Natural Selection
Topic 7.1 established the engine; Topic 7.2 asks what shape it leaves behind. When natural selection acts on a heritable trait spread across a population, it reshapes the whole distribution of that trait — the bell curve of how common each value is. Read the before-and-after distribution and you can name exactly how the environment pushed on it. Three patterns cover the cases the AP exam cares about, and telling them apart is the entire skill of this topic.
Directional selection favors one extreme, so the mean slides toward it — peppered moths going darker as soot darkens the bark, or beak depth creeping up in a drought. Stabilizing selection favors the middle and punishes both extremes, so the mean holds steady while the spread narrows — human birth weight is the classic case, too small and too large both costly. Disruptive selection favors both extremes over the average, splitting one hump into two — the intermediate form is the one selected against. The trap is reading the mode off the wrong cue: a shifting mean is directional, a shrinking variance is stabilizing, and a two-peaked split is disruptive, but only if you compare the full distributions rather than a single "winner."
Interactive · Selection Modes
Set an environmental pressure and run the population forward, then watch the trait distribution reshape itself. See the mean slide under directional pressure, the spread tighten under stabilizing pressure, and a single hump split in two under disruptive pressure — and name the mode from the shape, not from which individual happens to survive.
Selection Modes · Open the full sandbox →The mistakes here cluster around a few failure modes. Students still imagine evolution as goal-directed or progressive, as if a mode of selection were aiming at a target (U7-BIO1); they say an individual adapts on purpose when it is the population's distribution that shifts (U7-BIO2); they read "fittest" as "strongest" rather than best reproductive success in that environment (U7-BIO3); they think selection creates the new variation it favors when it only acts on the spread already present (U7-BIO5); and — the signature error of this topic — they misread which mode is shown, calling a narrowing curve directional or a two-peaked split stabilizing (U7-BIO6). Every scenario asks you to compare full distributions and name the mode from how the curve moved.
The work
3 ways in · any order
Lesson
Natural Selection
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Natural selection reshapes a trait's whole distribution, and the three modes each leave a signature: directional slides the mean toward one extreme, stabilizing favors the middle and narrows the spread, and disruptive favors both extremes and splits the curve in two. The lesson walks the ways students misread that shape — mistaking a narrowing curve for directional, a two-peaked split for stabilizing, or treating a mode as if it aimed at a goal. It closes with a ten-scenario applet that asks you to name the mode from the distribution, not from a single survivor.
Diagnostic
10-item topic check
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Ten items on the three modes of selection — that a mode is not aiming at a goal or climbing toward a "higher" form (U7-BIO1); that it is the population's distribution that shifts, not an individual adapting on purpose (U7-BIO2); that "fittest" means reproductive success in that environment, not strength (U7-BIO3); that selection acts on variation already present rather than creating it (U7-BIO5); and — the core skill — that you name directional, stabilizing, or disruptive from how the trait distribution shifts (U7-BIO6). Take it cold to surface which of these are still tangled, or after the lesson to confirm they hold.
Targeted Practice
Drill a single misconception
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Pick one of the failure modes you missed and drill it on its own. The round is adaptive: two correct in a row clears the misconception and moves you to the next.