Origin of
Species
Chapter 24
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What you need to know!
• The difference between microevolution and
macroevolution.
• The biological concept of species.
• Prezygotic and postzygotic barriers that
maintain reproductive isolation in natural
populations.
• How allopatric and sympatric speciation are
similar and different.
• How an autopolyploid or an allopolyploid
chromosomal change can lead to sympatric
speciation.
• How punctuated equilibrium and gradualism
describe two different tempos of speciation. 2
Species
• A population (or group of populations) who
can mate and produce fertile offspring
• Speciation: One species transforms into
another
• A Population transforms over time as
adaptations add up in its gene pool
(microevolution)
• Macroevolution: is major developmental
changes spawning new branches of species
– the evolution of the spinal cord: vertebrates
– the evolution of feathers: birds
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Adaptations
• A genetic change that makes
an organism better suited to
their environment
• Natural selection facilitates
adaptation
Special adaptations:
• Mimicry: mimicking another
organism that is potentially
dangerous
• Camouflage: blending in with
the environment
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Reproductive Isolation
Prezygotic (prevent mating and/or hinder
fertilization):
1. Habitat isolation: don’t share a habitat
2. Behavioral isolation: different mating
practices
3. Temporal isolation: sex at different times of
day or year
4. Mechanical isolation: not physically
possible
5. Gametic isolation: sperm cannot fertilize
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egg
Reproductive Isolation
Postzygotic (fertilized eggs don’t develop
into fertile adults):
1. Reduced hybrid viability: genetic
incompatibility terminates pregnancy
2. Reduced hybrid fertility: viable
offspring that cannot reproduce
(mule)
3. Hybrid breakdown: viable offspring
that grow and reproduce but their
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offspring are weak or sterile
Speciation
Allopatric Speciation
• random mating becomes impossible due to
geographic isolation (rivers, canyons,
mountains)
• New species evolve on the fringes of large
ancestral populations
Sympatric Speciation
• Sudden mutation within a population leads to
new species
• Polyploidy plants mutate by altering
chromosome number in a non-disjunction
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error
Adaptive Radiation
• Colonization of
new habitat
– New habitats
usually vary from
the original
habitat
• Example
Darwin’s Finches
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Rate of Change
• Species that change slowly are K
(carrying capacity) – selected
– elephants
• Species that change quickly are r (growth
rate) – selected
– Bacteria
• Gradualism: species descended from a
common ancestor and gradually diverge
• Punctuated equilibrium: period of
apparent stasis followed by rapid change
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