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Speciation
Professor Andrea Garrison
Biology 11
Illustrations ©2010 Pearson Education,
Inc. , unless otherwise noted
Speciation
• Natural selection not disputed
– Natural selection = microevolution (small genetic
changes)
• Special cases of selection due to chance
changes
– Genetic drift
• Founder Effect – subgroup leaves population and stays
together
• Bottleneck– population decimated by disaster
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Genetic Drift
• Chance loss of alleles
– Found in small populations
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Genetic Drift
• Founder Effect
– Establishment of small population whose gene pool differs from
parent population
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Genetic Drift
• Bottleneck Effect
– Natural disasters decrease population to extent some alleles
underrepresented by chance
– Difficult to maintain healthy population
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Speciation
• Natural selection drives evolution by favoring
genetic variations that enable organisms to
survive and reproduce
• Works on level of gene pool
– Gene pool = all alleles in population
– Affects entire population
• Agents of change
– Mutation (= chance changes in DNA)
– Sexual reproduction (speeds up opportunities for
selection of alleles)
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Speciation
• What is a species?
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Speciation
• What is a species?
– Group of similar organisms which can interbreed
and produce fertile offspring
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Speciation
• What is a species?
– Group of similar organisms which can interbreed
and produce fertile offspring
• Thus, different species are organisms that
differ in one or more characteristics and do
not interbreed extensively in nature, even if
they occur together
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Speciation
• Each species has its own niche
• What is a niche?
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Speciation
• Each species has its own niche
• What is a niche?
– The resources used by a species and the manner
in which those resources are used
– Encompasses all aspects of a species’ environment
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Speciation
• Each species has its own niche
• What is a niche?
– The resources used by a species and the manner
in which those resources are used
– Encompasses all aspects of a species’ environment
• Each species is biologically suited (physically,
metabolically, personality-wise) to occupation
of its own niche
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Speciation
• How does one species evolve from another?
A → A1
non-branching evolution
A1
A
divergence (branching evolution)
A2
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Speciation
• Key to speciation is presence or absence of
reproduction
– Local subgroups may have chance mutations
– Not new species until so different they cannot
interbreed
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Speciation
• Population = all members of a species within
given geographic area (often taken to mean
individuals interbreeding with one another)
• Non-branching evolution A → A1
– changes over time, A1 cannot interbreed with A
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Speciation
• Population = all members of a species within
given geographic area (often taken to mean
individuals interbreeding with one another)
• Divergence – A1 and A2 must be kept from
interbreeding until they are so different they
cannot interbreed
A1
A
A2
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Speciation
• Divergence occurs because local populations
breed only with their neighbors, and not those
members of the same species on “the other side
of the hill”
– If members of two subgroups interbreed, no
divergence
• They share any chance mutations
– If members of two subgroups do not interbreed,
divergence can occur
• No sharing of chance mutations
– Chance mutations are different
– Two different environments select differently for different
mutations
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Speciation
• Even some organisms recognized as separate
species today are actually capable of
interbreeding, and would converge back to
one species if the opportunity existed
• REPRODUCTIVE ISOLATING MECHANISMS
– Prevent reproduction between two populations
– Maintain separate species
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Reproductive Isolating Mechanisms
• Prezygotic mechanisms
– Geographical isolation
• Geographical ranges do not coincide
• Species never encounter each other
– Example: English Oak in Europe and Valley Oak in
California
• Never close enough to interbreed, although they could
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Reproductive Isolating Mechanisms
• Prezygotic mechanisms
– Ecological (habitat) isolation
• Two species live in same area, but different habitats
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Reproductive Isolating Mechanisms
• Prezygotic mechanisms
– Temporal isolation
• Closely related species mate at different times of the year
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Reproductive Isolating Mechanisms
• Prezygotic mechanisms
– Behavioral isolation
• Closely related species with different mating rituals
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Reproductive Isolating Mechanisms
• Prezygotic mechanisms
– Mechanical isolation
• Structural differences between species (copulatory organs, pollen
grains, etc.
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Reproductive Isolating Mechanisms
• Prezygotic mechanisms
– Gametic isolation
• Chemical differences between gametes (e.g., proteins on surface of
gametes)
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Reproductive Isolating Mechanisms
• Postzygotic mechanisms
– Reduced hybrid viability
• Hybrid is weak or dies
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Reproductive Isolating Mechanisms
• Postzygotic mechanisms
– Reduced hybrid fertility
• Hybrid is sterile
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Reproductive Isolating Mechanisms
• Postzygotic mechanisms
– Hybrid breakdown
• Hybrid reproduction
produces weak offspring
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