Download Geospiza fortis

Development of Darwin’s idea
An example of natural selection
• The premises
• 1. Populations exhibit phenotypic variation.
• 2. The phenotypic variation has a genetic component
(can be inherited).
• 3. Differential reproductive success among members of
the population.
• Survival and reproduction is, on-average, nonrandom
• 4. Phenotypic variation shifts between generations in
response to a changing environment.
Adaptive radiation
Galapagos finches
Medium ground finch
Geospiza fortis
generation time: 4.5 years
life span c. 16 years
Research of Peter and Rosemary Grant: 1973 - present
120 m
N = c. 1,200
7 cm/yr
Hot spot
2-3 my
4-5 my
1 my
1: Is the population phenotypically variable?
Geospiza fortis
2: Is the variation heritable?
(heritability: proportion of phenotypic variation due to genetic variation; c. 65%)
Evolution!
Effect of natural
selection
1977: drought
130 mm precipitation dropped
to 24 mm
The base level natural selector
Was there differential survival?
The interplay
2.
Number of finches
Seed abundance
1.
3.
The second level natural selector
Seed characteristics of surviving
plants
Had evolution taken place? Significant difference in beak size.
Note: natural
selection is always
one generation
behind the
expression of modified
phenotypes
Natural selection cannot anticipate future
“needs” of a population
• Evolutionary change is based selection in the
previous generation.
• 1. Parental population + environment (natural
selectors)
• 2. Part of population selected to reproduce
• 3. Transmission of heritable characteristics to the
new generation (e.g., size of the beak).
– But the change was based on phenotypic variation among
their parents.
Evolution tends to take a branching form.
: species originate by divergence from common ancestors
Darwin’s 1859 illustration (On the Origin of Species by Means of Natural Selection)