The Genetical Theory of Natural Selection

• Who wrote this (1858)?

– “It occurred to me to ask the question, why do some die and some live.

– And the answer was clearly, that on the whole the best fitted lived. From the effects of disease the most healthy escaped; from enemies, the strongest, the swiftest, or the most cunning; from famine, the best hunters or those with the best digestion; and so on.

– Then I at once saw that the ever present variability of all living things would furnish the material from which, by the mere weeding out of those less adapted to the actual conditions, the fittest alone would continue the race.

– There suddenly flashed upon me the idea of the survival of the fittest.

– The more I thought over it, the more I became convinced that I had at length found the long-sought-for law of nature that solved the problem of the Origin of Species.”

Where natural selection occurs

Fundamental patterns of intraspecific natural selection

Disruptive

Rare

Economist. In 1838, Darwin reads essay on population. Rate of populationgrowth is greater than the rate of increase of the food supply.

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 on parents of the present generation.

• 1. Parental population + environment (natural selectors)

• 2. Part of population selected to reproduce• 3. Transmit their heritable characteristics to the

new generation.• But any evolutionary changes were based on

selection from parental phenotypic variation.