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Section 11.1
KEY CONCEPT
A population shares a common gene pool.
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Genetic variation in a population increases the chance that
some individuals will survive.
Why it’s beneficial:
• Genetic variation leads to phenotypic variation.
• Increases the likelihood that some individuals can survive a
change in the environment.
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Genetic variation in a population increases the chance that
some individuals will survive.
How it’s stored in a population:
• 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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How it’s measured:
• Allele frequencies measure genetic variation.
– measures how common allele is in population
– can be calculated for each allele in gene pool
Genetic variation in a population increases the chance that
some individuals will survive.
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Genetic variation comes from several sources.
• Mutation is a random change in the DNA of a gene.
– can form new allele
– can be passed on to
offspring if in
reproductive cells
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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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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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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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– Stabilizing selection favors the
intermediate phenotype.
• Natural selection can take one of three
paths.
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• Natural selection can take one of three
paths.
– Disruptive selection favors both
extreme phenotypes.
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KEY CONCEPT
Natural selection is not the only mechanism through
which populations evolve.
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11.1 Genetic Variation Within Populations TEKS 7C, 7E, 7F
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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• 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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• 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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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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KEY CONCEPT
Hardy-Weinberg equilibrium provides a framework for
understanding how populations evolve.
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Hardy-Weinberg equilibrium describes populations that
are not evolving.
• Biologists use models to study populations.
• Hardy-Weinberg equilibrium is a type of model.
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Hardy-Weinberg equilibrium describes populations that
are not evolving.
• Genotype frequencies stay the same if five conditions are
met.
– very large population: no genetic drift
– no emigration or immigration: no gene flow
– no mutations: no new alleles added to gene pool
– random mating:
no sexual selection
– no natural selection:
all traits aid equally
in survival
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Hardy-Weinberg equilibrium describes populations that
are not evolving.
• Real populations rarely meet all five conditions.
– Real population data is
compared to a model.
– Models are used to
studying how populations
evolve.
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The Hardy-Weinberg equation is used to predict genotype
frequencies in a population.
• Predicted genotype frequencies are compared with actual
frequencies.
– used for traits in simple dominant-recessive systems
"The Hardy-Weinberg equation
is based on Mendelian genetics.
It is derived from a simple
Punnett square in which p is the
frequency of the dominant allele
and q is the frequency of the
recessive allele."
– must know frequency of recessive homozygotes
– p2 + 2pq + q2 = 1
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There are five factors that can lead to evolution.
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• Genetic drift changes allele frequencies due to chance
alone.
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• Gene flow moves alleles from one population to another.
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• Mutations produce the genetic variation needed for
evolution.
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• Sexual selection selects for traits that improve mating
success.
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• Natural selection selects for traits advantageous for
survival.
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• In nature, populations evolve.
– expected in all populations
most of the time
– respond to changing
environments
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KEY CONCEPT
New species can arise when populations are isolated.
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The isolation of populations can lead to speciation.
• Populations become isolated when there is no gene flow.
– Isolated populations adapt to their own environments.
– Genetic differences can add up over generations.
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• Reproductive isolation can occur between isolated
populations.
– members of different
populations cannot
mate successfully
– final step to
becoming separate
species
• Speciation is the rise of two or more species from one
existing species.
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Populations can become isolated in several ways.
• Behavioral barriers can cause isolation.
– called behavioral isolation
– includes differences in courtship or mating behaviors
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• Geographic barriers can cause isolation.
– called geographic isolation
– physical barriers divide population
• Temporal barriers can cause isolation.
– called temporal isolation
– timing of reproductive periods prevents mating
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KEY CONCEPT
Evolution occurs in patterns.
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Evolution through natural selection is not random.
• Natural selection can have direction.
• The effects of natural selection add up over time.
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• Convergent evolution describes
evolution toward similar traits in
unrelated species.
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• Divergent evolution describes evolution toward different
traits in closely related species.
How do convergent and divergent
evolution illustrate the directional
nature of natural selection?
ancestor
red fox kit fox
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Species can shape each other over time.
• Two or more species can evolve together through
coevolution.
– evolutionary paths become connected
– species evolve in response to changes in each other
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• Coevolution can occur in beneficial relationships.
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• Coevolution can occur in competitive relationships,
sometimes called evolutionary.
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Species can become extinct.
• Extinction is the elimination of a species from Earth.
• Background extinctions occur continuously at a very low
rate.
– occurs at roughly the same
rate as speciation
– usually affects a few species
in a small area
– caused by local changes in
environment
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• Mass extinctions are rare but much more intense.
– destroy many species at global level
– thought to be caused by catastrophic events
– at least five mass extinctions in last 600 million years
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Speciation often occurs in patterns.
• A pattern of punctuated equilibrium exists in the fossil
record.
– theory proposed by Eldredge and Gould in 1972
– episodes of speciation occur suddenly in geologic
time
– followed by long periods of little evolutionary change
– revised Darwin’s idea that species arose through
gradual transformations
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• Many species evolve from one species during adaptive
radiation.
– ancestral species diversifies into many descendent
species
– descendent species
usually adapted to
wide range of
environments