Download Modes of Natural Selection

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The Gene Pool
•Members of a species
can interbreed &
produce fertile
offspring
Species have a shared
gene pool
Gene pool – all of the
alleles of all individuals
in a population
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The Gene Pool
•Different species
do NOT exchange
genes by
interbreeding
Different species
that interbreed
often produce
sterile or less viable
offspring e.g. Mule
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Populations
•A group of the
same species living
in an area
No two individuals
are exactly alike
(variations)
More Fit
individuals survive &
pass on their traits
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Speciation
•Formation of new
species
•One species may
split into 2 or more
species
A species may
evolve into a new
species
Requires very long
periods of time
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Modern
Evolutionary
Thought
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Modern Synthesis Theory
• Combines Darwinian
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selection and
Mendelian inheritance
Population genetics study of genetic
variation within a
population
Emphasis on
quantitative
characters (height,
size …)
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Modern Synthesis Theory
• 1940s – comprehensive
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theory of evolution
(Modern Synthesis
Theory)
Introduced by Fisher &
Wright
Until then, many did not
accept that Darwin’s
theory of natural
selection could drive
evolution
S. Wright
A. Fisher
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Modern Synthesis Theory
• TODAY’S theory on evolution
• Recognizes that GENES are responsible for
the inheritance of characteristics
• Recognizes that POPULATIONS, not
•
individuals, evolve due to natural selection
& genetic drift
Recognizes that SPECIATION usually is
due to the gradual accumulation of small
genetic changes
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Modern Synthesis Theory
Darwin & Wallace (Natural Selection)
+ Mendel
(Genetics)
+ Wright & Fisher (Population Genetics)
+ Mayr & others
(Biogeography, Paleontology)
____________________________________
= Modern Evolutionary Synthesis
• a more complete understanding of how
evolution proceeds in the wild
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Macro- vs. Micro- Evolution
• Macroevolution - change that results in
the formation of new species, new
taxonomic groups, evolutionary trends,
adaptive radiation, and mass extinction.
• Microevolution: the generation-togeneration change in the frequency of
alleles in a population
– Evolution at its smallest scale
– The core definition of evolution
Microevolutionary changes give rise to
macroevolution
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Microevolution
• Changes occur in gene pools due to
mutation, natural selection, genetic drift,
etc.
• Gene pool changes
cause more variation
in individuals in the
population
• Example:
Bacteria becoming
unaffected by
antibiotics (resistant)
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Microevolution
of Species
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Causes of Microevolution
• Genetic Drift
- the change in the gene pool of a small
population due to chance
• Natural Selection
- success in reproduction based on heritable
traits results in selected alleles being passed
to relatively more offspring (Darwinian
inheritance)
- Cause ADAPTATION of Populations
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Causes of Microevolution
• Gene Flow
-is genetic exchange due to the migration of
fertile individuals or gametes between
populations
• Mutation
-a change in an organism’s DNA
-Mutations can be transmitted in gametes to
offspring
• Non-random mating
-Mate selection may be influenced by
geographic proximity
-Individuals may select a mate with traits
similar to their own
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Genetic Drift
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Genetic Drift
• Changes in a population’s allele
frequencies due to chance
• Drift can cause rapid change in
small populations
• A problem when populations get too
small
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Factors that Cause Genetic Drift
•Bottleneck Effect
-a drastic reduction in population
- (volcanoes, earthquakes, landslides …)
-Reduced genetic variation
-Smaller population may not be able to
adapt to new selection pressures
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Factors that Cause Genetic Drift
•Founder Effect
-occurs when a new colony is started
by a few members of the original
population
-Reduced genetic variation
-May lead to speciation
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Founder’s Effect
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Loss of Genetic Variation
• Cheetahs have little genetic variation in
their gene pool
• This can probably be attributed to a
population bottleneck they experienced
around 10,000 years
ago, barely avoiding
extinction at the end
of the last ice age
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Natural Selection
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Natural Selection
• Natural selection – differential success
in survival and reproduction
• N.S. leads to populations becoming
better adapted to their environments
(adaptation)
• N.S. works most powerfully in large
populations (effect of drift is small,
and changes due to N.S. are preserved)
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It IS possible to
measure Natural
Selection in the
wild.
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Modes of Natural Selection
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Modes of Natural Selection
• Directional Selection
- Favors individuals at one end of the
phenotypic range
- Most common during
times of environmental
change or when moving
to new habitats
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Modes of Natural Selection
• Disruptive selection
-Favors extreme over intermediate
phenotypes
-Occurs when
environmental change
favors an extreme
phenotype
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Modes of Natural Selection
• Stabilizing Selection
- Favors intermediate over extreme
phenotypes
- Reduces variation and maintains the
current average
- Example:
- Human birth weight
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Modes of Natural Selection
• Sexual Selection
- Traits that enable an organism to
reproduce, without necessarily helping
them to survive are selected for
- Any inherited trait that improves the
mating success of certain individuals
will become more
pronounced in
succeeding
generations.
Example: Bright plumage
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Gene Flow
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Gene Flow
• Migration of individuals can change
the allele frequencies in a population
• Immigration – movement into a
population
• Emigration – movement out of a
population
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Mutation
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Mutations
• Mutations are the origin of all
differences between alleles
• But mutations are rare, so they must
still spread by drift or selection if
they are going to impact allele
frequencies in a population
Sickle cell anemia is
the result of a single
point mutation
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Speciation
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Speciation
• Speciation is the formation of a new
species
• A Species is a population of
organisms whose members can
interbreed and produce fertile
offspring.
So, how does one species become two?
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Reproductive Isolation
• Reproductive Isolation occurs when a
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population splits into two groups and the
two groups no longer interbreed.
When populations become reproductively
isolated, they can evolve into two
separate species
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Mechanisms of Isolation
• Behavioral Isolation occurs when two
populations that are capable of
interbreeding develop differences in
courtship rituals or other behaviors
• Ex: Eastern meadowlarks will not respond to
western meadowlark songs, and vice versa.
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Mechanisms of Isolation
• Geographic Isolation occurs when two
populations are separated by geographic
barriers such as rivers, mountains or
bodies of water
•
Ex: The Kaibab squirrel is a subspecies of the
Abert’s squirrel that formed when a small
population became isolated on the north rim of
the Grand Canyon.
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Mechanisms of Isolation
• Temporal Isolation occurs when two or
more species reproduce at different
times.
• Ex: Two species of frogs inhabit the same
area, but one reproduces from January to
March, whereas the other reproduces from
March to May
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