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Figure 25.15
Medium ground finch
(Geospiza fortis)
Daphne Major
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
Figure 25.16
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.
Figure 25.16-1
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.
Figure 25.16-2
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.
Figure 25.16
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.
Figure 25.16
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.
What
statistical
value is a
measure of
population
variation?
clicker!
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.
Figure 25.16-4
is natural
variation
evident?
did the
drought select
individuals of
better fitness
for increased
reproduction?
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.
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
Figure 25.18
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
Figure 25.19
Bacteria
Which dimension
shows the
progression of time?
Last
universal
common
ancestor
(LUCA)
Archaea
Flowering
plants
Mosses
Tapeworms
The branches on the tree
represent the relatedness
of populations. All of the
species have evolved from
a common ancestor. None
is “higher” than any other
Vertebrates
Fungi