Start with inherited variation in a population
Natural selection needs differences among individuals before selection changes how common a feature becomes.
Section 18.3 · Selection
Core and Extended; Biology 0610, 2026–2028. Use the stated specimen features and evidence. Diagrams are schematic. Written practice is self-checked
Before you begin
- Use the stated case, sample and assumptions. Diagrams and numerical records in this pathway are original teaching models; written answers are self-checked.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What must be present before natural selection can change how common a feature is?
What you will learn
- Identify variation within one population
- Use inheritance evidence without guessing genotype
Original selection schematic. Population counts and scenarios are supplied teaching models; they are not field observations.
| Stage or record | Meaning |
|---|---|
| Recorded group | Olive 18; bronze 12 |
| Sample total | 30 beetles |
| Olive proportion | 18/30 = 60% |
| Evidence limit | Sample phenotype count; not a genotype census |
Identify variation within one population
Natural selection acts on populations that already contain differences. In an invented estuary beetle population, individuals vary in shell shade. The case states that the shade difference is inherited. That supplied statement matters: an observed colour difference alone would describe phenotype, but would not prove the difference is genetic or inherited.
The population is the group of the same species living and reproducing in the stated place and time. A sample can show variation without containing every possible phenotype. If a field record lists 18 olive and 12 bronze beetles, it describes those 30 recorded individuals. It does not prove that all beetles in the species have only those two shades.
Use inheritance evidence without guessing genotype
Keep observation and explanation separate. “Olive shells were recorded” is an observation. “Olive shell shade is inherited” needs evidence or an explicit assumption in the question. Do not infer a genotype from appearance unless the inheritance model supplies that link. This restraint prevents a population count from becoming an unsupported genetic claim.
For selection to alter a population over generations, some differences must be passed from parents to offspring. An environmental change does not create a useful feature because organisms need it. It changes the conditions under which existing differences may affect survival and reproduction; those links are examined in the next lessons.
Pause and explain
The beetle case states shell shade is inherited. What does the 18-to-12 sample add?
Worked example
An invented sample contains 18 olive and 12 bronze beetles; the case states shell shade is inherited. Find the olive proportion and say what the count establishes.
Show the worked solution
- Add the recorded groups: 18 + 12 = 30 beetles.
- Divide the olive count by the total: 18/30 = 0.60, or 60%.
Answer Olive beetles are 60% of this recorded sample. The stated inheritance assumption links shade with inherited variation; the count alone does not establish a genotype.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
In a sample of 24 inherited-feature beetles, 15 are olive. Calculate the olive percentage and state the evidence boundary.
Give me a hint
Use the olive count divided by all 24 beetles.
Compare my reasoning
- 15/24 = 0.625.
- 0.625 × 100 = 62.5%.
Olive beetles are 62.5% of the sample; this does not identify all genotypes or every phenotype in the population.
Look for these in your work
- Correct denominator of 24
- Percentage and sample boundary
A student observes darker shells after a dry month and concludes drought made each beetle change its genes. Evaluate the conclusion.
Give me a hint
Separate an observation over time from an inherited mechanism.
Compare my reasoning
- The record reports a change in the observed sample, not the same individuals changing genes.
- Possible explanations include different survival or sampling; inheritance would need separate evidence.
The conclusion is unsupported: a sample shift does not show drought directed individual gene change. Compare identified individuals and offspring evidence before claiming an inherited difference.
Look for these in your work
- Individual change distinguished from sample composition
- Evidence needed for inheritance
A new shoreline has pale and dark individuals of another insect species. The case says colour is inherited and predators detect colours differently against the substrate. Explain why this supplies a starting point for selection, but not proof that dark insects will always increase.
Give me a hint
Use the inherited variation and local context; do not treat a conditional advantage as universal.
Compare my reasoning
- There is inherited colour variation in the population before any claimed change.
- Predator detection may change reproductive outcomes in this shoreline, but another habitat or changing conditions could alter the outcome.
The population has inherited variation on which selection could act. Whether either colour increases depends on reproductive outcomes in the stated environment and over generations.
Look for these in your work
- Existing inherited alternatives identified
- Environmental and evidence limit stated
Common mistakes
- Treating a phenotype count as a genotype count
- Claiming the sample contains every phenotype in the species
- Saying the habitat caused beetles to choose a new allele
What can you explain now?
The beetle case states shell shade is inherited. What does the 18-to-12 sample add?
Compare with the explanation
A measured phenotype proportion in the recorded sample
It counts appearances and supplies a denominator; inheritance comes from the stated case.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Without your notes, identify variation within one population and use inheritance evidence without guessing genotype. Explain the mechanism, use the given population or breeding records and state what the evidence does not establish.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.