1. Learning question: What factors affect evolution?
Title: The roles of genes and environment in evolution 12th February 2014
Learning question: What factors affect evolution?
Homework: Module test next week – study this unit
Starter: what do these two organisms have in common?

2. Key words Genetic drift Selection pressure Stabilising selection
Directional selection
Evolutionary force
Environmental resistance
Isolating mechanisms

3. Learning Outcomes
Explain, with examples, how environmental factors can act as stabilising or evolutionary forces of natural selection.
Explain how genetic drift can cause large changes in small populations.

4. Variation and Natural Selection
The set of alleles in a population is it’s gene pool
Each individual can have any combination of alleles in the gene pool
producing variation
Some individuals more likely to survive
They reproduce and pass genes on to offspring
Advantageous alleles become more frequent in the population

6. Environmental Resistance
Environmental factors that limit the growth of a population offer environmental resistance
These factors can be biotic (living factors) or abiotic (non-living factors)

7. Selection pressures
An environmental factor that “selects” for some members of a population over others
Offers an advantage onto certain individuals

9. Discuss!
Explain to your neighbour what environmental resistance and selection pressures are.
If the person speaking is incorrect, the partner must explain the correct term and give an example.

10. Stabilising Selection
If the environment stays stable
The same alleles will be selected for in successive generations
Nothing changes, this is called stabilising selection

11. Stabilising Selection

12. Stabilising Selection

13. Directional Selection
Change in the environment resulting in a change in the selection pressures on the population
Previously disadvantageous alleles maybe selected for
Change in the genetically determined characteristics of subsequent generations of the species
A.k.a. evolution

14. Directional Selection

15. Directional Selection

16. Discuss!
Explain to your neighbour what stabilising selection and directional selection are.
If the person speaking is incorrect, the partner must explain the correct term and give an example.

17. Genetic Drift
A change in the gene pool and characteristics within the population.
This change has occurred by chance rather than as the result of natural selection.

18. Genetic Drift and Islands
Genetic drift is thought to happen relatively frequently in populations on islands.
Small populations
Geographically separated from other members of their species
Evidence
Many isolated islands have their own endemic species of plants and animals

19. Genetic Drift Reduces genetic variation
Reduce the ability of the population to survive in a new environment
May contribute to the extinction of a population or species
Could lead to the production of a new species

20. Genetic Drift – Frog Hoppers
The colours of the common frog-hopper are determined by seven different alleles of a single gene.
The range of colours and their frequencies, on different islands in the Isles of Scilly, are very variable,
There are different selection pressures on the different islands

21. Genetic Drift – Frog Hoppers

22. The answers

23. Your task Complete questions 1-4 from page 141 of your textbook.
Correct your work using the mark scheme at the back of the book.

31. Natural selection video