Download Unit 5 Evolution - History of Life on Earth

Unit 5
Mechanisms of Speciation
History of Life on Earth
Mrs. Petrov
What is a Species?
 One or more populations of individuals
that can reproduce under natural
conditions and produce fertile offspring.
Different species may arise from
_________________________________
__________________ between
populations
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Asexual Vs. Sexual
Populations
 The biological species definition does not apply
to ____________________ populations.
 In those cases, the ability to
________________________ is not a good
definition.
 Instead, we rely on _______________ &
________________________similarities.
Genetic Divergence
 Genetic divergence is the process whereby local
individuals of a population become
______________________
_________________________from other
individuals and thus experience changes in gene
frequencies between the groups
 If the environments are different between isolated
units then natural selection, mutations, and genetic
drift will work independently on each. This will
speed up genetic divergence and speciation.
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Isolating mechanisms
 Mechanisms that lead to, or cause genetic
_____________________ by preventing
___________________________between
two groups.
As long as two groups do not interbreed
their gene pools will continue to drift further
and further apart.
The longer two species are genetically
isolated from each other the more different
they become from each other.
Isolating mechanisms
 Isolating mechanisms can be divided into
two different types.
Isolating mechanisms that occur before
fertilization are called
__________________________________
Isolating mechanisms that occur after
fertilization are called
__________________________________
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Types of Prezygotic Isolating
mechanisms
 _________________________________________:
Potential mates meet but cannot figure out what to do
about it because patterns of courtship may be altered to
the extent that sexual union is not achieved
 ________________________________________:
(Time) Different groups overlap in range but may not
be reproductively mature in the same season.
 ________________________________________:
Potential mates attempt engagement but sperm cannot
be successfully transferred . This may be due to
differences in reproductive organs.
Types of Prezygotic Isolating
mechanisms cont.
 _________________________________:
Sperm is transferred but sperm and egg are
incompatible.
 __________________________________:
potential mates never meet because they live in
different habitats
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Types of Postzygotic isolating
mechanisms
 ______________________________: Egg is
fertilized but zygote does not develop properly,
dies before birth because parents are genetically
incompatible.
 ______________________________Hybrid
very weak and can’t live outside the uterus.
 _____________________________: Hybrid is
sterile.
Speciation
 Speciation occurs when a species gives
rise to one or more different species.
 There are three main speciation patterns
___________________________________
___________________________________
___________________________________
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Allopatric
 Allopatric (Allo= different, Patric=homeland)
Populations are separated due to
_____________________________________________
 Rivers, earthquakes, continental drift, glaciation,
archipelagos cause allopatric speciation.
 Examples:
 Cave fish,
 Darwin’s finches
 Antelope squirrels of the Grand Canyon
 Isthmus of Panama
Sympatric speciation
 Sympatric (Sym = same, Patric=homeland) speciation
occurs in the same geographical region _____________
____________________________________________
 A new species can arise in a single generation if a
genetic change produces a reproductive barrier between
mutants and the parent population.
 Example: Accidents during cell division that result in
extra sets of chromosomes (Polyploidy). Selffertilization can give rise to new individuals that are
unable to mate and form fertile off-spring with the
parent species.
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Parapatric Speciation
 Parapatric (Para = near, Patric=homeland) neighboring
populations become distinct species while maintaining
contact through Hybrid zones
 If hybrid zone is removed through increased natural
selection, natural disasters or some other means the
extremes of a population fail to mate.
 Examples
1
2
Toad population along the north rim of the Grand
Canyon.
___________________
Branching and unbranched
evolution
 ____________________
B
Branched evolution. Occurs as
populations split and become
reproductively isolated from each other.
A
C
__________________
A
B
C
Unbranched evolution. Occurs as
changes in allele frequency and
morphology accumulate over long
periods of time. New species do not live
within the same time period
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Transitional Forms
 Species which are
__________________
in body form between
two groups of
organisms.
 Species that are
intermediate in time in
the fossil record.
Archaeopteryx
Archaeopteryx
Intermediate between birds and
dinosaurse
 Archaeopteryx lived after the
development of dinosaurs but before the
advent of birds.
 Archaeopteryx contains features that are
both characteristic of reptilian dinosaurs
as well as modern birds. Some features
are half way inbetween.
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Feature
Birds
Archaeopteryx
Coelurosaurs
Body covering
Feathers
Scales (?)
Metatarsals
Fused
Not fused
Bones
Hollow and
pneumatic
Not hollow or
pneumatic
Clavicles
Fused
(wishbone)
Not fused
Sternum
Large, keel
Small, no keel
Abdominal
ribs
Absent
present
The tempo of speciation
 ______________________________________
New species develop slowly and gradually as
an entire species changes over time. This idea
was asserted by Darwin.
 ____________________________________
This theory states that new species arise
suddenly and rapidly as small subpopulations of
a species split from the populations of which
they were a part.
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What Leads to Speciation?
 _________________________________ is
the major mechanism of evolution.
 Other mechanisms? Of Course!
__________________________
Random chance that some more successful
Non-Random Mating: __________ preferences
Migrations: movement in and out of population
Mutations: change genetics (_______) randomly
Modeling Evolution
 The Hardy-Weinberg Model
This is a ___________ model: if
assumptions are met, it models that NO
evolution is occurring.
If a change from 1 generation to the next IS
observed, this demonstrates that evolution IS
occurring.
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H-W Assumptions
 _______________________________________
 _______________________________________
 _______________________________________
 _______________________________________
 _______________________________________
All conditions seldom met!
H-W Model Equations
p2 + 2pq + q2 = 1
p+q=1
p2= decimal of homozygous dominant individuals
2pq = decimal of heterozygous individuals
q2 = decimal of homozygous recessive individuals
p = decimal of dominant alleles
q = decimal of recessive alleles
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Practice Problem
 In a population of 100 trees, 30
individuals are homozygous recessive for
small leaves.
 If the population is in HW equilibrium
(no evolution occurring), how many
dominant alleles should be in the
population next generation?
 How many heterozygous individuals
should there be?
Natural Selection Vs. Genetic Drift
 Differential success in
____________________________ results in
certain alleles being passed to the next generation
in greater proportions
 For example, an allele that confers resistance to
DDT increased in frequency after DDT was used
widely in agriculture
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Genetic Drift
 The smaller a sample, the greater the chance of
deviation from a predicted result
 Genetic drift describes how allele frequencies fluctuate
_________________________________ from one
generation to the next
 Genetic drift tends to _______________genetic
variation through ____________ of alleles
Animation: Causes of Evolutionary Change
Right-click slide / select “Play”
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Case Study: Impact of Genetic Drift
on the Greater Prairie Chicken
 Loss of prairie habitat caused a severe reduction in
the population of greater prairie chickens in Illinois
 The surviving birds had low levels of genetic
variation, and only 50% of their eggs hatched
Figure 23.11a
Pre-bottleneck
(Illinois, 1820)
Post-bottleneck
(Illinois, 1993)
Greater prairie chicken
(a)
Range
of greater
prairie
chicken
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Figure 23.11b
Location
Illinois
1930–1960s
1993
Population
size
Number Percentage
of alleles of eggs
per locus hatched
1,000–25,000
<50
5.2
3.7
93
<50
Kansas, 1998
(no bottleneck)
750,000
5.8
99
Nebraska, 1998
(no bottleneck)
75,000–
200,000
5.8
96
(b)
Effects of Genetic Drift: A Summary
1. Genetic drift is significant in _____________
populations
2. Genetic drift causes allele frequencies to change at
__________________
3. Genetic drift can lead to a loss of genetic variation
within populations
4. Genetic drift can cause __________________
alleles to become _______________
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A Closer Look at Natural Selection
 Natural selection brings about
___________________ evolution by acting on an
organism’s ______________________
Relative Fitness
 The phrases “struggle for existence” and “survival
of the fittest” are misleading as they imply direct
competition among individuals
 Reproductive success is generally more subtle and
depends on many factors
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 ________________________fitness is the
contribution an individual makes to the gene pool of
the next generation, relative to the contributions of
other individuals
 Selection favors certain genotypes by acting on the
phenotypes of certain organisms
Directional, Disruptive, and
Stabilizing Selection
 Three modes of selection:
 Directional selection favors individuals at one end
of the phenotypic range
 Disruptive selection favors individuals at both
extremes of the phenotypic range
 Stabilizing selection favors intermediate variants
and acts against extreme phenotypes
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Frequency of
individuals
Figure 23.13
Original
population
Evolved
population
(a) Directional selection
Original population
Phenotypes (fur color)
(b) Disruptive selection
(c) Stabilizing selection
The Key Role of Natural Selection in
Adaptive Evolution
 Striking adaptations have arisen by natural
selection
 For example, cuttlefish can change color rapidly
for camouflage
 For example, the jaws of snakes allow them to
swallow prey larger than their heads
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Figure 23.14
Bones shown in
green are movable.
Ligament
 Natural selection increases the frequencies of alleles
that enhance survival and reproduction
 Adaptive evolution occurs as the ____________
between an organism and its environment increases
 Because the environment can change, adaptive
evolution is a ______________________________
process
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 Genetic drift does not consistently lead to adaptive
evolution as it can increase OR decrease the match
between an organism and its environment
Sexual Selection
 Sexual selection is natural selection for
_______________ success
 It can result in sexual
_________________________ : Distinct differences
between the sexes.
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 Intrasexual selection is competition among
individuals of one sex (often males) for mates of the
opposite sex
 Intersexual selection, often called mate choice,
occurs when individuals of one sex (usually
females) are choosy in selecting their mates
 Male showiness due to mate choice can increase a
male’s chances of attracting a female, while
decreasing his
_________________________________________
Figure 23.15
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 How do female preferences evolve?
 The “_________ ________________” hypothesis
suggests that if a trait is related to male health, both
the male trait and female preference for that trait
should increase in frequency
Balancing Selection
 Balancing selection occurs when natural selection
maintains stable frequencies of ___________________
____________________________________________
 Balancing selection includes
 Heterozygote advantage
 Frequency-dependent selection
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Heterozygote Advantage
 Heterozygote advantage occurs when heterozygotes
have a higher __________________than do both
homozygotes
 Natural selection will tend to maintain two or more
alleles at that locus
 The sickle-cell allele causes mutations in hemoglobin
but also confers _____________________________
__________________________________________
Figure 23.17
Key
Frequencies of the
sickle-cell allele
0–2.5%
2.5–5.0%
Distribution of
malaria caused by
Plasmodium falciparum
(a parasitic unicellular eukaryote)
5.0–7.5%
7.5–10.0%
10.0–12.5%
>12.5%
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Frequency-Dependent Selection
 In frequency-dependent selection, the fitness of a
phenotype _________________________ if it becomes
too _______________________in the population
 Selection can favor whichever phenotype is __________
common in a population
 For example, frequency-dependent selection selects for
approximately equal numbers of “right-mouthed” and
“left-mouthed” scale-eating fish
Figure 23.18
“Left-mouthed”
P. microlepis
Frequency of
“left-mouthed” individuals
1.0
“Right-mouthed”
P. microlepis
0.5
0
1981 ’82 ’83 ’84 ’85 ’86 ’87 ’88 ’89 ’90
Sample year
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