Revision: evolution (66 marks Paper 2) Artificial selection and Natural Selection



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REVISION: EVOLUTION (66 marks Paper 2)
Artificial selection and Natural Selection
One of the observations Darwin made during his study of pigeons was about artificial selection.
Question 1
Using examples, describe natural and artificial selection and also highlight the

differences between these two processes.


Artificial selection

- Organisms of a particular species/any example show a great deal of variation√

- Humans√/farmers/scientists select organisms

- with a particular desirable characteristic√

- and interbreed √ them

- with other organisms that also √ have the same desirable characteristic

- to improve this characteristic further √ in the offspring

- They may also choose organisms with different √ desirable characteristics



- to get offspring with a combination√ of these desirable characteristics after many generations √
Examples: Production of improved fruit √/ meat production
Below is another example of artificial selection



  1. Choose the animal or plants that have the best characteristics

  2. Breed them with each other

  3. Choose the offspring with the best combination of characteristics

  4. Allow them to breed with each other

  5. Repeat the procedure many times to improve the characteristics


Advantages



Disadvantages


        • Usually closely related organisms are bred – leading to inbreeding

        • Inbreeding reduces the gene pool of a species

        • This could cause the entire species to be wiped out if exposed to a disease against which it has no resistance


Natural Selection
Natural selection e.g. Finches √ / or any other example
- Organisms of a particular species (population) shows a great deal of variation

- Most species produce a large number of offspring

- Some individuals may have characteristics such as small beak size √ that are favourable

for the environmental conditions

- Others may have characteristics /any example that are unfavourable

- These offspring competewith each other for food, shelter, etc.

- Offspring that have desirable features for obtaining these resources will survive

- Selective pressure by the environment √ due to competition / changing environmental

conditions

- Organisms with favourable characteristics survive and reproduce √ to pass favourable

characteristics to their offspring √

- while organisms with unfavourable characteristics will die out √

- Over time the whole population will have this favourable trait √

- the gene pool of the population has changed √



VARIATION
Sources of variation


  • Mutation

  • Meiosis : - independent assortment / crossing – over

  • Random mating

  • Chance fertilization

Question 2

Below is a sequence of diagrams (A, B and C) representing the changes

in a population of cactus plants on which bucks feed over a period of time.




A




thorns



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