NOTES CH 17 Evolution of Populations...Evolution (Change over time) vs. Genetic Equilibrium if a...
Transcript of NOTES CH 17 Evolution of Populations...Evolution (Change over time) vs. Genetic Equilibrium if a...
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NOTES – CH 17 – Evolution of
Populations
● Vocabulary
– Fitness
– Genetic Drift
– Punctuated
Equilibrium
– Gene flow
– Adaptive radiation
– Divergent evolution
– Convergent
evolution
– Gradualism
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17.1 – Genes & Variation
● Darwin developed his theory of natural
selection without knowing how heredity
worked…or how variations arise
● VARIATIONS are the raw materials for
natural selection
● All of the discoveries in
genetics fit perfectly into
evolutionary theory!
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Genotype & Phenotype
● GENOTYPE: the particular combination of
alleles an organism carries
● an organism’s genotype, together with
environmental conditions, produces its
PHENOTYPE
● PHENOTYPE: all physical,
physiological, and behavioral
characteristics of an
organism (i.e. eye color, height)
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Natural Selection● NATURAL SELECTION acts directly on…
…PHENOTYPES!
● How does that work?...some individuals
have phenotypes that are better suited to
their environment…they survive & produce
more offspring (higher fitness!)
● organisms with higher
fitness pass more copies
of their genes to the next
generation!
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Do INDIVIDUALS evolve?● NO!
● Individuals are born with a certain set of genes (and therefore phenotypes)
● If one or more of their phenotypes (i.e. tooth shape, flower color, etc.) are poorly adapted, they may be unable to survive and reproduce
● An individual CANNOT
evolve a new phenotype
in response to its
environment
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So, EVOLUTION acts on…● POPULATIONS!
● POPULATION = all members of a species
that live in a particular area
● In a population, there exists a RANGE of
phenotypes
● NATURAL SELECTION acts
on this range of phenotypes
the most “fit” are selected
for survival and reproduction
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17.2: Evolution as Genetic
Change in Populations
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Mechanisms of Evolution
(How evolution happens)
1) Natural Selection (from Darwin)
2) Mutations
3) Migration (Gene Flow)
4) Genetic Drift
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DEFINITIONS:
● SPECIES:
group of organisms that breed with one
another and produce fertile offspring.
● POPULATION:
group of individuals of the same
species that live in the same area
(& therefore interact,
interbreed, & share
the same GENE POOL)
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● GENE POOL:
combined genetic information of all
members of a particular population.
● Allele frequency:
the number of times that an allele occurs in
a gene pool compared with the number of
times other alleles occur (usually
expressed as a %)
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1) Mechanism of Evolution:
NATURAL SELECTION
● All organisms struggle for survival by competing for
resources (especially in an overpopulated
environment) so…
low levels of fitness = die or leave few
offspring
high levels of fitness = survive and reproduce
most successfully
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1) Mechanism of Evolution:
Natural Selection
● NATURAL SELECTION: survival of the fittest
-Imagine that green beetles are easier for birds to spot (and hence, eat). Brown beetles are a little more likely to
survive to produce offspring
The brown beetles pass their genes for brown coloration on to their offspring
Next generation: brown beetles are more common than in the previous generation.
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What is Fitness?● FITNESS: how successful a particular
genotype is at leaving offspring in the next
generation (relative to other genotypes)
-If brown beetles consistently leave more
offspring than green beetles…
-The brown beetles have a greater fitness
relative to the green beetles.
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Fitness is a relative thing
● A genotype’s fitness depends on the
environment in which the organism lives.
● The fittest genotype during an ice age, for
example, is probably not the fittest genotype once
the ice age is over.
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FITNESS
● The fittest individual is not
necessarily the strongest,
fastest, or biggest
● A genotype’s fitness
includes its ability to survive,
find a mate, produce offspring
(pass its genes to the next generation)
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There cannot be NATURAL SELECTION
without GENETIC VARIATION in the
first place!
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Sources of Genetic Variation -
MUTATION
● MUTATION: change in the DNA
sequence that affects genetic
information (random—not predictable)
-a mutation could cause parents with genes
for bright green coloration to have offspring
with a gene for brown coloration
-that would make the genes for brown
beetles more frequent in the population.
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Sources of Genetic Variation -
MUTATION
● Single mutation can have a large effect
● in many cases, evolutionary change is based on
the accumulation of many mutations
can be beneficial, neutral, or harmful
mutations do not “try” to supply what the organism
“needs.”
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Sources of Genetic Variation -
MUTATION● The individuals which happen to have
the mutations giving them the best
adaptations to the environment will be
the ones that survive
hence the “good”
mutations will be “passed
down” to the next
generation.
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● Not all mutations matter to evolution
-All cells in our body contain DNA
-Mutations in non-reproductive cells won’t be
passed onto offspring
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Causes of mutations:
● Mistake in copying DNA
● External sources—radiation, chemicals
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Sources of Genetic Variation:
MEIOSIS!Gene shuffling: (How chromosomes line
up in meiosis)
-Crossing over can also occur
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This shuffling is important for EVOLUTION
because it can introduce new
combinations of genes every generation.
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3) Mech. of Evolution: MIGRATION
(a.k.a. “GENE FLOW”)
● Some individuals from a population of
brown beetles might have joined a
population of green beetles.
-would make the genes for brown beetles
more frequent in the green beetle
population.
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4) Mechanism for Evolution:
GENETIC DRIFT
● In a population, an allele can become more or less common
by chance (remember genetics and probability!)
● GENETIC DRIFT = The random change in the frequency of an allele (gene)
most effective with small populations
● SO…Gene pools can change
WITHOUT natural selection…
an allele can become common in a
population by chance alone.
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GENETIC DRIFT: example
● Imagine that a population of green and
brown beetles
● Several green beetles were killed when
someone stepped on them and therefore,
they had no offspring.
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● The next generation would have a few
more brown beetles than the previous
generation—but just by chance.
● These chance changes from
generation to generation are known
as GENETIC DRIFT.
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Genetic Drift Example:
FOUNDER EFFECT
● A small group of individuals move to new habitat (the “founding” group)
● Their alleles and allele frequencies may be different that that of the original population
● So the new population that they
found will have different allele
frequencies than the original
group… BY CHANCE!
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Genetic Drift Example:
BOTTLENECK EFFECT
● a population experiences an event (storm,
sickness, over hunted by humans) that
causes it to decrease in # to just a few
individuals
● the allele frequencies in the few surviving
individuals may be different than the
original population
● Example: cheetahs
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Bottleneck Effect
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Of all of the mechanisms covered, the
“strongest” influence is that of
NATURAL SELECTION…
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NATURAL SELECTION● Natural selection on single gene traits can lead to
changes in the allele frequency
-Ex: brown vs. green beetles
● Natural selection on polygenic traits—affects
distribution of phenotypes in 3 ways
1) Directional Selection
2) Stabilizing Selection
3) Disruptive Selection
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Modes of Selection:
● Imagine the range of phenotypes in a
population are graphed into a distribution
curve:
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1) DIRECTIONAL SELECTION:
● If organisms at one end of the curve have
higher fitness than organisms in the
middle or at the other end of the curve
– Finch beaks in the Galapagos
– Result: specific beak size increased
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1) DIRECTIONAL SELECTION:
Examples:
-bacterial resistance to antibiotics
-peppered moth
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Peppered Moth example:
● 100 years after the first dark
moth was discovered in 1848,
90% of moths were dark;
● the light variety continued to
dominate in unpolluted
areas outside of London.
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2) STABILIZING SELECTION:
● Individuals near center of curve have
higher fitness that individuals at either end
of the curve
-Human baby birth weight
Babies born vs. underweight less
likely to survive
Larger babies have a hard time
being born (think size of birth canal)
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3) DISRUPTIVE SELECTION:
● Individuals at the outer ends of the curve
are more fit than those in the center
– Intermediate type is selected against
● Ex: Bird beak size
– if medium seed size becomes less common,
birds that can eat the smallest and largest
seeds will survive
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Evolution (Change over time) vs.
Genetic Equilibrium
● if a population is NOT evolving, allele
frequencies in the gene pool do not
change, and the population is in GENETIC
EQUILIBRIUM.
● The Hardy-Weinberg principle states
that allele frequencies remain constant
unless 1 or more factors cause those
frequencies to change…
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● for Hardy-Weinberg equilibrium to occur, the
following conditions must be met:
1) Large population
2) No mutation
3) No gene flow (no immigration or emigration)
4) Random mating (no mating preference for
particular phenotype)
5) No natural selection (all genotypes have an =
chance of surviving & reproducing)
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HOWEVER, in nature:1) most populations are small & may mate with
one another
2) there are always mutations
(chance with every DNA replication)
3) gene flow often occurs between
populations
4) mating is non-random
5) natural selection is always occurring
**Therefore, in nature there will always be
changes in populations (“microevolution”)
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**So why study population genetics?
Why use the H-W Theorem?
1) shows how genetics is related to evolution;
2) provides a benchmark genetic equilibrium against
which change can be noted;
3) permits an estimation of gene frequencies;
especially useful in estimating the number of
carriers of lethal alleles in human populations.
Ex: Brachydactyly - fingers are abnormally short in
heterozygotes; condition is fatal during infancy to
homozygous recessive individuals due to major skeletal
defects
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Hardy-Weinberg Equation:
● p = frequency of dominant allele (A)
● q = frequency of recessive allele (a)
● p + q = 1
● frequency of possible diploid
combinations (AA, Aa, aa):
p2 + 2pq + q2 = 1
(AA) (Aa) (aa)
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Example Problem:
● If the frequency of a recessive allele is
35% in a population of 1500 people,
how many people would you predict
would be carriers of this allele, but
would not express the recessive
phenotype?
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q = 35% = 0.35
p = 1 - q = 1 - 0.35 = 0.65
p2 + 2pq + q2 = 1
freq. of Aa genotype = 2pq
= 2(0.65)(0.35)
= 0.455 = 45.5%
# of carriers = (0.455)(1500)
= 683 people
Solution:
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Example Problem:
● In a population with 2 alleles for a particular
locus, B and b, the allele frequency of B is
0.78. If the population consists of 172
individuals, how many individuals are
heterozygous? How many will show the
recessive phenotype?
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p = 0.78
q = 1 - p = 1 - 0.78 = 0.22
p2 + 2pq + q2 = 1
freq. of Bb genotype = 2pq
= 2(0.78)(0.22)
= 0.343 = 34.3%
# of heterozygotes = (0.343)(172)
= 59 individuals
Solution:
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p2 + 2pq + q2 = 1
freq. of recessive phenotype =
freq. of bb = q2 = (0.22)2
= 0.0484 = 4.84%
# of recessive ind. = (0.0484)(172)
= 8.3 individuals
(8 ind.)
Solution:
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Nonrandom Mating leads to
Sexual Selection!● SEXUAL SELECTION: the selection of
mates based on heritable traits (e.g. size,
strength, coloration)
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SEXUAL SELECTION: Females
-females tend to increase
their fitness by
increasing the quality of
their offspring by
choosing superior
male mates (and are
therefore “choosier” or
more selective when
finding a mate)
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SEXUAL SELECTION: Males
-males increase their fitness by maximizing the
quantity of offspring produced
**as a result, in vertebrate species,
the male is typically the
“showier” sex
-colorful plumage
-lion’s mane
-antlers
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17.3: The Process of Speciation
Central Idea:
● How does natural selection (and other
mechanisms of evolution) lead to the
formation of a new species?
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● SPECIATION: formation of a new species● Remember:
– Species: a group of organisms that breed with one another and produce fertile offspring
– Population: members of the same species that share a gene pool.
● If a genetic change increases fitness, that allele will eventually be found in many members of the population
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● As new species evolve, populations become reproductively isolated from each other.
● When members of two populations cannot reproduce to produce fertile offspring =
reproductive isolation● At this point, the 2 groups have separate gene
pools
● Question: How does reproductive isolation develop?
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Kaibab Squirrel
Albert’s Squirrel
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3 Kinds of Isolating Mechanisms:
1) Behavioral Isolation
-Two populations are physically able to interbreed but have different courtship rituals or other types of behavior
2) Geographic Isolation:
-Geographic barriers (rivers, mountains, roads) prevent genes from being exchanged, including advantageous mutations and variations
-BUT does not guarantee formation of a new species…WHY NOT?
3) Temporal Isolation
-Species reproduce at different times and therefore are unlikely to reproduce with each other
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17.4: Molecular Evolution
● the analysis of genomes enables us to
study evolution at the molecular level
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17.4: Molecular Evolution
● MOLECULAR CLOCK: uses mutation
rates in DNA to estimate the time that 2
species have been evolving independently
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17.4: Molecular Evolution
● the more differences there are between
the DNA sequences of the 2 species, the
more time has elapsed since the 2 species
shared a common ancestor
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Molecular Evolution &
Cladograms● we can use this
information to
generate a
CLADOGRAM – a
diagram that links
groups of
organisms showing
branch points from
a common
ancestor.