Compare trait distributions before and after selection

How can a heritable trait become more common without individuals choosing to change?

Submit: An evidence record, explanation and revision.

Related lesson: Evolution and Natural Selection

This is a teacher-led extension beyond the short lesson. The task supplies a performance opportunity; completion alone does not certify mastery, full NGSS coverage or state alignment.

Build the idea

Selection acts on variation already present in a population. When a heritable trait affects reproductive success in an environment, its distribution can change across generations. Counts must be converted to proportions when sample sizes differ. A pattern alone does not establish heritability or rule out migration and sampling bias.

Evidence and model inputs

Invented model of a heritable beak-depth trait: shallow/medium/deep counts before a dry period = 40/40/20 (100 birds); among surviving breeders = 10/20/20 (50 birds); next generation = 18/36/36 (90 birds). The model assumes trait inheritance and comparable sampling. It is not a report of a real island study.

Keep actual observations, published findings and invented model cases labeled separately. If a source or required equipment is unavailable, pause that part with a teacher; do not invent results.

Materials and preparation

Paper, graph paper or spreadsheet, and the linked readings.

Grade-level scope

Basic statistical comparisons; no Hardy–Weinberg calculations or allele-frequency derivations.

Do the work

1. Convert each row to percentages and graph the three distributions.
2. Compare deep-beak frequency before and among breeders, then in the next generation.
3. Explain how differential survival and reproduction could produce the pattern without changing an individual bird’s inherited beak category.
4. Identify the assumptions about inheritance and environment and a competing explanation.
5. Propose what field observations would be needed to test the model empirically.

Learner worksheet

Write on paper or type here and print. Entries stay in this page’s memory and are lost when you leave or reload; they are not saved or submitted.

Check your reasoning

1. What is deep-beak frequency before selection?
2. What is it among breeders and offspring?
3. Did the population necessarily double in size?
4. Do birds intentionally acquire the needed inherited trait?

Teacher review guide

Review the actual deliverable and discuss the reasoning. For each criterion, use 0 when evidence is absent or fundamentally incorrect, 1 when partly supported with a material error or omission, and 2 when accurate, supported and complete for the stated task. Maximum 8 points is a task score, not a proficiency certification. Give feedback and allow revision.

1. Computes and compares distributions correctly.
2. Links heritable variation with differential reproduction.
3. Distinguishes individual change from population change.
4. Evaluates assumptions and alternative explanations.

Sources and standard reference

• Official NGSS HS-LS4-3 performance expectation — inspect the practice, core idea, crosscutting concept and assessment boundary.

Instructional text and hypothetical cases are original. Linked source readings supply published evidence where specified. Teachers should judge fit with their course and state requirements.