Natural Selection II
RAPID SELECTION
(Intense selection, rapid evolution)
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.
Hawian silverswords
Darwin’s finches
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
Classic examples of intense natural selection and
subsequent rapid evolution
•
Antibiotic resistance
•
Antiviral resistance
•
Cichlids
•
Sticklebacks
•
Pocket mice
•
Darwin’s finches
•
Hawian silverswords
•
Fish in fisheries
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
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 aren’t? What are we
still uncertain about?
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
Claims of evolution we’ll 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?
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, thinwalled snails to low spiralled, thick walled
snails of the same species.
EXAMPLE 1
The range of Carcinus extended into Nova
Scotia by 1986.
Laboratory experiments showed that
Carcinus prefers high-spiralled, thinwalled snails to low spiralled, thick walled
snails.
Morphology of Littoria obtusa
after 1982 •Low-spiraled
•Thick-walled
Morphology of snails
collected before 1900.
EXAMPLE 1
The predator
provided strong
selection for lowspiraled, thickwalled 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.
Trinidadian guppies and escape ability
EXAMPLE 2
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.
Trinidadian guppies and escape ability
EXAMPLE 2
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
Trinidadian guppies and escape ability
EXAMPLE 2
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
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?
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 TODAY’S DISCUSSION
•Analyzed data on fertility events and
oxygen saturation genotype from a
subgroup of 20 - 59 year old married
women
Genotype AB and BB oxygen saturation level of hemoglobin 10%
higher than AA
SUMMARY OF RESULTS
High saturation genotypes were significantly correlated with:
(1) lower infant mortality
(2) higher number of surviving
offspring
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?
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
Change occurs at the population, not individual level
Frequency of B alleles increase in the populations
Differential survival and reproduction occurs in response to
selective pressures
The change in allele frequency in the populations is
NATURAL SELECTION due to the intense selective
pressure of hypoxia.
We don’t know whether increased survivorship is due to having healthier mothers,
each individual having higher oxygen saturation level, or a combination of the two.
AA
Starting population,
AB,
= AA
= BB and
After one generation, the B alleles
make up a higher proportion of the
population
With each successive generation,
the B alleles increase in frequency.
AB
We’ve provided one answer to the first question, “How is this
evidence for evolution?”
In the conclusion, we’ll 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
References of primary examples
O’Steen, 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: 14300–14304
BIBLIOGRAPHY of further examples
Antibiotic and antiviral resistance
Davies J. Inactivation of antibiotics and the dissemination of
resistance genes. Science 1994;264:375-82.
Palumbi, S.R. Humans as the world’s 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 10:1075-1086
Stickleacks
Boughman, J. W. et al.. 2005. Parallel evolution of sexual isolation in
sticklebacks. Evolution 59: 361–373.
Pocket mice
Nachman, M. W., H. E. Hoekstra, and S. D’Agostino. 2003. The
genetic basis of adaptive melanism in pocket mice. Proc. Natl. Acad.
Sci. USA 100:5268–5273.
Darwin’s 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
Sexual Isolation in
Drosophila Melanogaster
http://www.geocities.com/mssever/gallery/photo/welcome_to_zimbabwe.jpg
Incipient Speciation
Incipient Speciation
• Strong sexual isolation between the
Zimbabwe and the worldwide Drosophila
melanogaster
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)
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
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 > 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
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 => “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
Cosmopolitan
male
Zimbabwe
Female
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
But they haven’t 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