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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 radiationGalapagos finches
Medium ground finchGeospiza fortisgeneration time: 4.5 yearslife span c. 16 years
120 mN = c. 1,200
Research of Peter and Rosemary Grant: 1973 - present
Hot spot
7 cm/yr
4-5 my
2-3 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!
1977: drought130 mm precipitation droppedto 24 mm The base level natural selector
Was there differential survival?
Effect of naturalselection
1.
2.
3.
The interplay
Seed abundance
Number of finches
The second level natural selectorSeed characteristics of survivingplants
Had evolution taken place? Significant difference in beak size.
Note: naturalselection is alwaysone generationbehind theexpression of modifiedphenotypes
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.
: species originate by divergence from common ancestors
Evolution tends to take a branching form.
Darwin’s 1859 illustration (On the Origin of Species by Means of Natural Selection)