Transcript lecture12-BW

Lecture 12: Evolution
Key terms:
Reading:
Ch16: Microevolution
Ch17:Speciation
Ch18:Macroevolution
Biological Change Over
Time
Microevolution
Macroevolution
Changes
Change
with in
species
Well defined
mechanism
Easily observed
Based on selection
from one
species to another
Undefined
mechanism
Interpretation of:
– Cladistics
– Fossil record
– Geological data
Microevolutionary
Processes


Drive a population away from
genetic equilibrium
Small-scale changes in allele
frequencies brought about by:
– Natural selection
– Gene flow
– Genetic drift
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
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
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
No Variation
No Variation
No selection
No selection
No selection
What happens when the
rules are broken?
Rule #1 No Mutation


Biological information changes
Each gene has own mutation rate
– What determines rates?

Effect of mutations on selection
– Lethal
– Neutral
– Advantageous
Variation in the gene pool?
Recombination
1.
•
Independent assortment
2.
•
•
5.
Meiosis II (haploid germ cells)
Fertilization
3.
4.
Crossing over at meiosis I
Reorganizing
Information
Haploid + haploid = diploid
Changes in chromosome
number or structure
Mutations
Changing
Information
Rule #2 No Immigration

Immigration from a separate,
segregated populations
– New variation
Alleles
 Mutations


Effects of immigration
– Shifts allele frequency
– Introduces new mutations through
breeding
Gene Flow



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
Rule #3 Survival or
Reproductive Advantage
What does selection do for a population?
Survival advantage or
Reproductive advantage
Pillars of Natural Selection
1.
2.
3.
4.
5.
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.
Basics of Natural
Selection
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
Basics of Natural
Selection
Capacity and Competition



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
– 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
Allele frequencies
shift in one
direction
Number of individuals
in the population
Number of individuals
in the population

Range of values for the trait at time 1
Range of values for the trait at time 2
Number of individuals
in the population
Directional
Selection
Range of values for the trait at time 3

Intermediate
forms are favored
and extremes are
eliminated
Number of individuals
in the population
Stabilizing
Selection
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

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

Range of values for the trait at time 1
Range of values for the trait at time 2
Number of individuals
in the population
Disruptive
Selection
Range of values for the trait at time 3
Resistance
Antibiotic Resistance
Bacteria
Antiviral Resistance
HIV
Pesticide Resistance
Insects
Chemical kills
susceptible
individuals
Resistant individuals
survive
If resistance is
heritable, following
generations exhibit
the same trait.
Example: Pesticide Resistance
Evolution in Action
The DDT Paradigm
Preadapted to
survive
99% Non-resistant die
Spray Pesticide
100% resistant survive
Spray with an
Insecticide
Second generation
Second
generation
survivors
Spray with an
Insecticide
Third generation
Third
generation
survivors
Mutation rate = 1 x 10-4
100 butterflies
or 1 in 10,000
1 million
butterflies
Beneficial mutation
=
1 x 10-9 or 1 in
1,000,000,000
Insects Evolve at a High Rate
Breeding
“super-bugs”
in the home?
Sexual Selection



Selection favors certain secondary
sexual characteristics
Through nonrandom mating, alleles
for preferred traits increase
Leads to increased sexual dimorphism
Balanced Polymorphism


Polymorphism - “having many
forms”
Occurs when two or more
alleles are maintained at
frequencies greater than 1
percent
Sickle-Cell Trait:
Heterozygote Advantage


HbS
Allele
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




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
Founder Effect



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
Gene
Frequency
100%
50%
allele A
neither
lost nor
fixed
0
1
5
10
15
20
25
30
35
40
45
Generation
(500 stoneflies at the start of each)
50
Bottleneck Simulation
100%
Gene
Frequency
AA in five populations
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)
Rule #5 Random Mating
Inbreeding



Nonrandom mating between related
individuals
Leads to increased homozygosity
Can lower fitness when deleterious
recessive alleles are expressed
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
No Variation
No Variation
No selection
No selection
No selection
Macroevolution and
Speciation
1.
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.”
2.
Evolution simply means change over time.
Descent with modification occurs because
all organisms within a single species are
related through descent with modification
Biological Species
Concept
“Species are groups of interbreeding
natural populations that are
reproductively isolated from other
such groups.”
Ernst Mayr
Morphology & Species

Morphological traits may not be
useful in distinguishing species
– Members of same species may appear
different because of environmental
conditions
– Morphology can vary with age and sex
– Different species can appear identical
Variable Morphology
Grown in water
Grown
on land
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
Reproductive Isolation
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
Speciation
Allopatric
Different lands, (physical barrier)
Sympatric
Same lands (no physical or ecological
barrier
Parapatric
Same border (small hybrid zone)
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
Extensive Divergence
Prevents Inbreeding


Species separated by geographic
barriers will diverge genetically
If divergence is great enough it will
prevent inbreeding even if the barrier
later disappears
Hawaiian Islands

Volcanic origins, variety of habitats

Adaptive radiations:
– Honeycreepers - In absence of other bird
species, they radiated to fill numerous
niches
– Fruit flies (Drosophila) - 40% of fruit fly
species are found in Hawaii
Hawaiian Honeycreepers
FOUNDER SPECIES
Reproductive Isolation
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
Speciation without a
Barrier

Sympatric speciation
– Species forms within the home range of
the parent species

Parapatric speciation
– Neighboring populations become distinct
species while maintaining contact along
a common border
Speciation by Polyploidy



Change in chromosome number
(3n, 4n, etc.)
Offspring with altered chromosome
number cannot breed with parent
population
Common mechanism of speciation in
flowering plants
Possible Evolution of
Wheat
Triticum
monococcum
(einkorn)
14AA
Unknown
species of
wild wheat
X
14BB
T. turgidum
(wild emmer)
CROSS-FERTILIZATION,
FOLLOWED BY A
SPONTANEOUS
CHROMOSOME
DOUBLING
14AB
28AABB
X
T. tauschii
(a wild
relative)
14DD
T. aestivum
(one of the
common
bread
wheats)
42AABBDD
Parapatric Speciation
Adjacent
populations
evolve into
distinct species
while maintaining
contact along a
common border
BULLOCK’S
ORIOLE
BALTIMORE
ORIOLE
HYBRID ZONE
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?
Patterns of Change
in a Lineage

Cladogenesis
– Branching pattern
– Lineage splits, isolated populations diverge
– Homology and morphology

Anagenesis
– No branching
– Changes occur within single lineage
– Gene flow throughout process
Evolutionary Trees
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)
Gradual Model


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
Punctuation Model


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




Burst of divergence
Single lineage gives rise to
many new species
New species fill vacant adaptive
zone
Adaptive zone is “way of life”
Adaptive Radiation
Extinction




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
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
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
Jones vs. Smith
Returning a cracked kettle
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
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)
Fig. 17.11 p. 268
Fig. 17.12 p. 269
Mechanism of Evolution
Progeny
Large Populations
Genetic Variability
Parental
Generation
Selection
Genetic
Variability
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
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