chapter 16 evolution of populations section 1 genes and variation
TRANSCRIPT
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CHAPTER 16 EVOLUTION OF POPULATIONS
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SECTION 1 GENES AND VARIATION
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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?
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1859 Darwin published his theory of 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
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What is a species? Biological species concept
defined by Ernst Mayr population whose members can interbreed &
produce viable, fertile offspring reproductively compatible
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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 genescontains two or more alleles—or forms
of a certain gene—for each inheritable trait
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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
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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
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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
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Sample Population
48% heterozygous
black
36% homozygous
brown
16% homozygous
black
Frequency of Alleles
allele for brown fur
allele for black fur
Relative Frequencies of Alleles
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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.
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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
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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
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Gene Shuffling independent assortment during
meiosiscrossing-over during meiosisWhen alleles are recombined during
sexual reproduction, they can produce dramatically different phenotypes. Thus, sexual reproduction is a major source of variation within many populations.
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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
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SINGLE-GENE TRAIT trait controlled by a single gene that
has two allelescan have two phenotypes only
widow’s peak hairline
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Fre
qu
ency
of
Ph
eno
typ
e(%
)100
80
60
40
20
0 Widow’s peak No widow’s peak
Phenotype
Distribution of Phenotypes for Single-Gene Trait in a Population
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POLYGENIC TRAITSTraits controlled by two or more
geneseach gene has two or more allelesone polygenic trait can have many
possible genotypes and even more possible phenotypesheight in humans
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Fre
qu
enc
y o
f P
hen
oty
pe
Phenotype (height)
Generic Bell Curve for Polygenic Trait in a population
Most people fall in the middle of the bell curve
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KEY CONCEPT QUESTIONS What are the main sources of inheritable
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
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SECTION 2 EVOLUTION AS GENETIC CHANGE
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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?
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Natural selection affects which 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.
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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.
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Take a look:
Brown is the normal color
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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
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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.
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As you learned earlier: the action of multiple alleles on traits such
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.
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Witness to Evolution Peppered Moth
dark vs. light variants
Peppered moth
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Peppered moth
Year % dark % light1848 5 951895 98 21995 19 81
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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
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Natural selection can affect the distributions of phenotypes in any of three ways: directional selection, stabilizing selection, or disruptive selection.
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DIRECTIONAL SELECTION When individuals at one end of the
curve have higher fitness than individuals in the middle or at the other end
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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.
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STABILIZING SELECTIONWhen 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
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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.
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Effects of Selection Driving changes in a population
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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
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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
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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
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Cheetahs All cheetahs share a small
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
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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
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Sample of Original Population
Founding Population A
Founding Population B
Descendants
Genetic Drift
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Sample of Original Population
Founding Population A
Founding Population B
Descendants
Genetic Drift
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Sample of Original Population
Founding Population A
Founding Population B
Descendants
Genetic Drift
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FOUNDER EFFECT situation in which allele
frequencies change as a result of the migration of a small subgroup of a populationDarwin’s FinchesFruit flies on Hawaii
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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
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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
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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.
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How do allele frequencies change?
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Human evolution today Gene flow in human
populations is increasing today transferring alleles
between populations
Are we moving towards a blended world?Are we moving towards a blended world?
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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
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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
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SECTION 3 THE PROCESS OF SPECIATION
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KEY CONCEPT QUESTIONS What factors are involved in the
formation of a new species?
Describe the process if speciation in the Galapagos Finches.
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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
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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
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
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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
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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
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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 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, 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.
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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!
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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
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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
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Two scientists, Peter and Rosemary Grant, tested Darwin’s hypothesisconcluded there is great variation of
inheritable traits among Galapagos Finches
individual birds with different sized beaks had different chances of survival
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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
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Speciation of Darwin’s Finches
1. Founders arrive
2. Separation of populations
3. Changes in the gene pool
4. Reproductive isolation
5. Ecological competition
6. Continued evolution
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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
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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