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Review:
Galápagos Islands
plate motion
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Darwin
Wolf
Galápagos Islands
Cocos Plate
Nazca Plate
Pinta
Marchesa
Genovesa
Santiago
Fernandina
Santa Cruz
San Cristobal
Isabela
Santa Fe
Floreana
Española
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Figure 25.15 Freeman Biological Science 5th edition Pearson
Medium ground finch
(Geospiza fortis)
Daphne Major
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Darwin’s Four Postulates
• Evolution is a logical outcome of four
postulates…
– populations have natural variation
– the organism’s features are heritable
– more offspring are produced than can survive
– some individuals produce more offspring because
of the environment
• Modern biologists condense Darwin’s four
steps into two statements: Evolution by
natural selection occurs when:
1. Heritable variation leads to
2. Differential reproductive success
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Figure 25.16 Freeman Biological Science 5th edition Pearson
Did natural selection on ground
finches occur when the environment changed?
Beak characteristics changed in response to a drought.
No changes in beak characteristics occurred
in response to a drought.
Weigh and measure all
birds in the population
before and after the
drought.
Number of finches
Average
Average beak
depth in the
population
increased
1976: Before drought
N  751 (all birds on island)
1978: After drought
N  90 (survivors)
Beak depth (mm)
Natural selection occurred. The characteristics of the
population have changed.
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Figure 25.18 Freeman Biological Science 5th edition Pearson
Bmp4 is a gene whose expression is shown by
in situ hybridization:
Lower Bmp4 expression
Higher Bmp4 expression
(dark area) in embryo’s beak
(dark area) in embryo’s beak
2 mm
Shallow
adult beak
Geospiza fortis
Deep
adult
beak
2 mm
Geospiza magnirostris
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Island Development
• Island appears
• Marine/shore colonized
• Island colonized by
plants
• Terrestrial biota
establishes
• Biota stabilizes
• Island moves away
from hot spot
• Waif founders lack
competition
• Speciation: adaptive
radiation to fill empty
niches
• Endemic species
evolve
• Sequence of islands
should reflect patterns
of speciation
• Endemics found on
just one island, one
volcano
• Humans arrive
• Introduced exotic
species are released
• Endemics that may be
poor competitors…go
extinct
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Cladistic Analysis
• 1960s Development of Cladistic Analysis
– Determines Possible Pathways of Evolution
• Characters and Polarized Character States
– Anatomy, Morphology, Behavior, Chemical, Gene
Sequence
– Plesiomorphies-Ancestral character states
– Apomorphies-Derived character states
• Construction of Cladogram-computers!
– Reveal homoplasies (parallel, convergent),
reversals
– Most-parsimonious (fewest steps) cladogram
– Not necessarily unique!
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