An example of natural selection The premises 1. Phenotypic variation in a population. 2. Phenotypic variation has a genetic component 3. Differential reproductive success Survival and reproduction is, on-average, nonrandom 4. Phenotypic variation shifts between generations in response to a changing environment.
Medium ground finch Geospiza fortis generation time: 4.5 years life span c. 16 years
120 m N = c. 1,200 Research of Peter and Rosemary Grant: present Hot spot 7 cm/yr 4-5 my 2-3 my 1 my
Adaptive radiation Galapagos finches
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: drought 130 to 24 mm precipitation A natural selector Was there differential survival? Effect of natural selection
The interplay Seed abundance Number of finches 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 changes 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 Cladistic evolution vs. anagenesis or phyletic evolution
Darwin’s 1859 illustration ( On the Origin of Species by Means of Natural Selection )