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 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

Adaptive radiationGalapagos 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: drought130 to 24 mm precipitation A natural selector

Was there differential survival?

Effect of naturalselection

1.

2.

3.

The interplay

Seed abundance

Number of finches

Seed characteristicsof surviving plants

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 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)