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Lecture 12: Evolution
Biological Change Over
Time
Key terms:
Microevolution
Macroevolution
„Changes
„Change
„
„
Drive a population away from
genetic equilibrium
Small-scale changes in allele
frequencies brought about by:
– Natural selection
– Gene flow
from one
species to another
„Undefined
mechanism
„Interpretation of:
species
„Well defined
mechanism
„Easily observed
„Based on selection
Reading:
Ch16: Microevolution
Ch17:Speciation
Ch18:Macroevolution
Microevolutionary
Processes
with in
– Cladistics
– Fossil record
– Geological data
Microevolution
Genetics
„
„
„
Microevolution changes
a population not
individuals
Traits in a population
vary among individuals
Microevolution is
change in frequency of
traits
Natural Selection
„
„
„
Reproductive success
for winning phenotypes
Acts directly on
phenotypes and
indirectly on genotypes
The first changed
individual has no
advantage
– Genetic drift
The Gene Pool
„
„
All of the genes in the
population
Genetic resource that is
shared (in theory) by all
members of population
Phenotype Variation
„
„
„
„
„
Two copies of each gene (2
alleles)
Inherit different allele
combinations
Different combinations=
different phenotypes
Inherit genotype, NOT
phenotypes
Variation is inherited
Genotypes, Phenotypes and
Environmental Effects
Himalayan rabbit experiment
Pluck hare
2. Grow hair with cold pack
Rabbits share genotype but phenotype is
dependent on environmental conditions
1.
Fig. 10.18, p. 166
1
Genetic Equilibrium
Allele frequencies at a locus are not changing
5 Rules for Equilibrium
1.
2.
3.
4.
5.
No mutation
No immigration/ emigration
Gene doesn’t affect survival
or reproduction
Large population
Random mating
Interpreted
What happens when the
rules are broken?
No Variation
No Variation
No selection
No selection
No selection
Rule #1 No Mutation
Variation in the gene pool?
Recombination
1.
„
„
Biological information changes
Each gene has own mutation rate
– What determines rates?
„
•
Rule #2 No Immigration
Immigration from a separate,
segregated populations
– New variation
Alleles
„ Mutations
•
5.
Meiosis II (haploid germ cells)
Fertilization
3.
4.
Crossing over at meiosis I
Independent assortment
2.
Effect of mutations on selection
– Lethal
– Neutral
– Advantageous
„
•
Haploid + haploid = diploid
Changes in chromosome
number or structure
Mutations
Changing
Information
Gene Flow
„
„
„
„
Effects of immigration
– Shifts allele frequency
– Introduces new mutations through
breeding
Reorganizing
Information
„
Physical flow of alleles into a
population
Tends to keep the gene pools of
populations similar
Counters the differences between
two populations that result from
mutation, natural selection, and
genetic drift
2
3.
Survival advantage or
Reproductive advantage
4.
5.
Basics of Natural
Selection
Basics of Natural
Selection
Capacity and Competition
„
„
„
Capacity and Competition
„
All populations have the capacity to
increase in numbers
No population can increase
indefinitely
Eventually, the individuals of a
population will end up competing for
resources
„
„
– Increased adaptation to a specific
environment
Results of Natural
Selection
„
Three possible outcomes:
Directional selection
– Decreases variation in favor of an
extreme.
„
Stabilizing selection
– Selects most average/ common form
of a trait
„
Disruptive selection
– Selects against intermediate forms
The alleles that produce the most
successful phenotypes will increase in
the population
Less successful alleles will become less
common
Change leads to increased fitness
Directional
Selection
„
Allele frequencies
shift in one
direction
Number of individuals
in the population
2.
Individuals of all populations have the capacity to produce more
offspring than the environment is able to support, so individuals must
compete for resources.
Individuals of a population vary in size, form, and other traits. The
variant forms of a trait may be more or less adaptive under prevailing
conditions.
When a form of a trait is adaptive under prevailing conditions, and
when it has a heritable basis, its bearers tend to survive and
reproduce more frequently than individuals with less adaptive forms
of the trait. Over generations, the adaptive version becomes more
common in the population.
Natural selection is the result of differences in survival and
reproduction among individuals of a population that differ from one
another in one or more traits.
Natural selection results in modifications of traits within a line of
descent. Over time, it may bring about the evolution of a new
species, with an array of traits uniquely its own.
Range of values for the trait at time 1
Number of individuals
in the population
What does selection do for a population?
Pillars of Natural Selection
1.
Range of values for the trait at time 2
Number of individuals
in the population
Rule #3 Survival or
Reproductive Advantage
Range of values for the trait at time 3
3
Range of values for the trait at time 1
„
Range of values for the trait at time 2
„
Range of values for the trait at time 3
Range of values for the trait at time 1
Range of values for the trait at time 2
Range of values for the trait at time 3
Example: Pesticide Resistance
Resistance
Antibiotic Resistance
Bacteria
Antiviral Resistance
HIV
Pesticide Resistance
Insects
Forms at both
ends of the
range of
variation are
favored
Intermediate
forms are
selected
against
Number of individuals Number of individuals
in the population
in the population
Intermediate
forms are favored
and extremes are
eliminated
Number of individuals
in the population
„
Disruptive
Selection
Number of individuals
in the population
Stabilizing
Selection
Chemical kills
susceptible
individuals
Resistant individuals
survive
If resistance is
heritable, following
generations exhibit
the same trait.
Evolution in Action
The DDT Paradigm
Preadapted to
survive
99% Non-resistant die
Spray with an
Insecticide
Second generation
Spray Pesticide
100% resistant survive
Second
generation
survivors
4
Spray with an
Insecticide
Third generation
Mutation rate = 1 x 10-4
Third
generation
survivors
100 butterflies
or 1 in 10,000
Insects Evolve at a High Rate
1 million
butterflies
Beneficial mutation
=
1 x 10-9 or 1 in
1,000,000,000
Breeding
“super-bugs”
in the home?
African Finches
Sexual Selection
„
Selection favors
birds with very
large or very
small bills
Birds with
intermediatesized bill are less
effective feeders
Number of individuals
60
„
„
50
40
„
30
20
„
Selection favors certain secondary
sexual characteristics
Through nonrandom mating, alleles
for preferred traits increase
Leads to increased sexual dimorphism
10
10
12.8
15.7
18.5
Widest part of lower bill
(millimeters)
5
Sickle-Cell Trait:
Heterozygote Advantage
Balanced Polymorphism
„
„
Polymorphism - “having many
forms”
Occurs when two or more
alleles are maintained at
frequencies greater than 1
percent
„
„
Allele HbS causes
sickle-cell anemia when
heterozygous
Heterozygotes are
more resistant to
malaria than
homozygotes
Malaria case
Sickle cell trait
less than 1 in 1,600
1 in 400-1,600
1 in 180-400
1 in 100-180
1 in 64-100
more than 1 in 64
Rule #4 Large Population
What happens if the population or allele
frequency gets wacked?
Genetic Drift
„
„
„
„
„
„
„
Small number of
individuals start a new
population
Low probability that
allele frequencies are
the same as original
population
Effect is pronounced on
isolated islands
Bottleneck
„
„
„
A severe reduction in
population size
Causes pronounced
drift
Results
– All progeny will be very
similar.
– Gene pool very shallow
Large Population
Simulation
100%
Gene
Frequency
Founder Effect
Random change in allele frequencies
Most pronounced in small populations
Sampling error - Fewer times an
event occurs, greater the variance in
outcome
Fixation: one allele is established in a
population
50%
allele A
neither
lost nor
fixed
0
1
5
10
15
20
25
30
35
40
45
50
Generation
(500 stoneflies at the start of each)
6
Bottleneck Simulation
100%
Gene
Frequency
AA in five populations
Rule #5 Random Mating
50%
allele A lost
from four
populations
0
1
5
10
15
20
25
30
35
40
45
50
Generation
(25 stoneflies at the start of each)
Genetic Equilibrium
Inbreeding
Allele frequencies at a locus are not changing
5 Rules for Equilibrium
„
„
„
Nonrandom mating between related
individuals
Leads to increased homozygosity
Can lower fitness when deleterious
recessive alleles are expressed
1.
2.
3.
4.
5.
Macroevolution and
Speciation
1.
2.
Biological evolution is the theory that all
living things are modified descendants of a
common ancestor that lived in the distant
past, or “descent with modification.”
Evolution simply means change over time.
Descent with modification occurs because
all organisms within a single species are
related through descent with modification
No mutation
No immigration/ emigration
Gene doesn’t affect survival
or reproduction
Large population
Random mating
Interpreted
No Variation
No Variation
No selection
No selection
No selection
Biological Species
Concept
“Species are groups of interbreeding
natural populations that are
reproductively isolated from other
such groups.”
Ernst Mayr
7
Morphology & Species
„
Variable Morphology
Grown in water
Morphological traits may not be
useful in distinguishing species
Grown
on land
– Members of same species may appear
different because of environmental
conditions
– Morphology can vary with age and sex
– Different species can appear identical
Reproductive Isolation
Isolation and Divergence
Reproductive
Isolation
„
„
„
Cornerstone of the
biological species concept
Speciation is the
attainment of
reproductive isolation
Reproductive isolation
arises as a
by-product of genetic
change
Genetic Divergence
„
„
„
Gradual accumulation of
differences in the gene
pools of populations
Natural selection, genetic
drift, and mutation can
contribute to divergence
Gene flow counters
divergence
Can’t allow gene flow
Prezygotic Isolation
„ Ecological Isolation
„ Temporal Isolation
„ Behavioral Isolation
„ Mechanical
Isolation
„ Gametic Mortality
Zygote is a fertilized egg
Speciation
Allopatric
Different lands, (physical barrier)
Sympatric
Same lands (no physical or ecological
barrier
Parapatric
Same border (small hybrid zone)
Postzygotic Isolation
Zygotic mortality
„ Hybrid inviability
„ Hybrid sterility
„
Allopatric Effect
„
„
„
„
Speciation in geographically isolated
populations
Probably most common mechanism
Some sort of barrier arises and prevents
gene flow
Effectiveness of barrier varies with
species
8
Extensive Divergence
Prevents Inbreeding
„
„
Hawaiian Islands
Species separated by geographic
barriers will diverge genetically
„
Volcanic origins, variety of habitats
„
Adaptive radiations:
– Honeycreepers - In absence of other bird
species, they radiated to fill numerous
niches
If divergence is great enough it will
prevent inbreeding even if the barrier
later disappears
– Fruit flies (Drosophila) - 40% of fruit fly
species are found in Hawaii
Reproductive Isolation
Hawaiian Honeycreepers
Can’t allow gene flow
Prezygotic Isolation
„ Ecological Isolation
„ Temporal Isolation
„ Behavioral Isolation
„ Mechanical
Isolation
„ Gametic Mortality
Postzygotic Isolation
Zygotic mortality
„ Hybrid inviability
„ Hybrid sterility
„
Zygote is a fertilized egg
FOUNDER SPECIES
Speciation without a
Barrier
„
Sympatric speciation
– Species forms within the home range of
the parent species
„
Speciation by Polyploidy
„
„
Parapatric speciation
– Neighboring populations become distinct
species while maintaining contact along
a common border
„
Change in chromosome number
(3n, 4n, etc.)
Offspring with altered chromosome
number cannot breed with parent
population
Common mechanism of speciation in
flowering plants
9
Possible Evolution of
Wheat
Unknown
species of
wild wheat
Triticum
monococcum
(einkorn)
14AA
X
14BB
T. turgidum
(wild emmer)
CROSS-FERTILIZATION,
FOLLOWED BY A
SPONTANEOUS
CHROMOSOME
DOUBLING
14AB
28AABB
X
Parapatric Speciation
T. tauschii
(a wild
relative)
14DD
T. aestivum
(one of the
common
bread
wheats)
42AABBDD
Adjacent
populations
evolve into
distinct species
while maintaining
contact along a
common border
BULLOCK’S
ORIOLE
BALTIMORE
ORIOLE
HYBRID ZONE
Patterns of Change
in a Lineage
Are We All Related?
„
Are all species are related by descent?
Do we share genetic connections that
extend back in time to the first
prototypical cell?
Evolutionary Trees
Cladogenesis
– Branching pattern
– Lineage splits, isolated populations diverge
– Homology and morphology
„
Anagenesis
– No branching
– Changes occur within single lineage
– Gene flow throughout process
Gradual Model
„
extinction
(branch
ended
before
present)
new species
branch point
(a time of
divergence,
speciation)
a single
lineage
branch point
(a time of
divergence,
speciation)
a new
species
a single
lineage
dashed line
(only sketchy
evidence of
presumed
evolutionary
relationship)
„
Speciation model in which species
emerge through many small
morphological changes that
accumulate over a long time period
Fits well with evidence from certain
lineages in fossil record
10
Punctuation Model
„
„
Adaptive Radiation
Speciation model in which most
changes in morphology are
compressed into brief period near
onset of divergence
„
Supported by fossil evidence in some
lineages
„
„
„
Adaptive Radiation
Extinction
„
„
„
„
Who Survives?
„
„
„
Species survival is to some extent
random
Asteroids have repeatedly struck Earth
destroying many lineages
Changes in global temperature favor
lineages that are widely distributed
Burst of divergence
Single lineage gives rise to
many new species
New species fill vacant adaptive
zone
Adaptive zone is “way of life”
Irrevocable loss of a species
Mass extinctions have played a
major role in evolutionary
history
Fossil record shows 20 or more
large-scale extinctions
Reduced diversity is followed by
adaptive radiation
Critics of Evolution
1.
2.
3.
4.
Critics of Evolution do not propose any
alternative hypotheses that can be
tested by evidence.
The critics selectively use evidence as
the basis of their alternative
hypotheses.
Science is not democratic, the majority
of the scientific community rejects the
critics regardless of their evidence.
There is no controversy
11
Jones vs. Smith
Returning a cracked kettle
extinction
(branch
ended
before
present)
new species
1.
2.
3.
4.
Smith never borrowed the kettle
When Smith returned the kettle
it wasn’t broken
The kettle was already cracked
when Smith borrowed it
There is no kettle
branch point
(a time of
divergence,
speciation)
a new
species
branch point
(a time of
divergence,
speciation)
dashed line
(only sketchy
evidence of
presumed
evolutionary
relationship)
a single
lineage
a single
lineage
Fig. 17.11 p. 268
Mechanism of Evolution
Progeny
Large Populations
Genetic Variability
Parental
Generation
Selection
Genetic
Variability
Fig. 17.12 p. 269
Mechanism of Evolution
Factors that cause change
„
„
Mutations- new alleles
Genetic Drift- unselected random
change in allele frequencies
– Genetic Bottlenecks
„
„
„
„
Founder effect
Inbreeding
Gene Flow- moving alleles with mating
Natural Selection
Evolution changes allele frequencies in populations not individuals
12
Mechanism of Evolution
„
Variation
„
– Mutations- new
alleles
– Natural Selection
– Genetic Drift
– Gene Flow
„
Selection
– Directional Selection
– Stabilizing Selection
– Disruptive Selection
Survival
– Selective forces
Abiotic- weather,
nature
Biotic- diseases
Competition
„
Reproduction
– Advantageous traits
must be passed to
progeny
– Ability to pass on the
genotype to the next
generation is the
measure of success
13