1. Review:
Galápagos Islands
plate motion
subduction zone

2. Galápagos Islands Cocos Plate Nazca Plate Darwin Wolf Pinta Marchesa
Genovesa
Santiago
Fernandina
Santa Cruz
San Cristobal
Isabela
Santa Fe
Floreana
Española

3. Figure 25.15 Freeman Biological Science 5th edition Pearson
Medium ground finch
(Geospiza fortis)
Figure Studying Evolution in Action on the Galápagos.
Daphne Major 3

4. 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:
Heritable variation leads to
Differential reproductive success 4

5. 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.
Average
1976: Before drought
N  751 (all birds on island)
Figure A Natural Experiment: Changes in a Medium Ground Finch Population in Response to a Change in the Environment (a Drought).
Number of finches
1978: After drought
N  90 (survivors)
Average beak
depth in the
population
increased
Beak depth (mm)
Natural selection occurred. The characteristics of the
population have changed. 5

6. Figure 25.18 Freeman Biological Science 5th edition Pearson
Bmp4 is a gene whose expression is shown by in situ hybridization:
Lower Bmp4 expression
(dark area) in embryo’s beak
Higher Bmp4 expression
(dark area) in embryo’s beak
2 mm
2 mm
Deep
adult
beak
Figure Changes in Bmp4 Expression Change Beak Depth and Width.
Shallow
adult beak
Geospiza fortis
Geospiza magnirostris 6

7. 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

8. 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!