Results of Evolution….
Three Modes of Natural Selection
• Natural selection can alter the frequency distribution of
heritable traits in three ways depending on which
phenotype is favored:
– Directional Selection
– Disruptive Selection
– Stabilizing Selection
Directional Selection
• Directional selection occurs when conditions favor individuals
exhibiting one extreme of a phenotypic range.
• Commonly occurs when the population’s environment changes or
when members of a population migrate to a new (and different)
habitat.
Describe a trait in humans that follows this pattern.
Possible Effect of
Continual Directional Selection
If continued, the variance may decrease.
after
before
after
Phenotype (trait)
before
after
Frequency
Frequency
Frequency
before
Phenotype (trait)
Phenotype (trait)
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Disruptive or Diversifying Selection
• Disruptive selection occurs when conditions favor
individuals at both extremes of a phenotypic range
over individuals with intermediate phenotypes.
• The “intermediates” in the population have lower
relative fitness.
Describe a trait in humans that follows this pattern.
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Disruptive or Diversifying Selection
• Disruptive selection occurs when conditions favor
individuals at both extremes of a phenotypic range
over individuals with intermediate phenotypes.
• The “intermediates” in the population have lower
relative fitness.
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Stabilizing Selection
• Stabilizing selection removes extreme variants from
the population and preserves intermediate types.
• This reduces variation and tends to maintain the
status quo for a particular phenotypic character.
Describe a trait in humans that follows this pattern.
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Adaptive Radiation
The process by which a species evolves to live in
a different way.
Convergent Evolution
Occurs when organisms that are
not closely related
independently evolve similar
traits
Wings evolved separately in:
insects (arthropods)
birds (birds)
bats (mammals)
So, flight has evolved at least four separate times!
Analogous structures can lead to
convergent evolution!
Coevolution
• When two or more species affect each other’s
evolution
– What does ‘co’ mean?
• More likely to happen when the two species
are
– Predator/prey
– Competitive species
– Mutualistic species
Coevolution
• Plant has a hollow thorns and
pores that secrete nectar for
the ants
• Plants wouldn’t have these
hollow thorns if their
evolution hadn’t been
affected by the ants
Sexual Selection
• A form of selection in which individuals with certain
inherited characteristics are more likely than other
individuals to obtain mates.
• Can result in sexual dimorphism which is a difference
between the two sexes with regard to secondary sexual
characteristics.
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Preserving Genetic Variation
• Some of the genetic variation is populations represents
neutral variation, differences in DNA sequence that do
not confer a selective advantage or disadvantage.
• There are several mechanisms that counter the tendency
for directional and stabilizing selection to reduce
variation:
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–
–
–
Diploidy
Balancing Selection
Hererzygote Advantage
Frequency-Dependent Selection
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Diploidy
• In diploid eukaryotes each organism has two copies
of every gene and a considerable amount of genetic
variation is hidden from selection in the form of
recessive alleles.
• By contrast, haploid organisms express every gene
that is in their genome. What you see is what you
get. It reduces genetic variability.
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Balancing Selection
• Balancing selection occurs when natural selection
maintains two or more forms in a population.
• This type of selection includes heterozygote advantage
and frequency-dependent selection.
• Heterozygote advantage involves an individual who is
heterozygous at a particular gene locus thus has a
greater fitness than a homozygous individual.
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Heterozygote Advantage
• A well-studied case is that of sickle
cell anemia in humans, a hereditary
disease that damages red blood cells.
• Sickle cell anemia is caused by the
inheritance of a variant hemoglobin
gene (HgbS) from both parents.
• In these individuals, hemoglobin in
red blood cells is extremely sensitive
to oxygen deprivation, and this
causes red blood cells to have a
sickle shape
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Heterozygote Advantage
• A person who inherits the sickle
cell gene from one parent, and a
normal hemoglobin gene (HgbA)
from the other, has a normal life
expectancy.
• However, these heterozygote
individuals, known as carriers of
the sickle cell trait
• Why are they called carriers?
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Heterozygote Advantage
• The heterozygote is resistant to
the malarial parasite which kills a
large number of people each year
in Africa.
• There exists a balancing selection
between fierce selection against
homozygous sickle-cell sufferers,
and selection against the standard
HgbA homozygotes by malaria.
• The heterozygote has a permanent
advantage (a higher fitness)
wherever malaria exists.
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Heterozygote Advantage
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Frequency-Dependent Selection
• The fitness of a phenotype depends on how common it
is in the population.
• In positive frequency-dependent selection the fitness
of a phenotype increases as it becomes more common.
• In negative frequency-dependent selection the fitness
of a phenotype increases as it becomes less common.
• Choose an example of this and explain
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Balanced Polymorphism
Balanced polymorphism occurs in a given
population when two distinct types (or
morphs) exists and the allele frequencies do
not change. This may be due to
•Variation in the environment where one
morph may be favored over another.
•One morph may be better adapted to a certain
time of the year over the other.
The lady bird beetle has 2 morphs. The red
variety is more abundant in the spring and
winter, whereas the black morph is more
abundant in the summer and fall.
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If natural selection is so powerful,
why do we not have perfect
organisms?
Why Natural Selection Cannot Fashion Perfect
Organisms
1. Selection can act only on existing variations.
• Natural selection favors only the fittest phenotypes
among those in the population, which may not be the
ideal traits. New advantageous alleles do not arise on
demand.
2. Evolution is limited by historical constraints.
•
Each species has a legacy of descent with modification from
ancestral forms. Evolution does not scrap the ancestral anatomy.
For example in birds and bats, an existing pair of limbs took on
new functions for flight as these organisms evolved from nonflying
ancestors.
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Why Natural Selection Cannot Fashion Perfect
Organisms
3. Adaptations are often compromises.
•The loud call that enables a frog to attract mates also
attracts predators.
4.Chance, natural selection and the environment
interact.
•Chance can affect the subsequent evolutionary history of
populations. A storm can blow birds hundreds of kilometers
over an ocean to an island, the wind does not necessarily
transport those individuals that are best suited to the
environment!
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