11.1 Genetic Variations within Populations...Genetic variation comes from several sources....
Transcript of 11.1 Genetic Variations within Populations...Genetic variation comes from several sources....
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11.1 Genetic
Variations within
Populations
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KEY CONCEPT
A population shares a common
gene pool.
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What are gene pools?
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NEW VOCABULARY
• POPULATION – GROUP OF
INDIVIDUALS OF SAME SPECIES THAT
INTERBREED
• GENE POOL – COMMON GROUP OF
ALL GENES PRESENT IN A
POPULATION
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Genetic variation in a population
increases the chance that some
individuals will survive.
• Genetic variation leads to phenotypic
variation.
• Phenotypic variation is necessary for natural
selection.
• Genetic variation is stored in a population’s
gene pool.
– made up of all alleles in a population
– allele combinations form when organisms have
offspring
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Gene Pool
Combined
genetic
info. of
all
members
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Allele frequencies measure
genetic variation.
– measures how common allele is in population
– can be calculated for each allele in gene pool
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Sources of Genetic Variation:
Mutations: Any change in a sequence of DNA.
Mutations can occur because of:
•Mistakes in the replication of DNA.
•Radiation or chemicals in the environment.
•Random.
•Viruses.
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Genetic variation comes from
several sources.• Mutation is a random change in the DNA of
a gene.
• Recombination forms new combinations of
alleles.
– can form new allele
– can be passed on to
offspring if in
reproductive cells
– usually occurs during meiosis
– parents’ alleles
arranged in new
ways in gametes
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Genetic variation comes from
several sources.
• Hybridization is the crossing of
two different species.
–occurs when individuals can’t find
mate of own species
–topic of current scientific research
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11.2 Natural
Selection in
Populations
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KEY CONCEPT
Populations, not individuals,
evolve.
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Natural selection acts on
distributions of traits.
• A normal distribution graphs as a bell-
shaped curve.
• Traits not undergoing
natural selection have a
normal distribution.
– highest frequency near
mean value
– frequencies decrease
toward each extreme
value
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Examples of selection pressures...
Can change a population’s physical traits
• Predators - variants with adaptations allowing them to
escape predators have more offspring
- e.g. speed, defensive weapons, camouflage, mimicry
• Prey/Food - variants with adaptations allowing them to
obtain food have more offspring
- e.g. Speed, senses for finding prey/food, weapons for killing prey or obtaining food, camouflage for stealth
• Climate - those who can survive new climate best have
more kids
- e.g. ice age, change in climate due to migration.
• Mates - variants with adaptations allowing them to attract a
mate to have offspring
- e.g. strong, attractive, good provider
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Example #1: Escaping Predation
Peppered Moth (see video clip)
• Early trees had light-colored bark
• Only the light-colored moths survived.
Selection was for less melanin.
• After industrialization, the tree bark was
darker.
• Only the darker colored moths now
survived. Selection was for more melanin.
New info on the Pepper Moth experiment
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Example #2
• The leaf bug
• The selection pressure is
predators
• It’s strategy is to mimic a
leaf
Pray Mantis Camouflage
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Main Types of Selection Pressures
• Directional Selection
– Natural selection favors one extreme of the population
for that trait
– often happens when environment changes in a
consistent way- e.g.climate gets colder.
• Disruptive Selection
– Natural selection favors both extremes selected
– Causes species to diverge
• Stabilizing Selection
– Natural selection favors the average for population
selected
• Sexual Selection
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Natural selection can change
the distribution of a trait in one
of three ways.
• Microevolution is evolution within a
population.
– observable change in the allele
frequencies
– can result from natural selection
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Natural selection can
take one of three paths.
– Directional selection favors phenotypes
at one extreme.
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Directional Selection• Neck of Giraffe
• Antibiotic resistance of bacteria
• Moth color (melanin)
• Camouflage/Mimics
• Many sexually selected traits
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Directional Selection: Mimicry
(mimic environment)
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– Stabilizing selection favors
the intermediate phenotype.
Natural selection can take one
of three paths.
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Stabilizing Selection• When the extremes of the
trait aren’t as well suited
Examples
• bird clutch size
• Elk Antlers size
• Giraffe neck length
• Tail length in birds
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Natural selection can take
one of three paths.
–Disruptive selection favors
both extreme phenotypes.
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Disruptive Selection• Causes divergence within the
species
• Occurs when two different
types of resources in one
area
• Results in specialization for
each branched group
• May lead to formation of
new species
• E.g. Darwin’s Finches
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Stabilizing selection - selection against the two extremes in a
population (e.g., birth weight in humans,
clutch size in birds)
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Directional selection - selection for one extreme in a population,
against the other extreme
(e.g., pesticide resistance in insects
antibiotic resistance in bacteria)
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Disruptive selection - selection for the two extremes in a
population, against the average forms
(e.g., limpets w/ 2 color forms: light & dark
in mosaic environment; flies on two hosts:
apple & hawthorn)
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11.3 Other
Mechanisms of
Evolution
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KEY CONCEPT
Natural selection is not the
only mechanism through
which populations evolve.
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Gene flow is the movement of
alleles between populations. • Gene flow occurs
when individuals join
new populations and
reproduce.
• Gene flow keeps
neighboring
populations similar.
• Low gene flow
increases the chance
that two populations
will evolve into
different species. bald eagle migration
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Genetic drift is a change in allele
frequencies due to chance.
• Genetic drift causes a loss of genetic diversity.
• It is most common in small populations.
• A population bottleneck can lead to genetic
drift.
– It occurs when an event
drastically reduces
population size.
– The bottleneck effect is
genetic drift that occurs
after a bottleneck event.
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Genetic Drift
CHANGE IN GENOTYPE SOLELY
BY CHANCE EFFECTS!
RANDOM!
promoted by:
Population Bottleneck -drastic reduction in
population size
Founder Effect - isolated colonies founded by
small number of individuals
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Fig. 23-9
Originalpopulation
Bottleneckingevent
Survivingpopulation
Population Bottleneck
Fig. 23-10
Rangeof greaterprairiechicken
Pre-bottleneck(Illinois, 1820)
Post-bottleneck(Illinois, 1993)
(a)
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The founding of a small population
can lead to genetic drift.
– It occurs when a few individuals start a
new population.
– The founder effect is genetic drift that
occurs after start of new population.
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Founder Effect
Example: Silverswords in Hawaii
Tarweeds (Mainland) Silverswords (Hawaiian Islands)
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Genetic drift has negative
effects on a population.
–less likely to have some
individuals that can adapt
–harmful alleles can become more
common due to chance
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Summary: Evolution can occur by two major mechanisms:
• Natural Selection (non-random)
• Genetic Drift (random)
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Sexual Selection
Selection resulting in greater reproductive
fitness in certain individuals of one sex
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Sexual selection occurs when certain
traits increase mating success.
• Sexual selection
occurs due to
higher cost of
reproduction for
females.
– males produce
many sperm
continuously
– females are more
limited in potential
offspring each cycle
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There are two types of
sexual selection.
– intrasexual
selection:
competition among
males
– intersexual
selection: males
display certain traits
to females
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Sexual SelectionIntrasexual selection – within one sex;
competition between members of one sex
(usually males)
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Sexual SelectionIntersexual selection – between two sexes;
preference by one sex for features of the
other sex. Usu. female choice.