selection:

Question 1

What is directional selection

Answer 1

= * Mean in population represents optimum for existing conditions
* Environmental change
= may produce new selection pressure that favours an extreme characteristic
* When conditions change optimum necessary for survival also changes
* Some organisms will possess the new optimum
* Over time selection means these will predominate and the mean will shift

Question 2

What is stabilising selection

Answer 2

= * Occurs in all populations where environment is stable.
* Selective pressure at both ends of distribution.
* Favours the average.
* Tends to eliminate extremes.
* Reduces variability, the size of the range within population.
* Reduces opportunity for evolutionary change.

Question 3

What is disruptive selection

Answer 3

= Disruptive is the opposite of stabilising selection.
The selection pressure favours the extreme characteristics.