Population Genetics
Population genetics zooms out from the single organism to the allele pool — the full set of gene variants a population carries — and tracks how allele frequencies shift from one generation to the next. Topic 7.4 asks you to hold two engines of change apart. Natural selection is the non-random one: some variants leave more offspring because of what they do in a particular environment, so their frequency climbs for a reason. Genetic drift is the random one: allele frequencies wobble generation to generation purely by the luck of which individuals happen to reproduce, with no reference to fitness at all.
The size of the population is what sets how loud drift gets. In a large population the random ups and downs of thousands of reproductive events mostly cancel out, and allele frequencies stay close to stable. In a small population the same chance events don't average away — a handful of individuals failing to breed can swing a frequency hard, drive an allele all the way to fixation, or lose one entirely, even when that allele was neither helpful nor harmful. So drift affects small populations far more than large ones, and it can push a population in a direction selection never "chose."
Interactive · Allele Pool
Set a starting allele frequency and a population size, then run the pool generation by generation. Watch a small population let random sampling swing the frequencies — sometimes all the way to fixation or loss — while a large population holds steady, and see how selection and drift pull differently.
Allele Pool · Open the full sandbox →The mistakes here cluster around treating drift like a second kind of selection. Students describe genetic drift as if it were goal-directed or adaptive — as though the alleles that drift to high frequency must be the beneficial ones (U7-BIO8) — when drift is random and indifferent to fitness. And they assume drift acts on every population equally, or wave it off entirely, missing that its effect scales inversely with population size and dominates precisely when populations are small (U7-BIO9). Every scenario in this topic asks you to keep the random and the non-random engines apart, and to remember that population size sets the volume knob on chance.
The work
3 ways in · any order
Lesson
Population Genetics
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Population genetics tracks allele frequencies across a whole gene pool, and Topic 7.4 turns on telling its two engines apart: selection changes frequencies non-randomly by reproductive success, while genetic drift changes them at random. The lesson walks the ways students blur the two — reading drift as if it were adaptive, and assuming it matters equally in every population. It closes with a ten-scenario applet that asks you to keep drift random and to remember that its effect grows as populations shrink.
Diagnostic
10-item topic check
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Ten items on how allele frequencies change — that genetic drift is random and not a disguised form of selection, so the alleles that drift up aren't necessarily the beneficial ones (U7-BIO8); and that drift's effect depends on population size, hitting small populations hard while large ones stay stable (U7-BIO9). 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.