Discriminate mechanisms that fit the same allele-frequency endpoint

1. Orient: Discriminate mechanisms that fit the same allele-frequency endpoint

A model can reproduce an observation while giving the wrong causal explanation. Population frequencies can change through selection, migration, mutation and drift, often together. An exact fit to one endpoint is therefore a starting point for discrimination, not a proof of mechanism.

2. Explain the population mechanism

For a haploid selection-only model beginning at p0.2, invert p′=wp/[wp+(1−p)] to obtain w=p′(1−p)/[p(1−p′)]. A migration-only replacement model with donor frequency pD instead requires m=(p′−p)/(pD−p). The supported endpoints give valid positive contributions and replacement fractions for both explanations.

At endpoint0.4 with donor0.8, selection alone fits w=8/3 while migration alone fits m=1/3. These fitted parameters represent different causal stories. The model deliberately makes them indistinguishable by the single final-frequency observation, then asks which additional evidence would separate them.

Independent survival/reproduction measurements, migrant-assignment evidence, repeated cohorts and replicate populations can challenge an explanation. Mutation-rate evidence and finite-sampling variation may matter too. Define what would falsify each model before viewing additional outcomes; do not declare the preferred story true because it sounds adaptive.

3. Work a complete evidence calculation

From p0.2 to0.4, selection w=(0.4×0.8)/(0.2×0.6)=8/3. With donor0.8, migration m=(0.4−0.2)/(0.8−0.2)=1/3. Substitution into both equations returns0.4 exactly; the fit alone chooses neither.

4. Predict and investigate

Predict first: Can two models with different biological mechanisms both make an exact prediction of the same endpoint?

Fit both explanations at endpoint0.4, then change donor frequency while holding the endpoint fixed. Explain why the selection fit stays unchanged while the migration fit changes. Specify one independent observation and a predicted direction for each competing explanation.

Retain the original population definition, first prediction, controls and full numerical ledger, including conflicting or null results. These are original bounded teaching calculations, not measured populations.

5. Transfer and explain

Apply the mechanism to a changed case

Initial p0.2 rises to0.5 with donor pD0.8. Find the selection-only w and migration-only m.

Retain your calculation, units and assumptions in the lesson notebook before opening this public practice response.

Compare a worked response

w=(0.5×0.8)/(0.2×0.5)=4; m=(0.5−0.2)/(0.8−0.2)=0.5. Both explain the endpoint under their assumptions; actual contribution or movement evidence is needed to discriminate.

6. Reflect and retain evidence

State a competing mechanism, an observation that would distinguish it, and a limitation of the supplied sampling or life-cycle model. Preserve first reasoning and a criterion-linked human review in the General Biology II workspace. Public practice feedback and simulated evidence do not certify an independently demonstrated comparative argument, field investigation or practical skill.

Supporting scientific reference, checked 30 October 2026. All prose, numerical packets and diagrams are original. No organism collection, culture, breeding, release, personal genetic inference or clinical procedure is requested.