RAPID SELECTION

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Natural Selection II

• RAPID SELECTION
• (Intense selection, rapid evolution)

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• Natural Selection
• Occurs when individuals differ in their
  reproductive success
• Results in changes in allele frequencies in a
  population (genetic change over time)
• Eventually, this leads to morphological changes
  and, if accompanied by reproductive isolation,
  speciation.

Darwins finches
Hawian silverswords
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Artificial versus rapid natural selection Is
artificial selection analogous to natural
selection?
YES!

• same basic mechanism (differential reproductive
  success)
• NS fitness related to environment
• AS fitness related to human preference

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Classic examples of intense natural selection and
subsequent rapid evolution

• Antibiotic resistance
• Antiviral resistance
• Cichlids
• Sticklebacks
• Pocket mice
• Darwins finches
• Hawian silverswords
• Fish in fisheries

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• Some lesser-known examples
• Snails shell morphology in northern New England
• Escape ability in trinidad guppies
• Copepods and dinoflagellates (if time)

• FEATURE EXAMPLES
• Oxygen saturation and offspring survival (Becky)
• Illustrates mechanisms of evolution
• Reproductive isolation in African Drosophila
  (Raul)
• Example of speciation

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Questions How do these examples provide evidence
for evolution? How can we use this information
to help non-scientists understand
evolution? What are the claims of evolution?
What arent? What are we still uncertain
about?
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• Conditions that favor rapid evolution
• Quick generation time
• Strong selective pressure
• Available ecological niches (as on newly formed
  islands)

long term evolution condensed into a short amount
of time
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• Claims of evolution well address in following
  examples
• Differential survival and reporduction occurs in
  response to selective pressures
• or is change guided by a divine hand?
• Change occurs at the population, not individual
  level
• or do individuals pass on acquired
  characteristics to their offspring?
• Natural selection acts only on heritable
  variation
• or do individuals pass on acquired
  characteristics to their offspring?
• Natural selection is adequate to explain
  speciation and higher taxonomic level differences
  (common ancestry)
• or do species have separate origins?

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EXAMPLE 1
Morphology of Littoria obtusa in 1900

• High-spiraled
• Thin-walled

The range of Carcinus, an intertidal crab and
predator of Littoria obtusa, expanded into
northern New England in 1900. The crabs prefer
high-spiralled, thin-walled snails to low
spiralled, thick walled snails of the same
species.
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EXAMPLE 1
The range of Carcinus extended into Nova Scotia
by 1986. Laboratory experiments showed that
Carcinus prefers high-spiralled, thin-walled
snails to low spiralled, thick walled snails.
Morphology of Littoria obtusa after 1982

• Low-spiraled
• Thick-walled

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EXAMPLE 1
Morphology of snails collected before 1900.
The predator provided strong selection for
low-spiraled, thick-walled snails. Within 82
years, Littoria obtusa underwent dramatic
morphological changes.
Evidence that populations change in response to
selective pressures
Morphology of snails collected after 1982.
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EXAMPLE 2
Trinidadian guppies and escape ability
EXPERIMENTAL BACKGROUD Guppies were introduced
into both high and low predation habitats. Twenty
years later, 6 guppies were removed from a
low-predation and 6 from a high predation
habitat. They were placed in an enclosure with
their natural predator, a cihlid. When the
cichlid had consumed 1/2 of the guppies, the
remaining guppies were removed and scored as to
whether they came from high or low predation
habitats. FINDINGS Guppies from high-predation
populations had a much higher survival rate.
Thus, they had improved escape ability.
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EXAMPLE 2
Trinidadian guppies and escape ability
BUT Is escape ability a heritable trait?
F2 generation were tested from low and high
predation habitats (bred and raised in
predator-free enclosures). FOUND F2s of guppies
from high predation populations had significantly
greater escape ability
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EXAMPLE 2
Trinidadian guppies and escape ability
Conclusions Escape ability is acquired rapidly
under the strong selection pressure of
predation. Escape ability is a heritable
trait. Evidence that natural selection acts on
heritable variation
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Offspring survival is greater among women with
higher oxygen saturation levels at high elevations
Background Populations at high altitudes are
exposed to hypoxia (Hypoxia stresses the oxygen
delivery system) Individuals differ in percent
oxygen saturation of hemoglobin despite a uniform
environment Oxygen saturation levels are
determined by a single locus following Mendelian
patterns of inheritance and dominance (locus is
inferred only, not yet cloned) Could hypoxia be
a selective pressure for higher oxygen saturation
levels?
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• THE STUDY
• Collected geneaological, oxygen saturation
  genotype, and female fertility data from 3812
  people in 14 villages between 3800 and 4200 m in
  the Tibet Autonomous Region of China from
  November 1997 to August 2000

FOCUS OF TODAYS DISCUSSION

• Analyzed data on fertility events and oxygen
  saturation genotype from a subgroup of 20 - 59
  year old married women

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Genotype AB and BB oxygen saturation level of
hemoglobin 10 higher than AA
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SUMMARY OF RESULTS
High saturation genotypes were significantly
correlated with
(1) lower infant mortality
(2) higher number of surviving offspring
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For which claims of evolution does this example
provide evidence?

• Four main claims of evolution indroduced at the
  beginning
• Differential survival and reproduction occurs in
  response to selective pressures
• or is change guided by a dinivne hand?
• Change occurs at the population, not individual
  level
• or do individuals pass on acquired
  characteristics to their offspring?
• Natural selection acts only on heritable
  variation
• or do individuals pass on acquired
  characteristics to their offspring?
• Natural selection is adequate to explain
  speciation and higher taxonomic level differences
  (common ancestry)
• or do species have separate origins?

x
x
x
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How is this an example of evolution?
Natural selection acts only on heritable variation
Number of individuals with the BB and AB
genotypes increase dramatically with each
generation Frequency of B alleles increase in
the populations The change in allele frequency
in the populations is NATURAL SELECTION due to
the intense selective pressure of hypoxia.
Change occurs at the population, not individual
level
Differential survival and reproduction occurs in
response to selective pressures
We dont know whether increased survivorship is
due to having healthier mothers, each individual
having higher oxygen saturation level, or a
combination of the two.
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AA
AB
Starting population, BB and AB, AA
After one generation, the B alleles make up a
higher proportion of the population
With each successive generation, the B alleles
increase in frequency.
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• Weve provided one answer to the first question,
  How is this evidence for evolution?
• In the conclusion, well consider how to use
  this information to help people understand
  evolution.
• anti-evolutionary arguments are often based on
  misconceptions about what evolution is and is
  not
• education is needed

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References of primary examples OSteen, S. et
al. Rapid evolution of escape ability in
Trinidadian guppies. 2002 Evolution 56
776-784 Seely, R.H Intense natural selection
caused a rapid morphological transition in a
living marine snail. 1986 PNAS 83
6897-6902. Beall, C.M. et al. Higher offspring
survival among Tibetan women with high oxygen
saturation genotypes residing at 4,000 m. 2004
PNAS 101 1430014304
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BIBLIOGRAPHY of further examples Antibiotic and
antiviral resistance Davies J. Inactivation of
antibiotics and the dissemination of resistance
genes. Science 1994264375-82. Palumbi, S.R.
Humans as the worlds greatest evolutionary
source. Science2001 293 1786-1790 Rambaut, A.
et al. The causes and consequences of HIV
evolution. Nature Reveiews Genetics 2004 5
52-61 Ciclids Kocher TD. 2004. Adaptive evolution
and explosive speciation the cichlid fish model.
Nature Reviews Genetics 5 288-298 Danley PD and
Kocher TD. 2001. Speciation in rapidly diverging
systems lessons from Lake Malawi. Molecular
Ecology 101075-1086
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Stickleacks Boughman, J. W. et al.. 2005.
Parallel evolution of sexual isolation in
sticklebacks. Evolution 59 361373. Pocket
mice Nachman, M. W., H. E. Hoekstra, and S.
DAgostino. 2003. The genetic basis of adaptive
melanism in pocket mice. Proc. Natl. Acad. Sci.
USA 10052685273. Darwins finches Grant, P.R.
Ecology and Evolution of Darwin's Finches(With a
New Preface and Afterward) 1999 (book) Princeton
University Press Hawiian Silverswords Baldwin,
B.G. and M.J. SandersonAge and rate of
diversification of the Hawaiian silversword
alliance (Compositae) 1998 Evolution 95
9402-9406 Fish in fisheries Olsen, E.M. et al.
Maturation trends indicative of rapid evolution
preceded the collapse of northern cod. 2004
Nature 428 932-935
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Incipient Speciation

• Sexual Isolation in
• Drosophila Melanogaster

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come_to_zimbabwe.jpg
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Incipient Speciation

• Strong sexual isolation between the Zimbabwe and
  the worldwide Drosophila melanogaster

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Strong Case

• Zimbabwe population is not a different species
• Limited gene flow between populations
• Unique genetic variability in Zimbabwe
• Some DNA differences are nearly fixed
• Mating Preference (Wu et al. 1995)

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No Choice Experiment

• Five females and males in same vial
• Recorded copulations after 1hr and 1 day
• Seven Z female lines rarely mated with C males
• Two Z female lines mated more frequently with C
  males (intermediate types)
• C females mated with Z males but not as quickly
  as with C males
• Results generally agree, measure possibly
  affected by experimental conditions

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Multiple Choice Experiment

• Z and C flies given different colored food
• Place males and females from Z and C together
• Discrimination Index (DI) measures within-type
  preference (DI gt 3 means strong isolation)
• All Z and C lines show significant sexual
  isolation
• Bidirectional, stronger between Z females and C
  males
• Intermediate types show intermediate preferences
  
• Complete isolation between D. melanogaster and D.
  simulans

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Mating Behavior is Heritable

• Hybrid males attract Z females better than C
  males but worse than Z males
• Hybrid male success depends on competitor
  strongly suggests female choice
• Hybrid females are less discriminate toward C
  males most will settle for C males if given no
  choice
• Genetic trickery gt zimbabweness in major
  autosomes
• No evidence of hybrid sterility or inviability
• 2006 handful of loci involved in male mating
  success and female preference, complex network

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Cosmopolitan male
Zimbabwe Female
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Reproductive Isolation in Sockeye Salmon

• Between ancestral river and an introduced lake
  population in less than 13 generations (70
  years)
• Lake population is genetically distinct despite
  some gene flow
• Lake males have deep bodies, immigrant males have
  shallower bodies for river swimming
• Lake females are smaller than immigrant females
  large size in river facilitates nest building
• Lake body plans confer reproductive advantage

Hendry et al. 2000 Science
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But they havent speciated

• Be glad
• Rare glimpse into the speciation process
• Evolution at work differential reproductive
  success within members of a species
• Rapid vs. great opportunities to study speciation
  events