/nrg/journal/v8/n7/fig_tab/nrg2126_F1.html
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Plasmodium falciparum
 Inside the red blood cells, Plasmodium
feeds on the hemoglobin, and the
amino acids that make it up to produce
more of itself.
 The Plasmodium transforms the cell
into a machine that will help it
reproduce and avoid transport to the
spleen.
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Sickle Cell Anemia
 A single base-pair substitution causes the
cellular machinery to insert the wrong amino
acid to the beta globin.
 The defective hemoglobin collapses onto
itself taking on a needle shape.
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http://t1.gstatic.com/images?q=tpn:ANd9GcRX7NQlvgnBnolAaG-2HxbUcBtuHROXt1yfk8ViEBlhslsZPKTE&t=1
Sickle Cell Anemia
 If this disease is fatal, why is the allele
for it maintained in the population?
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Sickle Cell Anemia
 Homozygotes (aa) are unlikely to
survive to reproductive age.
 Heterozygotes, however, have a good
deal of resistance to malaria.
 When the Plasmodium invades the
body and into a heterozygote’s RBC,
the cell sickles and loses its ability to
pump potassium.
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Sickle Cell Anemia
 The Plasmodium depends on the potassium
for survival and cannot survive and
reproduce without it.
 These individual’s immune systems quickly
recognize the sickled cells as foreign and
remove them along with the parasite.
 Thus the individual’s bout with malaria is
greatly reduced.
 The mechanism for protection is not
completely understood.
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Other Blood Disorders
 Other blood disorders are prevalent in
areas where malaria is found.
 Ovalocytosis.
 Thalassemia.
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Heterozygous Advantage
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Neutral Variation
 Some of the genetic variation has little or no
effect on reproductive success. Much of the
difference we see is found in untranslated
parts of the genome.
 Confers no advantage--are called
pseudogenes.
 Genetic drift can increase or decrease the
frequency of pseudogenes. Difficult to
measure--very debatable.
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Sexual Selection
 Sexual selection is natural selection for
mating success. It can result in sexual
dimorphism--differences between the
sexes in secondary characteristics.
 There are two types of sexual selection:
 Intrasexual selection.
 Intersexual selection.
 Males are usually the showier sex.
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Intrasexual Selection
 In this, we have direct competition of
one sex for mates of the opposite sex.
 A male often patrols a group of females
and prevents other males from mating
with her. He is often the psychological
winner via a ritual that discourages
competitors. This prevents harm to him
and increases his own fitness.
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Intersexual Selection
 Individuals of one sex are choosy in
selecting mates from the other sex. In
most cases, a female’s choice depends
on the showiness of a male.
 Example: Peacocks display sexual
dimorphism and both inter- and intrasexual selection.
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An Interesting Aside
 Regarding showiness, the most
intriguing thing is that it is often a
hindrance to their survival. The
benefits, however, seem to outweigh
the costs. When a female chooses a
showier male, she is often choosing the
healthiest mate with the best genes.
 This allows the male to pass his genes
on to his offspring.
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Selective Breeding and
Evolution
 Other forms of selective breeding of
domesticated animals shows that
artificial selection can cause evolution.
 Horses, cats, pigeons, dogs, etc.
 Here is an example:
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Selective Breeding and
Evolution

Here is another example involving
plants.
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Natural Selection

It doesn’t fashion perfect organisms:
1. Evolution is limited by historical
constraints.
2. Adaptations are often compromises.
3. Change and natural selection interact.
4. Selection can edit only existing variation.
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1. Evolution is Limited By
Historical Constraints.
 Each species comes from a long line of
ancestral forms.
 Ancestral anatomy isn’t scrapped by a
new form, it’s a slow change.
 This helps to explain why you don’t see
an example of every species that has
ever lived preserved in the fossil record.
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2. Adaptations are Often
Compromises
 What makes us better in some ways,
hinders us in others.
 Take the tail of the peacock as an
example. It makes the bird better in
that it allows it to get a mate, it hinders
it in its ability to evade predators.
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3. Chance and Natural
Selection Interact
 Chance events can alter a gene pool
such as when a storm blows birds or
insects over an ocean and to a new
environment. The genes which arrive
may not be the best in the former
population.
 The organisms pass through a
“bottleneck.”
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4. Selection Can Only Edit
Existing Variation
 Natural selection favors the fittest
phenotype in any population, and new
variations can’t arise on demand.
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