Connect heritable variation to relative reproductive success

Mechanism before a label

Natural selection requires variation among individuals, a heritable component of that variation, and differences in reproductive contribution associated with it. Individuals do not acquire a needed allele because they try; population composition changes across generations when variants contribute unequally.

Fitness is reproductive contribution in a specified environment, not strength, longevity or a moral ranking. Survival matters only through its consequences for reproduction. A phenotype favored in one environment can be disadvantaged in another, and trade-offs prevent a universal best organism.

Directional selection shifts a trait distribution, stabilizing selection favors intermediate values and disruptive selection favors extremes under the specified conditions. These descriptions require a distribution and relative reproduction, not merely a change in one average.

Make resource limitation and molecular variation explicit

Competition for limited resources can cause unequal survival and reproduction. Heritable differences in molecular amount or function can change how an organism obtains or uses those resources; molecular difference is not itself fitness until connected to reproductive consequences in an environment. OpenStax natural selection, checked 31 October 2026.

Original changed comparison: A transporter variant increases nutrient uptake in a scarce-resource condition, but producing it has an energetic cost. In a nutrient-rich condition, uptake no longer limits reproduction. Must the same variant remain favored?

Check the environment-dependent prediction

No. The benefit can shrink while the production cost remains, so the reproductive ranking can reverse. Measure reproductive contribution and relevant resource conditions rather than equating more transporter or faster uptake with universally greater fitness. The proposed mechanism is conditional and does not supply observed selection coefficients.

Predict, compare and calculate

The following record is an original synthetic teaching dataset. Its assigned conditions are not a claim that an actual organism was measured. Write your prediction before revealing the changed condition.

Variant contribution	Value (individuals)
A starting individuals	50
B starting individuals	50
A offspring contribution	100
B offspring contribution	50

A haploid asexual teaching population with completely inherited labels and equal initial abundance.

Your prediction before comparison

This scratch prediction is not saved automatically. Copy it to the lesson notebook to retain it.

Evidence conditionReference comparisonChanged condition / follow-up

Connect heritable variation to relative reproductive success

Supplied synthetic evidence: A haploid asexual teaching population with completely inherited labels and equal initial abundance.

Variant contribution: reference condition

Quantity	Value	Unit
A starting individuals	50	individuals
B starting individuals	50	individuals
A offspring contribution	100	individuals
B offspring contribution	50	individuals

Task: A variant survives twice as well but produces half as many offspring per survivor. Under a simple product model, compare its total contribution with the reference.

This is not a diploid genotype model; no dominance is implied.

Compare with the conditional interpretation

A contributes two thirds of offspring under the stated assumptions.

Reference A frequency among offspring is 100/(100+50)=2/3. Relative offspring contributions per starting individual are 2 and 1; scaling by the maximum gives relative fitnesses 1 and 0.5.

Case index for this investigation

0. Connect heritable variation to relative reproductive success
1. Distinguish selection by people from inheritance and trade-offs
2. Separate drift, gene flow and mutation from selection
3. Calculate allele frequencies and test equilibrium claims
4. Combine fossils, homology and molecular evidence
5. Read a longitudinal evolution study without inventing observations
6. Read branching relationships rather than tip order
7. Explain reproductive barriers and divergence
8. Relate variation, changing environments and extinction
9. Separate origin-of-life hypotheses from ancestry evidence

No numerical confidence, laboratory credit or independent mastery is inferred from opening a supplied case. Save your first prediction and comparison in the notebook.

Reset this unsaved comparison

Test your model prediction

Scratch inputs are not saved or graded. Retain your prediction and calculations in the lesson notebook. No simulation output is a measured organism or population.

Initial A frequencyA reproductive weightB reproductive weight

Weighted A contribution=1.000; B=0.500. Next A frequency=A/(A+B)=0.6667; change=0.1667.

Ideal haploid asexual inheritance, deterministic expected reproduction, no mutation/migration/drift. Swap the reproductive weights and explain why the direction changes. Making both weights zero leaves no offspring and an undefined frequency.

Reset practice

Worked reasoning

Reference A frequency among offspring is 100/(100+50)=2/3. Relative offspring contributions per starting individual are 2 and 1; scaling by the maximum gives relative fitnesses 1 and 0.5.

A contributes two thirds of offspring under the stated assumptions.

Apply to a new case

A variant survives twice as well but produces half as many offspring per survivor. Under a simple product model, compare its total contribution with the reference.

Compare your reasoning with the worked response

The product 2×0.5=1 gives equal total contribution under those assumptions. Survival alone did not establish greater reproductive success.

This public response supplies feedback, not a score for independent reasoning. Record which part of your prediction changed and why.

Evidence and investigation boundary

This is not a diploid genotype model; no dominance is implied. Do not infer the same response for every population or future environment.

For a supervised investigation, predeclare the question, biological unit of replication, treatment/comparison assignment, quantities and units, exclusion rules, graph and uncertainty method. Keep the raw records and the first prediction separate from later revisions. A simulated case does not supply laboratory hours or establish AP authorization. No physical manipulation is required by this lesson.

Source and continuation

Framework topics: 7.1–7.2. Current AP Biology framework; supporting scientific reference. Sources checked 30 October 2026; all teaching wording, questions and numbers here are original. Return to AP Biology to compare this mechanism with another scale of organization.