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17.3 The Process
of Speciation
The Process of Speciation
Lesson Overview
Evolutionary Classification
• Cladogram-shows how
evolutionary lines branched off
from common ancestors
The Process of Speciation
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• Humans more closely related to
turtle than a frog
• Tuna more closely related to
human than shark
The Process of Speciation
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Genetic equilibrium – a MODEL to explain
what would happen to a hypothetical, nonevolving population
Allele frequencies NOT changing
Hardy-Weinberg principle – states that allele
frequencies in a population should remain
constant unless something causes them to
change
The Process of Speciation
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Hardy-Weinberg
p2 + 2pq + q2 = 1 AND p + q =1
P = frequency of dominant allele
Q = frequency of recessive allele
(frequency of AA) +(frequency of Aa) +
(frequency of aa) =
100% AND (frequency of A) + (frequency of
a) = 100%!
The Process of Speciation
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Hardy-Weinberg
Genetic equilibrium occurs in large
populations!
5 conditions can disrupt genetic
equilibrium and cause evolution to
occur:
1. Nonrandom mating (sexual selection)
2. Small population size – leads to of
genetic drift
3. Migration (immigration or
emmigration) – aka gene
flow into or
out of a population
4. Mutations
5. Natural selection – different fitness
exists for different
alleles
The Process of Speciation
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Speciation
Reproductive isolation occurs when 2
populations can’t interbreed - causes
speciation!
• Once reproductive isolation occurs,
natural selection increases the
differences between the separated
populations.
1. Behavioral isolation – different
courtship.
2. Ecological/habitat isolation – can
only mate in specific or preferred habitats
3. Mechanical isolation – no sperm is
transferred.
4. Gametic isolation – no fertilization
of egg occurs.
5. Temporal isolation – reproduce at
different times.
Geographic isolation – population becomes
divided
(isolated) by a physical barrier.
The Process of Speciation
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Rates of speciation
Gradualism – slow, steady change leading
to new species
Punctuated equilibrium –brief periods of
rapid change leads to the formation of new
species
Rapid change occurs when a small
population is isolated from the rest of the
population or migrates
The Process of Speciation
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Patterns of evolution
Divergent evolution – related species evolve
different traits.
usually a result of the same species to
different and isolated environments
through genetic drift and natural selection.
adaptive radiation
can also happen due to random mutation
Divergent evolution is the accumulation of
differences between groups which can lead to the
Ex. Dinosaurs
Darwin’s finches
video
The Process of Speciation
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Convergent Evolution
Convergent evolution – similar
structures are produced in
distantly related organisms
Convergent Evolution :two unrelated
species independently evolve similar
traits to cope with specific
evolutionary challenges.
Sometimes convergent evolution is
so powerful that creatures that
began as entirely different animals
start to look almost the same,
Ex. Mammals that feed on
ants/termites evolved
The Process of Speciation
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The Process of Speciation
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Behavioral Isolation
Behavioral isolation occurs when two populations that are
capable of interbreeding develop differences in courtship
rituals or other behaviors.
The Process of Speciation
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Geographic Isolation
Geographic isolation occurs when two populations are
separated by geographic barriers such as rivers, mountains,
or bodies of water.
For example, 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.
Separate gene pools formed, and genetic changes in one
group were not passed on to the other.
The Process of Speciation
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Temporal Isolation
Temporal isolation happens when two or more species
reproduce at different times.
For example, three species of orchid live in the same rain
forest. Each species has flowers that last only one day and
must be pollinated on that day to produce seeds. Because
the species bloom on different days, they cannot pollinate
each other.
The Process of Speciation
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Testing Natural Selection in
Nature
What did the Grants’ scientific
investigation show about Galápagos
finches?
long-term studies demonstrating
evolution in action in Galápagos
finches. Demonstrated how very rapid
changes in body and beak size in
response to changes in the food supply
are driven by natural selection.
Variation within a species increases the
likelihood that the species can adapt
and survive environmental change.
The Process of Speciation
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Natural Selection
The Grants’ data showed individual finches with
different-sized beaks had different chances of surviving
drought. When food was scarce, individuals with the
largest beaks were more likely to survive.
The Grants observed that average beak size in that finch
population increased dramatically over time.
The Process of Speciation
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Finch Research in Perspective
The Grants documented directional selection in nature.
Their data also shows that competition and climate change
drive natural selection.
In addition, their work shows that variation within a species
increases the likelihood that the species can adapt and
survive environmental change.