Download Natural Selection

Natural Selection
• Directional Selection
• Stabilizing selection
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Fre que ncy
– One trait is favored over others
– Results in directional changes
Sta bilizing Se le c tion
– Promotes phenotypic uniformity
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• Disruptive selection
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Dire c tiona l Se le c tion
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Fre quency
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Disruptive Se le c tion
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Frequenc y
– Intermediate traits are disfavored,
while two extremes are favored
– Directional selection acting in several
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– A common allelic trait is favored, less
common are disfavored
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Stabilizing Selection
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Reduces genetic variability by selecting out extremes (pulls in the “skirts”
of the distribution.
Average individuals have higher fitness than individuals at the extremes.
The survival of sparrows during the winter of 1898 on sparrows
– Wing length and other characteristics were correlated with
survivorship.
– Birds with average traits had a higher survival rate that smaller or
larger birds.
Sta bilizing Se le c tion
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Frequency
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Birth We ight (lbs)
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Pe rc e nt Morta lity
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The birth weight of nearly 7000
human female babies recorded by
Kam and Pinrose
After one month, survival of these
babies was determined as a function
of birth weight: Overall survival was 95.9%
Babies less than 4.5 lb had 41.4%
survival - babies
7.5 -8.5 lb had 98.5 survival
Babies greater than 10.0 lb had
90.5% survival
An example of stabilizing selection?
Pe rc e nta ge of Popula tion
Birth weight and Infant Survival
Stabilizing selection for number of offspring per birth
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Clutch Size in Birds
¾ Too few eggs mean reproductive potential
is not met = lowered fitness
¾ Too many eggs means low fledgling
survivorship = lowered fitness
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The fact that humans usually give birth to
one child is undoubtedly explained by low
survivorship of multiple births in earlier
human populations.
Directional Selection
Selection that favors one extreme
Responsible for evolutionary change
Examples:
¾ Artificial selection is a special form of directional selection in which
humans provide the selective pressure
¾ Penicillin resistant Micrococcus Industrial melanism
¾ Adaptation of humans and horses to environmental change
Dire c tiona l Se le c tion
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Antibiotic Resistance
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In the absence of antibiotics,
mutant bacteria are out
competed by normal bacteria.
When an antibiotic is introduced
into the the environment of
mutant bacteria are are favored
over “normal” bacteria that lack
the mutation.
Directional selection of this type
has produced drug resistant
strains of the organisms
responsible for diseases such as
tuberculosis, staph infections.
The Peppered Moth Story
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The peppered moth story is an
example of directional selection
Light colored moths were favored
before pollution
Soot darkened the bark of trees
and favored dark colored moths.
Later, when pollution was reduced
the reproductive success of light
colored moths again increased
A good example of how the fitness
values associated with a trait is
contingent of current
environmental conditions.
Horse Evolution
• A well-documented example of
directional section in response to
environmental change.
• Grasslands expanded at the end
of the Miocene epoch
• These environmental changes
produced selective pressures that
favored fewer toes and higher,
longer wearing teeth in horses.
Human Evolution
• Humans were exposed to
the same environmental
changes as horses.
• Contraction of tropical
forests and environmental
instability resulted in
directional selection for:
¾ Bipedalism
¾ Larger brains
¾ A flexible adaptation based
on the cultural transmission
of learned information
Diversifying Selection
• Also know as destabilizing or
disruptive selection
• Promotes phenotypic differences,
favors the presence of multiple
alleles
• Thought to be important in species
formation
• The fishing net analogy
Disruptive artificial selection on
bristle number in fruit flies
Balancing Selection
• Balancing selection occurs
when the heterozygote has
higher fitness than
homozygotes
• This is called heterozygote
advantage
• Balanced Polymorphism
Balanced Polymorphism
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Fitness
¾ The situation in which
heterozygotes have a selective
advantage over homozygotes
¾ A balanced polymorphism exists
when an equilibrium among
different alleles at a given
chromosomal locus
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Genotype
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Sickle Cell Anemia: an example of balancing selection
• Best known example is that of sickle-cell trait in humans
¾ People homozygous for the trait (ss) are severely anemic
¾ Normal people (SS) are susceptible to malaria
¾ Heterozygous people are slightly anemic but have the advantage of
malaria resistance
¾ Human modification of the environment may be reducing the advantage
of having the sickle cell trait
The Spread of the S Allele
Sexual Selection
• Darwin’s (1871)
definition of sexual
selection:
"sexual selection depends not
on a struggle for existence in
relation to other organic
beings or to external
conditions, but on a struggle
between the individuals of one
sex, generally the males for
possession of the other sex."
Types of Sexual Selection
• Intrasexual selection:
competition for mates
among members of the
same sex: this may favor
horns, large canines etc.
• Intersexual selection
("sex appeal"): This kind
of sexual selection may
explain traits such as the
peacock's tail.
Sexual Selection and Sexual Dimorphism
• Sexual dimorphism:
morphological differences
between the sexes
• Over mates tends often
favors larger male size in
comparison to that of
females as is exemplified
by the elephant seal
• The angler fish is an
exception
Sexual Selection and Natural Selection
• Sexual selection and natural
selection can reinforce each
other as well as act at crosspurposes
• A trait that increases success in
attracting mates (a longer tail)
may increase vulnerability to
predators
• Ultimately an equilibrium will
be reaches between the
pressures of sexual selection and
natural selection.
Sexual selection and mating patterns
• Polygyny (“many wives”):
increases intrasexual selection and
sexual dimorphism
• Monogamy (“one mate”): reduces
intrasexual selection and favors
reduced sexual dimorphism
Sexual Selection in Humans
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How can we explain the physical differences between human males and
females
¾ To what extent are they a result of natural selection?
¾ To what extent are they a result of sexual selection?
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Darwin’s perspective: "We may conclude that the greater size, strength,
courage, pugnacity, and energy man, in comparison with woman, were
acquired during primeval times, and have subsequently been augmented,
chiefly through the contests of rival males for the possession of the females."
Charles Darwin, Descent of Man (1871)
Sex Differences in the Human Pelvis
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Bre a dth
Shoulde rs
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Natural selection favored a
pelvic outlet large enough to
give birth to large-headed
babies.
Sexual selection may have
reinforced this: males may
have preferred mates with
wider hips
Wide hips: present mechanical
difficulties during running (loss
of energy through angular
momentum. This would
counterbalance their obstetrical
advantage
Narrow hips: women may
have preferred males with
narrower because of its
association with running ability
and hunting prowess
Width
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Chimpa nze e
Gorilla
Woma n
Sexual Selection in Humans
“You’re really cute when you’re angry”