1. Pages 231 - 234 Specification area 4.4
Types of Selection
Pages
Specification area 4.4
Test coming up…

2. Starter
Natural selection results in organisms which are better adapted to their environment. The adaptations may be anatomical, physiological or behavioural
Discuss the difference between the three emboldened words

3. Syllabus

4. Objectives Describe what selection is
Describe the environmental factors which exert selection pressure
Explain what Stabilising and Directional selection are

5. Distribution of phenotypes that are under “polygenic” control
A “NORMAL” DISTRIBUTION

6. What is a “Selection Pressure”?
“Any factor in the environment that can affect the chance of an organism’s survival”
Selective Pressure Leads to Evolution. Selective pressures drive natural selection. Some members of the population will not survive and reproduce and thus will not pass on their genes into the next generation (
During the video – list as many examples of selection pressures that you can
Predation
Climate (any named variable)
Food supply
Disease
Natural changes to habitat
Habitat changes due to man’s activities

7. Types of selection Directional
Directional – occurs when individuals that vary in one direction from the mean of the population and with a particular advantage are selected for (e.g. antibiotic resistant bacteria example page 232.) Occurs when the selection pressure changes
Mean changes
Standard deviation stays same

8. Directional Selection

9. Evolution and antibiotic resistance in bacteria – an example of “Directional Selection”1 2 3

10. Stabilising
Stabilising – favours average individuals e.g. birth mass of infants example page Extreme variation / individuals at BOTH ends of the distribution are selected against when conditions remain constant for long periods of time
Mean stays same
Standard deviation gets smaller
Click on image for video describing an example of stabilising selection

11. Stabilising Selection
– favours average individuals e.g. birth mass of infants example page Extreme variation / individuals are selected against at BOTH ends of the distribution when conditions remain constant
Stabilising Selection
Use page 233 to describe the data presented as evidence for stabilising selection in human birth mass

12. Questions for class Summary q page 234 They must be cuckoo p234
As hwk is revision

13. Extension - Heterozygous advantage

14. Homework
Revise for test

15. Extension activity Rich questions:
Fossils indicate that crocodiles and sharks have remained relatively unchanged for millions of years. Does this indicate that they are no longer subject to natural selection?
Are humans still evolving? If so how?

16. Keywords Directional selection Stabilising selection
Selection pressure
Normal Distribution curve