KHAEMENES ACADEMY · BIOLOGY 10 · WEEK 29 · DAY 03

Directional, Stabilizing & Disruptive Selection

Interpret changes in trait distributions instead of memorizing three graph shapes.

Safety and evidence boundary

Use paper-token simulations and published or fictional trait-frequency data. Do not intentionally expose organisms to harmful agents or breed animals for class. Avoid purpose-based language: populations change through differential reproduction, not because organisms need or want traits.

Explicit science instruction

Selection can alter trait distributions. Directional selection shifts the mean toward one end; stabilizing selection reduces extremes around an intermediate region; disruptive selection can favor separated ranges over intermediates. These labels describe patterns under conditions, not permanent properties.

Worked example with reasoning

A fictional bird beak distribution begins mean 10 mm. After a drought leaving mostly large hard seeds, survivors/reproducers average 12 mm: consistent with directional selection if beak size is heritable and other evidence supports differential reproduction.

Guided question

Why can't a before/after histogram alone prove selection caused the change?

Independent work

  1. Classify three distribution changes.
  2. Calculate mean shifts from supplied data.
  3. Identify heritability evidence needed.
  4. Propose drift or migration alternatives.

Misconception check

Stabilizing selection does not stop evolution, and disruptive selection does not automatically create new species.

Formative understanding check

Which pattern favors intermediate phenotypes relative to extremes?

Worked response

Stabilizing selection.

Open-book formative practice only; not an official mastery attempt or grade.

Mentor evidence conference

Ask Archaemenes to challenge whether your population-level claim follows from the data and whether alternative evolutionary mechanisms remain possible.

Open Archaemenes

Teacher guidance

Foundation: use annotated histograms. Core: connect patterns to mechanisms cautiously. Extended: compare selection gradients and changing environments conceptually.

Preserve learning evidence

Keep calculations, graphs, models, corrections and limitations in the matching Week 29 evidence field.