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 population growth 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 radiation Galapagos finches

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

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 mm precipitation dropped to 24 mm The base level natural selector Was there differential survival? Effect of natural selection

The interplay Seed abundance Number of finches 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 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.