evolution of populations chapter 16
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Evolution of Populations Chapter 16. Dr. Donna Howell Biology I Blacksburg High School. Genes and Variation. In the 1930’s, experts finally connected the work of Gregor Mendel and Charles Darwin. They realized for the first time that changes in genes produced variation in offspring. - PowerPoint PPT PresentationTRANSCRIPT
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Evolution of PopulationsChapter 16
Dr. Donna HowellBiology I
Blacksburg High School
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Genes and Variation
• In the 1930’s, experts finally connected the work of Gregor Mendel and Charles Darwin.• They realized for the
first time that changes in genes produced variation in offspring.• They then said that
natural selection works on this variation.
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Variation and Gene Pools
• One way scientists study genetic variation is through the study of populations – both animal and human.• Because members of a
population interbreed, they share a common group of genes called a “gene pool.”• The relative frequency
of an allele is the number of times that the allele occurs in a gene pool.
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Variation and Gene Pools• Gene pools
are important in the field of evolution because evolution is the change in relative frequency of alleles in a population.
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Sources of Genetic Variation
• One major source of genetic variation is mutations in DNA.• These can occur
due to mistakes in DNA replication or due to environmental factors such as chemicals, radiation, etc.• Can affect an
organisms “fitness.”
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Sources of Genetic Variation
• Another source of genetic variation is the shuffling of genes.• This occurs during
the production of eggs and sperm.• The 23 pairs of
chromosomes can produce 8.4 million different genetic combinations.
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Single-Gene and Polygenic Traits
• The number of phenotypes produced for a given trait depends on how many genes control the trait.• Some traits are
controlled by a single gene. • Others are
controlled by two or more genes.
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Single-Gene and Polygenic Traits
• An example of a single-gene trait is the gene that codes for the widow’s peak in your hair.• An example of a
polygenic trait is skin color in humans.• Because multiple
genes code for this, there is a very wide range of possible skin colors.
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Natural Selection on Single-Gene Traits
• Natural selection doesn’t act on genes – it acts on whole organisms because either the organism lives or dies with all of its genes.• Natural selection
on single-gene traits results in changes in allele frequencies and thus evolution.
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Natural Selection on Single-Gene Traits• An example is the
population of lizards to the right. • Mutations have
occurred that produced red and black lizards.• The red ones are
more visible to predators, so disappear.• The black ones can
get warmer in the sun, eat more, and thus survive.
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Natural Selection on Polygenic Traits• When there is
more than one gene that controls a trait, natural selection is more complex.• Can affect in three
ways:
• Directional selection• Stabilizing selection• Disruptive selection
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Natural Selection on Polygenic Traits• Directional
selection is when individuals at one end of the curve have higher fitness than individuals at the middle or other end of the curve.• Example: a food
shortage causes the supply of small seeds to run low, and beak sizes get bigger as a result because only big seeds left.
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Natural Selection on Polygenic Traits• Stabilizing
selection is when individuals near the center of a curve have higher fitness that those at either end.• Example: human
babies born at average mass are more likely to survive than babies born either much smaller or much larger than average.
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Natural Selection on Polygenic Traits• Disruptive
selection is when individuals at the upper and lower ends of the curve have higher fitness than individuals near the middle• Example: average-
sized seeds become less common, and larger and smaller seeds become more common.
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Genetic Drift• Genetic drift is a
random change in allele frequency due to a smaller population.• Can occur when a
small group of individuals colonizes a new habitat.• The founder effect
occurs when a small subgroup of a population migrates away from the rest.
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Genetic Equilibrium• Genetic
equilibrium is when allele frequencies in a population remain constant.• If allele
frequencies remain constant, evolution does not occur.• The Hardy-
Weinberg Principle says that allele frequencies will remain constant unless something causes it to change.
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Genetic Equilibrium• Five conditions are
required for genetic equilibrium to take place:
1. Random mating2. Population must be
large3. No movement into
or out of population4. No mutations5. No natural selection
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Genetic Equilibrium• Five conditions are
required for genetic equilibrium to take place:
1. Random mating2. Population must be
large3. No movement into
or out of population4. No mutations5. No natural selection
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Speciation• Natural selection
can lead to new allele frequencies in a population.• But how does this
lead to the formation of whole new species?• The formation of
new species is called speciation.
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Speciation – Reproductive Isolation• As new
species evolve, populations exhibit reproductive isolation.
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Speciation – Reproductive Isolation• One way
reproductive isolation can occur is through behavioral isolation.• Ex:
different courtship rituals or mating songs
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Speciation – Reproductive Isolation• Another
way reproductive isolation can occur is geographic isolation.• Ex: get
stuck on two different sides of a river
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Speciation – Reproductive Isolation• Another
way reproductive isolation can occur is by temporal (time) isolation.• Ex:
reproduce at different times of year
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Speciation of Darwin’s Finches• Darwin
studied a group of finches in the Galapagos, and documented the process of speciation.
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Speciation of Darwin’s Finches• Darwin
found that speciation occurred by:• Founding of
new population• Geographic
isolation• Changes in
gene pool• Reproductive
isolation• Ecological
competition
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1. Founders Arrive• To begin, a
few species of finches either flew or were blown to another island. • They survived
and reproduced.
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2. Geographic Isolation• Later, some
birds crossed to another island and stayed there. • They survived
and reproduced and no longer shared the same gene pool with birds on the original island.
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3. Gene Pool Changes• Over time,
populations on the different islands became adapted to their local environments.• Natural
selection caused beaks to change according to what birds ate.
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4. Reproductive Isolation• Once the gene
pool changes, different species can no longer mate with one another and produce offspring.• Example: mate at
different times of the year, different courtship rituals, etc.
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5. Ecological Competition• Now that
different species are living together, they must compete for available food (seeds). • Their beaks will
evolve to eat whatever kinds of food they can find.
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The End