Transcript Number of individuals in the population

Darwin &
Microevolution
Chapter 19-20
Charles Darwin (1809-1882)
Former divinity and medical student
 Secured an unpaid position as ship's
naturalist on the H.M.S. Beagle
 Voyage provided Darwin a unique
opportunity to study plants, animals, and
their environment
 Gathered a great deal of evidence he
would later incorporate into a theory of
evolution
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Voyage of the Beagle
5 yr. mission to chart South America
EQUATOR
Galapagos
Islands
Darwin’s Theory of
Natural Selection
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Individuals in a population have variable levels
of success in reproducing
Left unchecked, populations tend to expand
exponentially, leading to a scarcity of resources
In the struggle for existence, some individuals
are more successful (fit) than others, allowing
them to survive and reproduce
Those organisms best able to survive and
reproduce will leave more offspring
Darwin’s Theory of
Natural Selection
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Over time there will be heritable changes in
phenotype (genotype) of a species
These changes may result in a transformation of
the original species into a new species similar to,
but distinct from, its parent species
Common Descent, due to these changes
similar species have common ancestors. This
means that nearly all of life is linked
What is Evolution?
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Evolution is a process that results in heritable
changes in a population spread over many
generations
Evolution can be precisely defined as “any
change in the frequency of alleles within a
gene pool from one generation to the next”
Populations, not individuals evolve
Traits within a population vary among individuals
Variation
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Within a population most phenotypic traits are
polymorphic, they have two or more forms
Those traits that have many forms show
continuous variation
Individual inherit different combinations of alleles
leading to different phenotypes
All these genes & their alleles within a
population is known as the gene pool.
This variation is the raw material for
evolution
This variation is also what allows for natural
selection
What Determines Alleles in an
Individual?
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Mutations, many are lethal, but some can be
neutral & some may confer an advantage
Crossing over at meiosis I, shuffles alleles
Independent assortment, genes that may work
together, but are on different chromosomes will
not be inherited together
Fertilization, sexual reproduction
Change in chromosome number or structure,
often deleterious, but can be advantageous
Genetic Equilibrium
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Point when a population is not evolving
The opposite of evolution
Allelic frequencies are not shifting
Calculated using the Hardy-Weinberg equations
p+q=1
p2 + 2pq + q2 = 1
Where p = frequency of Dominant allele (A)
and q = frequency of recessive allele (a)
Genetic Equilibrium
Five conditions need to be met:
 No mutations
 Random mating
 Gene does not affect survival or
reproduction
 Large population
 No migration
Hardy-Weinberg Equilibrium
p A
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q a
p A
AA(p2)
Aa(pq)
q a
Aa(pq)
aa(q2)
p2 = frequency of AA (Homozygous dominant)
2pq = frequency of Aa (heterozygous)
q2 = Frequency of aa (homozygous recessive)
Hardy Weinberg Equilibrium:
Example
Starting population:
490 AA butterflies
Dark-blue wings
420 Aa butterflies
Medium-blue wings
90 aa butterflies
White wings
Hardy Weinberg Equilibrium:
Example
Frequencies in Gametes:
F1 genotypes:
Gametes:
0.49 AA
A
0.42 Aa
A
A
0.09 aa
a
a
a
0.49 + 0.21
0.21 + 0.09
0.7A
0.3a
Hardy Weinberg Equilibrium:
Example
THE NEXT GENERATION
490 AA butterflies
STARTING POPULATION
420 Aa butterflies
490 AA butterflies
Dark-blue wings
90 aa butterflies
420 Aa butterflies
Medium-blue wings
90 aa butterflies
White wings
NO CHANGE
THE NEXT GENERATION
490 AA butterflies
420 Aa butterflies
90 aa butterflies
NO CHANGE
Mechanisms of Evolution
Evolution of a population over time may
occur as a result of
New mutations
Natural selection
Nonrandom mating (Sexual selection)
Genetic drift because of small
population
Gene flow – immigration and emigration
(Opposite of Genetic Equilibrium)
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Mutations
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Mutations that alter protein structure enough
to impact its function
 more
likely to be harmful but may be beneficial
 our genome is product of thousands of
generations of selection
Fuel for evolution
 Mutant allele may enable an organism to fit
its environment better & increase
reproductive success
 especially likely if environment is changing
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Natural Selection
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Difference in the survival & reproductive success
of different genotypes and/or phenotypes
Over time, 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
Selection is not a “force” it is merely the favoring
of some genetic changes over others
Types of Natural Selection
Directional Selection
 Stabilizing Selection
 Disruptive Selection
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Shift in the variation in a
consistent direction
within the phenotypic
range
Examples:
 Pesticide
resistance in
insects
 Antibiotic resistance in
bacteria
in the population
Number of individuals
Range of values for the trait at time 1
Number of individuals
in the population
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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
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Loss of extreme
forms with
stabilization of an
intermediate form
Example: Infants
greater or less than
7.5 lbs have
increased mortality
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
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favors individuals at
the extremes with a
reduction of
intermediate forms
Example: African
Finches selection
favors small or large
beak size
Number of individuals Number of individuals
in the population
in the population
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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
Sexual Selection
A type of natural selection
 Selection that is driven by the competition
for mates
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Gene Flow
Movement of alleles into a population
 Tends to keep the gene pools of
populations similar
 Counters changes due to mutation, natural
selection, and genetic drift
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Genetic Drift
Random change in allele frequencies
brought about by chance
 Effect is most pronounced in small
populations
 This can cause similar, but isolated
populations to become dissimilar due to
the loss or fixation of alleles
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Bottleneck
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Genetic drift is most pronounced when small
populations grow into larger ones, usually after a
catastrophe
A bottleneck as only a few alleles survive and
are now disproportionally expressed in the
population
A founder effect results in rare or even
disadvantageous alleles being found in a
population at a level higher than normally
expected
Macroevolution:
Evidence for evolution
Biogeography
 Fossil Record
 Comparative anatomy
 Comparative embryology
 Molecular Biology
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Evolution evidence:
Biogeography
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Geographical distribution of species
Indicates that populations that are isolated from
one another geographically evolve separately
Evolution evidence:
The Fossil Record
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Fossils are created when
organisms become buried
in sediment, bone and
other hard tissue is
converted to rock
The fossils contained in
sedimentary rock layers
reveal a history of life on
earth
Evolution evidence:
Comparative Anatomy
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Comparing body forms and structures of
major lineages
By studying homology or how similarly
derived body parts have evolved, we can
put together an evolutionary tree and find
common ancestors even if the body parts no
longer serve the same function
Evolution evidence:
Comparative anatomy
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Homologous
structures
 Similar
anatomy,
different functions
 Indicate divergent
evolution
Evolution evidence:
Comparative anatomy
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Analogous structures
 Similar
function,
different anatomy
 Indicated Convergent
Evolution
 Example: bird’s wing
vs. fly’s wing
Evolution evidence:
Comparative anatomy
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Vestigial Organs
 Remnants
of evolution
 Organs that were
required in ancestor
that are not needed in
present-day organism
 Examples: appendix,
tailbone
Evolution evidence:
Comparative Embryology
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Development of early
embryos of resemble
each other due to a
common developmental
plan
During later embryonic
development this plan
becomes modified to
create the different body
types we see
Evolution evidence:
Molecular Biology
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Tracking mutations in
sequences of DNA
and/or proteins can
trace the evolutionary
history of organisms
The more nucleotides/
amino acids in
common, the more
closely related the
species