Transcript CHAPTER 16 EVOLUTION OF POPULATIONS

CHAPTER 16
EVOLUTION
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to add textOF POPULATIONS
SECTION 1
GENES AND VARIATION
KEY CONCEPT QUESTIONS
 What are the main sources of
inheritable variation in a population?
 How is evolution defined in genetic
terms?
 What determines the number of
phenotypes for a given trait?
 1859 Darwin published his theory of

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
evolution
1866 Mendel worked with peas to explain
inheritance
These two ideas did not come together
until the 1930’s
Today, genetics, molecular biology, and
evolutionary theory work together to
explain how inheritable variation appears
and how natural selection operates on
that variation
What is a species?
 Biological species concept



defined by Ernst Mayr
population whose members can interbreed &
produce viable, fertile offspring
reproductively compatible
 POPULATION

a collection of individuals of the
same species in a defined area
 GENE POOL
the combined genetic information of
all the members of a particular
population
 common group of genes
 contains two or more alleles—or
forms of a certain gene—for each
inheritable trait

Changes in populations
 Evolution of populations is really
measuring changes in allele frequency

all the genes & alleles in a population =
gene pool
 Factors that alter allele frequencies
in a population
natural selection
 genetic drift

 founder effect
 bottleneck effect

gene flow
Populations evolve
 Natural selection acts on individuals

differential survival
 “survival of the fittest”

differential reproductive success
 who bears more offspring
 Populations evolve
genetic makeup of
population changes
over time
 favorable traits
(greater fitness)
become more common

Bent Grass on
toxic mine site
Individuals DON’T evolve!!!
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 RELATIVE FREQUENCY
the number of times that allele
occurs in a gene pool compared
with the number of times other
alleles occur
 expressed in percents

Relative Frequencies of Alleles
Sample Population
48%
heterozygous
black
16%
homozygous
black
36%
homozygous
brown
Frequency of Alleles
allele for
brown fur
allele for
black fur
 What are the main sources of
genetic variation in a population?

The two main sources of
genetic variation are mutations
and the genetic shuffling that
results from sexual
reproduction.
Mutation & Variation
 Mutation creates variation

new mutations are constantly appearing
 Mutation changes DNA sequence
changes amino acid sequence?
 changes protein?

 change structure?
 change function?

changes in protein may
change phenotype &
therefore change fitness
Sex & Variation
 Sex spreads variation
one ancestor can have many
descendants
 sex causes recombination
 offspring have new combinations
of traits = new phenotypes

 Sexual reproduction recombines alleles
into new arrangements in every
offspring
 Gene Shuffling
independent assortment during
meiosis
 crossing-over during meiosis
 When alleles are recombined during
sexual reproduction, they can
produce dramatically different
phenotypes. Thus, sexual
reproduction is a major source of
variation within many populations.

Variation impacts natural selection
 Natural selection requires a source of
variation within the population
there have to be differences
 some individuals must be more fit than
others

 SINGLE-GENE TRAIT
trait controlled by a single gene that
has two alleles
 can have two phenotypes only
 widow’s peak hairline

Distribution of Phenotypes for
Single-Gene Trait in a Population
Frequency of Phenotype
(%)
100
80
60
40
20
0
Widow’s peak
Phenotype
No widow’s peak
 POLYGENIC TRAITS
Traits controlled by two or more
genes
 each gene has two or more alleles
 one polygenic trait can have many
possible genotypes and even more
possible phenotypes
 height in humans

Frequency of Phenotype
Generic Bell Curve for Polygenic
Trait in a population
Phenotype (height)
Most people fall in the middle of the
bell curve
KEY CONCEPT QUESTIONS
 What are the main sources of inheritable
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
variation in a population?
 Mutations and sexual reproduction
How is evolution defined in genetic terms?
 genetic makeup of population changes
over time
 favorable traits (greater fitness) become
more common
What determines the number of phenotypes
for a given trait?
 The number of genes that control the trait
SECTION 2
EVOLUTION AS GENETIC CHANGE
KEY CONCEPT QUESTIONS
 How does natural selection affect
single-gene and polygenic traits?
 What is genetic drift?
 What 5 conditions are needed to
maintain genetic equilibrium?
 Natural selection affects which
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individuals having different phenotypes
survive and reproduce and which do
not
In this way, natural selection
determines which alleles are passed
from one generation to the next.
Any factor that causes alleles to be
added to or removed from a population
will change the relative frequencies of
alleles.
 Whenever an individual dies without



reproducing, its genes are removed from the
population.
But if an individual produces many offspring,
the proportion of that individual’s genes in
the gene pool will increase.
In genetic terms, evolution is any change in
the relative frequencies of alleles in a
population’s gene pool.
Thus, evolution acts on populations, not on
individuals.
Take a look:
 Brown is the normal color
 Besides a mutation for red color, what other
mutation occurred in the lizard population?
A mutation for black color
 How does color affect the fitness of the
lizards?
Both red and brown lizards are less fit than
black lizards
 What do you predict the lizard
population will look like by generation
50? Explain.

The lizard population will have
more black lizards, fewer brown
lizards, and no red lizards by
generation 50. The environment
determines the favorable color.
 As you learned earlier:
 the action of multiple alleles on traits such
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as height produces a range of phenotypes
that often fit a bell curve
The fitness of individuals close to one
another on the curve will not be very
different.
But fitness can vary a great deal from one
end of such a curve to the other.
And where fitness varies, natural selection
can act.
Witness to Evolution
 Peppered Moth

dark vs. light variants
Peppered moth
Peppered moth
Year
1848
1895
1995
% dark
5
98
19
% light
95
2
81
Peppered moth
 Why did the population change?
early 1800s = pre-industrial England
 low pollution
 lichen growing on trees = light colored bark
 late 1800s = industrial England
 factories = soot coated trees
 killed lichen = dark colored bark
 mid 1900s = pollution controls
 clean air laws
 return of lichen = light colored bark
 industrial melanism

 Natural selection can affect the
distributions of phenotypes in any
of three ways: directional
selection, stabilizing selection, or
disruptive selection.
 DIRECTIONAL SELECTION

When individuals at one end of the
curve have higher fitness than
individuals in the middle or at the
other end
 Ex) The supply of small seeds runs low in

a particular environment. Take a look at
the graph below and explain what is
happening.
- the birds with larger beaks are more
likely to survive and reproduce because
their beaks are adapted to the available
food. There is a shift in the beak size of a
population.
 STABILIZING SELECTION

When individuals near the center
of the curve have higher fitness
than individuals at either end of the
curve
 EX) Figure shows that human babies
born at an average mass are more likely
to survive than babies born either much
smaller or much larger than average
 DISRUPTIVE SELECTION

when individuals at the upper and
lower ends of the curve have higher
fitness than individuals near the
middle
 EX) average-sized seeds become less
common, and larger and smaller seeds
become more common. As a result, the
bird population splits into two subgroups
specializing in eating different-sized
seeds.
Effects of Selection
 Driving changes in a population
 GENETIC DRIFT
random change in allele frequencies
that occurs in small populations
 In small populations, individuals that
carry a particular allele may leave
more descendants than other
individuals do, just by chance. Over
time, a series of chance occurrences
of this type can cause an allele to
become common in a populationa

Genetic drift
 Effect of chance events
founder effect
 small group splinters
off & starts a new
colony
 bottleneck
 some factor (disaster)
reduces population to
small number & then
population recovers
& expands again

Bottleneck effect
 When large population is drastically reduced
by a disaster
 famine, natural disaster, loss of habitat…
 loss of variation by chance
 alleles lost from gene pool
 narrows the gene pool
Cheetahs
 All cheetahs share a small
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number of alleles
 less than 1% diversity
 as if all cheetahs are
identical twins
2 bottlenecks
 10,000 years ago
 Ice Age
 last 100 years
 poaching & loss of
habitat
Conservation issues
 Bottlenecking is an
important concept in
conservation biology of
endangered species
loss of alleles from gene
pool
 reduces variation
 reduces ability to
adapt
 at risk populations

Genetic Drift
Sample of
Original Population
Descendants
Founding Population A
Founding Population B
Genetic Drift
Sample of
Original Population
Descendants
Founding Population A
Founding Population B
Genetic Drift
Sample of
Original Population
Descendants
Founding Population A
Founding Population B
 FOUNDER EFFECT

situation in which allele
frequencies change as a result of
the migration of a small subgroup
of a population
 Darwin’s Finches
 Fruit flies on Hawaii
Founder effect
 When a new population is started by only a
few individuals
 some rare alleles may be at high
frequency; others may be missing
 skew the gene pool of
new population
 human populations that
started from small group
of colonists
 example: white people
colonizing New World
 HARDY-WEINBERG PRINCIPLE

allele frequencies in a population will
remain constant unless one or more
factors cause those frequencies to
change
 GENETIC EQUILLIBRIUM
in which allele frequencies remain
constant
 If the allele frequencies do not change,
the population will not evolve

Five conditions are required to maintain
genetic equilibrium (alleles don’t
change) from generation to generation:
1. There must be random mating
2. The population must be very large
3. There can be no movement into or out
of the population
4. No mutations
5. No natural selection.
How do allele frequencies change?
Human evolution today
 Gene flow in human
populations is
increasing today

transferring alleles
between populations
Are we moving towards a blended world?
KEY CONCEPT QUESTIONS
 How does natural selection affect
single-gene and polygenic traits?
 affect the distributions of phenotypes
in any of three ways: directional
selection, stabilizing selection, or
disruptive selection.
 What is genetic drift?
 random change in allele frequencies
that occurs in small populations
Focus Questions
 What 5 conditions are needed to
maintain genetic equilibrium?
 Random mating
 Large population
 No movement into or out
 No mutations
 No natural selection
SECTION 3
THE PROCESS OF SPECIATION
KEY CONCEPT QUESTIONS
 What factors are involved in the
formation of a new species?
 Describe the process if speciation in
the Galapagos Finches.
Speciation
 New species are created by a series of
evolutionary processes
 populations become isolated
 reproductively isolated
 geographically isolated
 isolated populations
evolve independently
 Isolation
 allopatric
 physical separation
 sympatric
 still live in same area
Allopatric speciation
 Allopatric = “other country”

geographic separation
 migration
 physical barrier
Harris’s antelope
squirrel inhabits
the canyon’s
south rim (L). Just
a few miles away
on the north rim
(R) lives the
closely related
white–tailed
antelope squirrel
 GEOGRAPHIC ISOLATION

two populations are separated by
geographic barriers such as rivers,
mountains, or bodies of water
 Squirrels at the Grand Canyon
 Darwin’s Finches
Sympatric speciation
 Sympatric = “same country”
some type of isolation even
though populations live in
same area
 what causes this isolation?
 behavioral differences
 non-random mating
 physiological differences
 chromosomal changes
 polyploidy
 mostly in plants: oats,
cotton, potatoes,
tobacco, wheat

 TEMPORAL ISOLATION
two or more species reproduce at
different times
 Orchids and pollination
REPRODUCTIVE ISOLATION

when members of two populations
cannot interbreed and produce fertile
offspring
BEHAVIORAL ISOLATION
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occurs when two populations are
capable of interbreeding but have
differences in courtship rituals or other
types of behavior
 Mating songs of meadowlarks
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Darwin studied birds on the Galapagos
Islands.
 He thought they were blackbirds,
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warblers, and other kinds of birds!
The species he examined differed greatly
in the sizes and shapes of their beaks and
in their feeding habits, as shown on p.
406.
 Some species fed on small seeds, while
others ate large seeds with thick shells.
One species used cactus spines to pry
insects from dead wood. One species,
not shown here, even pecked at the
tails of large sea birds and drank their
blood!
 Once Darwin discovered that these
birds were all finches, he hypothesized
that they had descended from a
common ancestor. Over time, he
proposed, natural selection shaped the
beaks of different bird populations as
they adapted to eat different foods
Darwin’s hypothesis relied on two
testable assumptions.
 First, in order for beak size and shape to

evolve, there must be enough inheritable
variation in those traits to provide raw
material for natural selection.
Second, differences in beak size and
shape must produce differences in fitness
that cause natural selection to occur
 Two scientists, Peter and Rosemary
Grant, tested Darwin’s hypothesis
concluded there is great variation of
inheritable traits among Galapagos
Finches
 individual birds with different sized
beaks had different chances of
survival

When food for the finches was
scarce, individuals with the largest
beaks were more likely to survive
 Beak size also plays a role in mating
behavior, because big-beaked birds
tend to mate with other big-beaked
birds
 they found natural selection takes
place frequently and sometimes very
rapidly

Speciation of Darwin’s Finches
1.
2.
3.
4.
5.
6.
Founders arrive
Separation of populations
Changes in the gene pool
Reproductive isolation
Ecological competition
Continued evolution
KEY CONCEPT QUESTIONS
 What factors are involved in the
formation of a new species?
 Allopatric isolation
 physical separation
 Sympatric isolation
 still live in same area
KEY CONCEPT QUESTIONS
 Describe the process of speciation
in the Galapagos Finches.
• Founders arrive
• Separation of populations
• Changes in the gene pool
• Reproductive isolation
• Ecological competition
• Continued evolution