Download Evolution of Populations

The Evolution of Populations
Chapter 23
“Nothing in Biology makes sense except in the light of
evolution”
T. Dobzhansky
Population Genetics

What was missing from Darwin’s explanation of natural
selection?
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Population Genetics:
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A way to explain how chance variations can show up in a
population, while also accounting for precise transmission
from parent to offspring…
Emphasizes the variation within populations and recognizes
the importance of quantitative characters - those
characteristics that vary along a continuum
Modern synthesis:


ties in ideas from paleontology, taxonomy, biogeography, and
population genetics
Scientists contributing: Dobzhansky, Wright, Mayr, Simpson,
Stebbins (page 446)
Quantitative Characters and Discrete
Characters

Quantitative Characters:
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traits that vary along a continuum in a population
(like plant height)
Discrete Characters:

traits that can be classified on an either-or basis (like
flower color)
Genes & Variation
While developing his theory of evolution, Darwin
worked under a serious disadvantage – he did not
know how heredity worked

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Without understanding heredity, Darwin was unable to
explain 2 important factors:
1.
The source of variation central to his theory
2.
How hereditable traits were passed from one generation to
the next
Today, genetics, molecular biology, and evolutionary
theory work together to explain how inheritable
variation appears and how natural selection operates
on that variation (i.e. how evolution takes place)
Gene Pools

A gene pool is the combined genetic information of
all the members of a particular population

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A gene pool typically contains 2 or more alleles (or
forms of certain genes)
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Recall that a population is a collection of individuals of the
same species in a given area which share a common
group of genes
Example: mouse populations may have 2 or more alleles
for fur color – the gene pool for the trait fur color is the
combination of all the alleles in the population
The relative frequency of an allele is the number of
times that allele occurs in a gene pool compared to
the number of times other alleles occur
Relative Frequency of Alleles
Sources of Genetic Variation

The two main sources of genetic variation are
mutations and the genetic shuffling that
results from sexual reproduction

A mutation is any change in a sequence of DNA


Mutations that affect an organisms phenotype can
lead to an increase in fitness for that organism
Most inheritable differences are the result of gene
shuffling that occurs during sexual reproduction

Example: the 23 pairs of chromosomes found in
humans can produce 8.4 million different combinations
of genes
Mutations


Mutation is the ultimate source of new
alleles within a population
Mutation is also a source of evolution
A A A
A A
A A A
A A A
a a
A a A
A
T=0
a
T=1
Sexual Recombination

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Most of the genetic variation
in a population results from
the unique combination of
alleles that each individual
receives.
Three mechanisms
contribute to the shuffling of
alleles during sexual
reproduction:



Crossing over
Independent assortment of
alleles
Fertilization
Population Genetics & Hardy Weinberg

Before we consider the mechanisms that cause a
population to evolve, it will be helpful to examine, for
comparison, the gene pool of a NONEVOLVING
population.

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Such a gene pool is described by the Hardy-Weinberg
theorum.
The theorum states that the frequencies of alleles and
genotypes in a population’s gene pool REMAIN
CONSTANT over generations unless acted upon by
mechanisms other than Mendelian segregation and
recombination of alleles.
 Put another way – the shuffling of alleles due to meiosis
and random fertilization has no effect on the overall
gene pool of a population.
Population Genetics

Hardy-Weinberg Principle – states that allele
frequencies tend to remain constant in populations
unless something happens OTHER THAN Mendelian
segregation and sexual recombination.

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This situation in which allele frequencies remain constant is
called genetic equilibrium
 If allele frequencies do not change, the population will not
evolve
Hardy-Weinberg is a mathematical model that
describes the changes in allele frequencies in a
population

Allows us to predict allele and genotype frequencies in
subsequent generations (testable)
Hardy-Weinberg Principle

Model assumptions (conditions required to maintain
genetic equilibrium from generation to generation):
1.
2.
3.
4.
5.
random mating population
Large population size – n > 100
No emigration or immigration (no movement into or out of
the population)
No mutations
No natural selection (all genotypes have an equal chance
of survival and reproduction)

If all 5 conditions are met, there should be NO
EVOLUTION – no selection, no gene flow, no
genetic drift, no mutation

Describes a NON-EVOLVING POPULATION
Hardy-Weinburg Principle

The Hardy-Weinburg Principle is neat
because it can serve as a null hypothesis
for evolution

It can show that evolution IS OCCURING
within a population
Hardy-Weinburg Principle
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Let p= frequency of allele A
Let q= frequency of allele a
Let p2= frequency of genotype AA
Let 2pq= frequency of genotype Aa
Let q2= frequency of genotype aa
Law says, given assumptions, that within 1 generation
of random mating, the genotype frequencies are found
to be in the binomial distribution p2+2pq+q2=1
(genotype frequencies) and p+q=1 (allele frequencies)
Hardy-Weinberg Example

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The allele for the ability to roll one’s tongue is
dominant (R) over the allele for the lack of
this ability (r).
In a population of 500 individuals, 25% show
the recessive phenotype. How many
individuals would you expect to be
homozygous dominant and heterozygous?
How Hardy-Weinberg Works
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The equation: p2 + 2pq + q2 = 1
Therefore, p + q = 1
500 organisms, 25% are rr; thus q2 = .25
so 125 organisms are rr
If q2 = .25, then q=.5
Thus, p + .5 = 1, leaves p = .5
So, p2 = .25, so 125 organisms are RR
2pq leaves the heterozygotes, so
2(.5)(.5) = .5 or 50%, so 250 organisms are Rr
Causes of Microevolution

Natural selection, genetic drift, and gene flow can
alter allele frequencies in a population and cause
MOST evolutionary changes.

Microevolution:


generation to generation change in a population’s
allele frequencies
Three main causes:
1.
2.
3.
natural selection
genetic drift
gene flow
Natural Selection on Polygenic Traits

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Natural selection: differential survival and
reproduction among members of a population
Natural selection is NOT random – it leads to adaptive
evolution – evolution that results in a better match
between organisms and their environment.
Can affect the distribution of genotypes in any of three
ways:
1.
2.
3.
Stabilizing selection
Directional selection
Disruptive selection
Genetic Drift

Natural selection is not the only force that can
lead to evolution:
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In addition to natural selection, genetic drift is a
way by which allele frequencies can change
In the real world, population sizes fluctuate

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Because populations fluctuate in size, sometimes
there can be changes in allele frequencies due to
random chance
These changes are called random genetic drift
Genetic Drift

In small populations, individuals that carry a
particular allele may leave more descendants
than other individuals, just by change

Over time, a series of chance occurrences of this
type can cause an allele to become common in a
population
The Power of Genetic Drift

Genetic drift is a powerful force when a
population size is very small

Can and does lead to allele fixation
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Allele fixation means that a population changes
(evolves) from many alleles represented to only 1
allele represented
Depends on starting frequency (which allele
becomes fixed)
Consequences of Genetic Drift

Consequences of genetic drift:
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Can and does lead to fixation of alleles
Effect of chance is different from population to
population
Small populations are effected by genetic drift more
often than larger ones
Given enough time, even in large populations genetic
drift can have an effect
Genetic drift reduces variability in populations by
reducing heterozygosity
REAL WORLD EXAMPLES OF GENETIC DRIFT:
1.
2.
The Bottleneck Effect
The Founder Effect
Real World Examples of Genetic Drift

The Bottleneck Effect

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Occurs when only a few
individuals survive a random
event, resulting in a shift in
allele frequencies within the
population
Small population sizes
facilitate inbreeding and
genetic drift, both of which
decrease genetic variation
Reduces genetically
variability because at least
some alleles are likely to be
lost from gene pool
Figure 23.5 The bottleneck effect: an analogy
Real World Examples of Genetic Drift
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The Founder Effect
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Occurs when individuals
from a source population
move to a new area and
start a new population
This new population is
often started by relatively
few individuals that do
not represent the
population well in terms
of all alleles being
represented
The Founder Effect
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
What determines which variants survive
the event or get to the new location?

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Random chance
Genetic drift has the larges effect on small
populations (10-100 individuals)
The Founder Effect
Sample of
Original Population
Descendants
Founding Population A
Founding Population B
The Founder Effect
Sample of
Original Population
Descendants
Founding Population A
Founding Population B
The Founder Effect
Sample of
Original Population
Descendants
Founding Population A
Founding Population B
Gene Flow

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Gene flow can also change allele frequencies
Gene flow is the physical flow of alleles into or
out of a population.
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Immigration – alleles coming in (added)
Emigration – alleles moving out (lost)
Gene flow counteracts differences that arise
through mutation, natural selection, and genetic
drift.
Gene flow helps keep separated populations
genetically similar – reduces differences between
populations
The Power of Natural Selection

Natural selection is the ONLY mechanism that
consistently causes adaptive evolution.
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Evolution by natural selection is a blend of chance and
“sorting” – chance is the creation of new genetic variations
and sorting as natural selection favors some alleles over
others.
 Because of this sorting effect – ONLY natural
selection consistently increases the frequencies of
alleles that provide reproductive advantage and thus
leads to adaptive evolution.
Relative fitness is the contribution an individual makes to
the gene pool of the next generation, relative to the
contributions of other individuals.
 Relative fitness conferred by a particular allele
depends on the entire genetic and environmental
context in which it is expressed.
Three Modes of Selection

The effect of selection on a varying
characteristic can be:

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Stabilizing
Directional
Disruptive (Diversifying)
This effect on allele frequency depends on
which phenotypes in a varying population are
being favored.
Stabilizing Selection
 selection is against phenotype with
arrows
 selection is against both extreme
phenotypes
 intermediate survives and reproduces
at a higher rate than others
 phenotypic extremes are eliminated,
variance has decreased
 population has stabilized around mean
 But…remember that mutation and
gene flow can increase variance by
counteracting selection
Stabilizing Selection

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When individuals near the center of the curve
have higher fitness than individuals at either end
of the curve
Intermediate forms of a trait are favored and
alleles that specify extreme forms are eliminated
from a population
Counteracts the effects of mutation, gene
flow, and genetic drift – preserves the most
common phenotypes.
Section 16-2
Stabilizing Selection
Directional Selection
 selection against 1
extreme in favor of the
other extreme
 after time we see a shift
in the direction of the
population toward 1 of the
2 homozygous extremes
Variation is reduced here
and alleles can be lost
from the population
Directional
Selection
Directional Selection
Disruptive/Diversifying Selection
 selection is against phenotype
with arrows
 selection is against intermediate
phenotype in favor of BOTH
extremes
 number of intermediates after a
few generations is low, but
variation is maintained here
 in the real world, this can lead to
speciation
 if this occurs long enough and
there is barrier to gene flow,
speciation can occur
Disruptive or Diversifying Selection

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When individuals at the upper and lower ends of
the curve have higher fitness than individuals
near the middle.
Forms at both ends of the range of variation are
favored and intermediate forms are selected against
– selection creates two, distinct phenotypes

Ex. Bird beak size – no middle sized seeds, only large seeds
and small seeds; thus, small and large beaks are favored
Disruptive Selection
Selection Graphs
Figure 23.12 Modes of selection
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Key Role of Natural Selection
in Adaptive Evolution

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Natural selection increases the frequencies of
alleles that enhance survival and
reproduction, thus improving the match
between organisms and their environment.
The physical and biological components of an
organism’s environment may change over
time.

As a result, what constitutes a “good match”
between an organism and its environment can be
a moving target – making adaptive evolution a
continuous, dynamic process!
Sexual Selection


Sexual selection may lead to pronounced secondary
differences between the sexes:
Sexual selection is a form of natural selection in
which individuals with certain inherited
characteristics are more likely than other individuals
to obtain mates

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Maintained by natural selection
May lead to pronounced differences between sexes
– sexual dimorphism

a marked difference between the two sexes in
secondary sexual characteristics
Figure 23.16x1 Sexual selection and the evolution of male
appearance
Types of Sexual Selection
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Intrasexual Selection means selction within the
same sex – typically males
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Individuals of one sex compete directly for mates of the
opposite sex
Often it is based on rituals and displays that don’t risk injury
Intersexual Selection is also called “mate choice” –
typically females
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Female choice is typically based on showiness of the
male’s appearance and/or behavior
Males will often weight the attraction of predators versus
the attraction of mates
The Preservation of Genetic Variation

Tendency for directional and stabilizing
selection to reduce variation is countered by
mechanisms that preserve or restore it:
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Diploidy
Balanced Polymorphism
Neutral Variations
Diploidy

Diploidy refers to organisms carrying genes
in pairs:

Recessive traits can be preserved in
heterozygotes – this maintains a large pool of
genes that may not be useful today, but could be
in the future.
Balanced Polymorphism

Balancing selection maintains two or more
forms in a population:
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Heterozygous Advantage: sometimes a
heterozygote has an advantage to homozygotes
and survives
Frequency Dependent Selection: the fitness of a
phenotype declines if it becomes too common in a
population
Figure 23.0 Shells
Figure 23x2 Polymorphism
Neutral Variation

Changes in the DNA (typically non-coding)
that provide no selective advantage or
disadvantage.
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However, these variations MAY influence survival
and reproduction in ways that are difficult to
measure.
The variation may also be neutral in ONE
environment but beneficial in ANOTHER
environment.
The point is that this variation is an enormous
reservoir of raw material for natural selection!
Figure 23.7 A nonheritable difference within a population
Genetic Variation is the raw
material for natural selection
Genetic variation occurs within
and between populations
many are at molecular level and
cannot be seen…not all are heritable –
some are environmentally induced
(Ex. Map butterflies – figure 23.7)
Measuring Genetic Variation

Population Geneticists use whole gene
measurements and molecular measurements –
gene diversity and nucleotide diversity
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
Ex. Fruit flies – gene diversity using loci, nucleotide
diversity using DNA fingerprinting
Note: humans have little genetic variation
compared to other species – same nucleotide
sequence at 999 out of every 1000 nucleotide
sites in your DNA
Geographic Variation


Differences in gene pools between populations or
subgroups of populations
Due to fact that at least SOME environmental
factors are likely to differ from one place to
another; thus, natural selection can contribute to
this….

Ex. In population, one type of geographic variation is a
Cline - graded change in trait along a geographic axis
Figure 23.8 Clinal variation in a plant
What Can’t Natural Selection Do?

NATURAL SELECTION CANNOT FASHION PERFECT
ORGANISMS!!!
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Limited by historical constraints
Ex. Body structure for erect posture
Adaptations are often compromises
Ex. Seal on land vs. water
Not all evolution is adaptive
Ex. Storm blows ALL orgs to new place, not just best suited
Selection can only edit existing variations
See page 461