Download Population Genetics

What Darwin Never Knew
How Genetics is Altering
Evolutionary Thought
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The Tree of Life in the 1830’s
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Tree of Life
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Modern Tree of Life
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Question
• What do these trees tell you about
science?
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Peas
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Historical Development
The Monk and his Peas.
• 1850s Gregor Mendel interbred peas and
kept track of their physical traits.
• In certain traits he found specific ratios
based on the way he interbred the plants.
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Historical Development
• This was the first step in the discovery of
genetics.
• If Mendel discovered genes as the
inherited variation in the 1850s, why didn’t
Darwin include this in On the Origin of
Species?
Darwin Published in 1859
Mendel’s ideas not popular until early 1900’s
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The Gaps in the Darwin’s Knowledge
• How do inherited traits or adaptations pass
from one generation to the next?
• Variation extremely important!
What is the source?
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Historical Development
Birth of Genetics
• Early 1900s Thomas Hunt Morgan bred
fruit flies and studied mutations.
• Mutations do not always create a new
species as had been suggested.
• Source of the variation that Darwin
suggested in his original proposal!
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Historical Development
• It took well over 50 years for scientists to
popularize and identify the inherited
variation that Darwin discussed in his
book.
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Review
• Natural Selection & Evolution act on
natural variation of the population
• This natural variation comes from
differences in DNA sequences.
• DNA is the inherited molecule.
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Differences in DNA Sequences?
• Mutations!
• Discovery of DNA in 1950 lead scientists
to identify what was inherited and how it
was effected.
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DNA and Natural Selection
• Found sources of variation.
Mutations
Gene Shuffling
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Mutations
• These occur randomly in the genetic code.
• They do not seek to “help” organisms.
• Those that increase an organisms ability
to survive and reproduce may be passed
on.
Fitness?
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What is a gene pool?
The gene pool can be defined as:
The total information from all the genes and alleles of the
breeding individuals in a population at a particular time.
The gene pool’s composition
What happens If there is a
consistent change in
allele frequency?
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Current Understanding of Evolution by
Natural Selection
• NOT within one organism!
One organism CANNOT evolve biologically.
• Happens to a population over multiple
generations.
• The change is the allele frequency, the
time is the multiple generations.
Bunny Simulation!
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Genetic Drift
• Natural Selection is NOT the only source
of genetic change.
• Flip a coin…what are my chances?
• Chance events associated with survival
and reproduction not directly effected by
selection pressures.
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Genetic drift
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Population bottlenecks
A population bottleneck occurs when a large, genetically
diverse population is drastically reduced by a catastrophic,
non-selective event, such as a volcanic eruption. Limiting
Factor?
The total genetic diversity of the few survivors is likely to be
much lower than that of the original population.
The cheetah population has an
exceptionally low genetic
diversity. This is thought to be
due to a very narrow bottleneck,
where only a single family group
survived the last ice age.
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The founder effect
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Introducing the Hardy-Weinberg principle
The Hardy-Weinberg principle is a mathematical model
used to calculate the allele frequencies of traits with
dominant and recessive alleles.
The model assumes that the population:

is large

has random mating

is experiencing no selection

has no mutation, emigration or immigration.
If these assumptions are met then the allele frequencies
of the population will remain stable over time.
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Factors affecting the gene pool
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Types of selection
Selection causes some traits to survive and spread, while
others are lost. A selection pressure determines which traits
are successful.
There are three types of selection: disruptive, stabilizing
and directional.
Starting population has
a normal distribution
of traits.
number of
individuals
Selection can be represented using graphs showing the
distribution of individuals with a particular trait.
mode
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trait
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Types of selection
Selection can cause the mode and/or distribution to change.
mode
number of
individuals
mode
number of
individuals
number of
individuals
mode
trait
disruptive
Selection
pressure toward
the extremes
creates two
modal values.
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trait
stabilizing
Selection pressure
toward the center
increases the
number of
individuals at the
modal value.
trait
directional
Selection
pressure toward
one extreme
moves the mode
in this direction.
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Selection pressures
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Humanity as a selection pressure
Human activity provides some of the strongest selection
pressures in the world today.
The widespread use of antibiotics
has exerted a very strong
selection pressure on bacteria.
Any that can survive exposure to
antibiotics can rapidly divide and
produce a resistant population.
MRSA is an example of an
antibiotic resistant bacteria.
What are the implications of the
evolution of antibiotic resistance
for humans?
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Inbreeding and artificial selection
In artificial selection humans decide which members of a
population will breed.
This allows the alleles for
desirable characteristics to be
maintained in the population
and others eliminated. This
technique is used by farmers
to produce animals and plants
with a high yield.
Artificial selection can lead to inbreeding, which reduces
genetic diversity. This can increase the risk of a disease
affecting the whole population.
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Convergent Evolution
• unrelated species become more and more
similar in appearance as they adapt to the
same kind of environment
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Divergent Evolution
• the process of two or more related species
becoming more and more dissimilar.
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Coevolution
• See Clip
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Coevolution
• joint change of two or more species in
close interaction
– Predators and their prey sometimes coevolve
– parasites and their hosts often coevolve
– plant-eating animals and the plants upon
which they feed also coevolve
– plants and the animals that pollinate them
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