Separate evolutionary mechanisms

Supplied materials

• All measurements and observations below are supplied classroom examples. They are not data from actual learners or a claim about a particular real site.
• Paper or a browser document for a labeled drawing, table or explanation.
• Worked problem inputs: A small population’s frequencies shift after a random survivor sample, with no measured trait advantage. Compare drift and selection.
• Assessment case 1: An inherited variant produces more offspring under one condition and rises over generations. What supports selection over drift alone? Which inference or prediction is best supported?
• Assessment case 2: Which scientific label fits the specifically described step or feature? Supplied case: Chance sampling alters a small population.
• Assessment case 3: Which scientific label fits the specifically described step or feature? Supplied case: Migrants bring a variant into population.
• Assessment case 4: Which scientific label fits the specifically described step or feature? Supplied case: A new DNA sequence change appears.
• Assessment case 5: Which scientific label fits the specifically described step or feature? Supplied case: Random founder sample changes frequencies.
• Assessment case 6: Migrants enter a population, breed and contribute alleles. What population pathway is established? Which inference or prediction is best supported?

Learn the idea

Population frequencies can change by selection, random drift, gene flow or new mutation. Distinguishing mechanisms requires evidence. A change that looks useful is not proof it arose because it was needed.

Selection means trait-linked reproductive differences change frequencies. Drift means random sampling changes frequencies. Gene flow means migration transfers genetic variants. Mutation means new sequence variation arises.

For the supplied mechanism problem, A small population’s frequencies shift after a random survivor sample, with no measured trait advantage. Compare drift and selection. Random sampling fits drift. Selection would require evidence connecting inherited traits to reproductive differences. A frequency shift alone cannot establish that advantage.

1. Read the supplied problem and mark the structures, processes or quantities named: A small population’s frequencies shift after a random survivor sample, with no measured trait advantage. Compare drift and selection.
2. Draw the relevant boundary, pathway or comparison and label it using the scientific ideas above.
3. Write a causal explanation or calculation, showing how each observation supports the result.
4. Compare your reasoning with the worked solution; name a condition that would change or limit the inference.

Check your reasoning

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Take it further

New case: migrants introduce an allele absent from local records. Expected: gene flow is a candidate mechanism; do not call migration evidence proof of local selection advantage.

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