Darwinian natural selection
The logic of The Origin of Species
by Means of Natural Selection
Darwin’s four postulates
1. Individuals within populations are variable
variation
2. Some of these variations are passed on to
offspring
heritability
from the Illustrated London News (1851), in Secord (1981)
Darwin’s four postulates
3. In every generation, more offspring are
produced than can survive. Some
individuals survive and reproduce better
than others.
•
Most populations are stable in number over
years, despite great reproductive potential
(adopted from Malthus)
Darwin’s four postulates
4. The individuals that survive and reproduce
the most are those with favorable variations.
They are naturally selected.
1.
Survival and reproduction are not random
2.
They are linked to the variation among
individuals
Darwinian natural selection
Selection on beak shape in
Galápagos (“Darwin’s”) finches
There are 15 species of
“Darwin’s” finches, derived from
the grassquit (genusTiaris) of
mainland Central and South
America
This complex of species is closely related but shows
remarkable variation in beak size and shape
This great
variety of bill
characteristics
allows use of a
broad
spectrum of
diets
Cactospiza pallida
15 species are on the
Galápagos islands off Ecuador,
with one species on Isla Cocos
off Costa Rica
*
The ground finches:
a phylogenetic
group primarily of
fruit and seed eaters
The medium ground
finch Geospiza fortis
cracks seeds at the
base of its thick bill
Peter and Rosemary
Grant and colleagues
have tagged ~100%
of the G. fortis
population on
Daphne Major
(~1200 birds
average), and studied
them for > 30 years
Testing postulate 1
Variation
1976 Data
Figure 3.6 Variation in beak depth in G. fortis. The
finches obey Darwin’s 1st postulate: the population on
Daphne Major is highly variable for beak shape (e.g. depth)
and beak size.
Testing postulate 2
Heritability
Figure 3.7 Heritability of beak depth in G. fortis. Peter Boag has
shown that variation is passed to offspring (Darwin’s Postulate 2).
Tagging allows them to follow parent/offspring relationships.
Offspring beak depth resembles that of their parents.
Testing postulate 3
Do only a fraction of offspring
survive to reproduce?
The Galápagos
have wet (Jan May) and dry
seasons (June Dec)
Extreme drought
occurred during
the 1977 wet
season (1/5th
average rainfall)
The drought led to a severe reduction in the number of
seeds available to G. fortis.
During the
drought, the usual
variety of plants
did not set seed.
Tribulus cistoides
is an annual plant
whose seeds
were the majority
available during
the 1977 drought
This led to
larger and
harder seeds
available on
average to
ground finches.
Only large birds
with deep,
narrow beaks
can crack open
and feed on
these harder
seeds.
Only 16% of the ground finch population survived the
drought to reproduce in later years. Darwin’s postulate 3
held true—more offspring were produced than survived.
Testing postulate 4
Did natural selection occur?
Figure 3.9 Beak
depth before
and after the
drought. Was
survival nonrandom with
respect to beak
shape (Darwin’s
postulate 4)?
Yes—the
average survivor
of the drought
had a deeper
beak than pre1977.
This proves that the 1977
drought produced natural
selection for deeper beaks
But did the population of G. fortis
evolve?
Evolution is a change, over
generations, in the genetic
composition of a population.
Evolution is a response to
natural selection—did this
occur?
To find out, Grant and Grant
measured beaks of the
offspring of the survivors.
Offspring beaks had become
deeper, proving that genetic
change over generations
(evolution) had occurred.
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