Population Evolution

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Population Evolution

• Biology Chapter 16

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Genetic Variation
Populations always show variation in traits no
two individuals are EXACTLY the same

• Variations are caused by two main sources
• Mutations
• Gene shuffling during sexual reproduction

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Gene Pools
Gene pools consist of all genes (AND all alleles)
within a population Gene frequency is the number
of times that an allele occurs in a gene pool
EXAMPLE of gene pools - A homozygous black mouse
has two alleles for black fur. A heterozygous
black mouse has one allele for black fur and one
allele for brown fur. A homozygous brown mouse
has two alleles for brown fur.

• Gene Pool
• Homozygous Black
• Homozygous Brown
• Heterozygous Black

BB bb Bb
12 Heterozygous Black
How many total genes? How many total alleles? How
many B alleles? How many b alleles?
25 50 20 30
4 Homozygous Black
9 Homozygous Brown
Allele B frequency 20/50 40 0.4 Allele b
frequency 30/50 60 0.6
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Changes in allele frequency indicate that a
change in the allele populations has occurred
through evolution
Sources of Genetic Variation

• Mutations
• ANY change in the DNA sequence due to mistakes,
  environmental sources (radiation, etc.)
• Can have no effect or drastic effects
• Gene shuffling as a result of sexual reproduction
• Crossing over during meiosis I
• Random contributions from mother father

Most genetic variation results in little or no
observable changes in organisms
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Trait distribution in populations
Polygenic traits
Single-gene traits
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Natural Selection effects on single- gene traits
CAN lead to allele frequency changes evolution!
What happened? Environmental changes allowed
black lizards to be less conspicuous to predators
than the previous environment. Therefore, red and
light colored lizards stood out more and were
more easily eaten by predators than black lizards
causing a change in the allele frequency.
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Natural Selection effects on polygenic traits
More complex than single-gene trait results
Directional selection
Disruptive selection
Stabilizing selection
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Directional Selection Example
BEFORE Industrial Revolution (and soot)
AFTER Industrial Revolution (and soot)
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Stabilizing Selection Example
Healthy birth weight leads to better survival
rates
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Disruptive Selection Example
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Genetic Drift

• In a population, an allele can become more or
  less common solely due to chance genetic
  drift. (Individuals can immigrate., emigrate,
  etc.)
• Greatest effect in small populations

Founder Effect Individuals leave a population
to initiate a new population, or change another
population
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Genetic Equilibrium
No change in allele frequency genetic
equilibrium
Hardy-Weinberg Principle a mathematical way to
quantitatively determine whether a population is
showing evolution by studying allele frequencies

• The determination only works when there is no
  evolution (genetic equilibrium). Need to have
  these conditions
• Random mating
• Very large population
• No immigration nor emigration
• No mutations
• No natural selection

Genetic Equilibrium
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Speciation
The process of forming new species
If an allele mutation allows better fitness for
a population, then that allele will become more
common

• WHAT causes a species to form two (or more) new
  species?
• Isolating Mechanisms causing reproductive
  isolation
• Behavioral
• Geographic
• Temporal (time)

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Behavioral Isolation
A form of reproductive isolation in which two
populations that may be able to mate have
differences in courtship rituals or other types
of behavior that prevent them from interbreeding
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Geographic Isolation
A form of reproductive isolation where two
populations are separated by a geographical
border keeping them from interbreeding.
Northern Spotted owl video
Mexican Spotted owl video
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Temporal Isolation
A form of reproductive isolation in which the two
species reproduce at different times of the day,
season, or year. Wood and leopard frogs are an
example of two similar species whose ranges
overlap.
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Darwins Finches Speciation Example
Gene pool changes
Founders arrive
Geographic isolation
Speciation the formation of new species takes
place over many years and is influenced by many
factors. Eventually, changes in the gene pool
happens as a result of the birds adapting to
their local environment. If some birds from
island B fly back to island A, they may have
changed enough that they can no longer interbreed
(reproductive isolation). If they both exist in
island A, they will compete for local resources,
perhaps causing further evolutionary changes.
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Unanswered Questions
How does evolution add information? Mutation
explains how existing genetic information is
changed, but it doesn't explain where new genetic
information comes from. How can evolution be so
quick? Millions of years can seem like a long
time to the average person, but in terms of
evolution, it's quite quick. Current evolutionary
theory hasn't explained how a one-celled organism
can evolve into an organism as complex as a human
being in the time available in Earth's history.
Where did the first living cell come from? In
order for mutation and natural selection,
processes essential to evolution, to operate,
life must already exist. The current hypothesis
is that life formed spontaneously from chemical
reactions about 4 billion years ago, but is this
really Possible?
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Whats next?