Darwin's Logic and quantifying natural selection Flashcards
First Three facts
1) ALl species have such great potential fertility that their population size would increase exponentially if all individuals that were born reproduces successfully
2) Except for minor annual fluctuations and occasional major fluctuations, populations normally display stability in size
3) Natural resources are limited. In a stable environment they remain relatively constant
Inference 1
Since more individuals are produced than can be supported by the available resources but population size remains stable, it means that there must be a fierce struggle for existence among the individuals of a population, resulting in the survival of only a part, often a very small part of the progeny of each generation .
Fact 4, 5
4) no two individualls are exactly the same, rather every population displays enormous variablitity
5) Much of variation is heritable
Summary: three postulates
1) Individuals are variable in some traits
2) At least some of this variation is heritable
3) there is a struggle for survival or reproduction and some individuals fare better than others
Strong heritablity (graph parent vs offspring)
Positive slope, low scatter of points along trend line
“S”
Selection differential : the difference between the mean of the population before selection and the mean of the population after selection
“R”
Response to selection: The difference between the mean of the population before selection and the mean of the offspring from the surviving parents
Directional Selection
favors phenotypes at on end of a distribution, evoultion in that direction
Stabilizing Selection
favors values toward the middle of the distribution. Fitness of organisms at either end is lower
Disruptive Selection
favors phenotypes towards the ends of the distribution
