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17.2.1 Explain that natural selection occurs because populations have the capacity to produce many offspring that compete for resources; in the ‘struggle for existence’, individuals that are best adapted are most likely to survive to reproduce and pass on their alleles to the next generation.
17.2.2 Explain how environmental factors can act as stabilising, disruptive and directional forces of natural selection.
17.2.3 Explain how selection, the founder effect and genetic drift, including the bottleneck effect, may affect allele frequencies in populations.
17.2.4 Outline how bacteria become resistant to antibiotics as an example of natural selection.
17.2.5 Use the Hardy–Weinberg principle to calculate allele and genotype frequencies in populations and state the conditions when this principle can be applied.
17.2.6 Describe the principles of selective breeding (artificial selection).
17.2.7 Outline the following examples of selective breeding:
the introduction of disease resistance to varieties of wheat and rice
inbreeding and hybridisation to produce vigorous, uniform varieties of maize
improving the milk yield of dairy cattle.
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It is very important that learners are confident in using some of the key terms they encountered in Topics 6 and 16, including allele, frequency, dominant, recessive, homozygous, heterozygous, mutation, and so on. Ask learners individually to choose a term and offer a definition for it. (F)
Use active learning to demonstrate a model that represents the natural selection of the HbS allele. Place around 20–30 beads or sweets of two different colours in a non-transparent bag to represent the alleles HbA and HbS. There should be an equal number of both. Ask a learner to take two sweets at random. If an HbA and an HbS are taken, count this twice. This models the advantage experienced by heterozygous individuals. If two HbS alleles are drawn out, place these out of sight. This models the disadvantage experienced by recessive homozygous individuals. Discuss how this models natural selection and point out the most common phenotype in the sweets that remain on the table. Learners record numbers of each genotype in each generation and construct graphs to show the effect of selection over time.
Work through the interactive activity on natural selection with learners: https://phet.colorado.edu/en/simulation/legacy/natural-selection
Learners record a step-by-step guide to explain how natural selection occurs. This could consist of a series of diagrams, a flow chart with statements separated by arrows or a short story. Examples of case studies include: warfarin resistance in rats; melanism in peppered moths; cyanogenic clover; antibiotic resistance in bacteria; resistance in insects to insecticides. Alternatively, link with areas of significant research and provide suggested websites. For example, Peter and Rosemary Grant’s research into finches of the Galapagos islands:
www.biointeractive.org/classroom-resources/origin-species-beak-finch
the adaptive radiation of Anolis lizards on Caribbean Islands:
www.biointeractive.org/classroom-resources/lizard-evolution-virtual-lab (I)
To support learning of the material in this unit, share an automated Hardy-Weinberg calculator with learners, e.g. www.perinatology.com/calculators/Hardy-Weinberg.htm. Learners may use this to check their answers after manually calculating values. Tutorials and quizzes on the Hardy–Weinberg equilibrium:
http://highered.mheducation.com/sites/dl/free/0767424263/322933/fuentes_4_1.html www.wwnorton.com/college/biology/discoverbio3/core/content/ch17/animations.asp
Learners prepare Venn diagrams that compare the processes of natural and artificial selection. They could prepare these on a large piece of paper or card with a range of materials, and subsequently show them in a ‘marketplace’ activity. This involves one member of each group standing by their poster and offering an explanation to other learners as they move around the room. Challenge learners to include in their descriptions references to inbreeding depression, outbreeding and hybrid vigour. Ask learners to include plenty of examples in their work. Examples of natural selection are described above. Examples of artificial selection:
www.irri.org/disease-and-pest-resistant-rice
www.thoughtco.com/wheat-domestication-the-history-170669 https://learn.genetics.utah.edu/content/evolution/corn/
http://www.holsteinusa.com/holstein_breed/breedhistory.html (I)
Challenge learners to write a ‘how to’ guide to help their peers in the following year group to answer questions using the Hardy–Weinberg equations. They should describe the problems they encountered and mistakes they initially made during this lesson. Their work must be written in a casual, friendly tone. (F)
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