Read a probability distribution of drift rather than one lucky trajectory

1. Orient: Read a probability distribution of drift rather than one lucky trajectory

Genetic drift is variation in inherited frequencies due to finite sampling. It can occur without a selective advantage. Different replicate populations started from the same frequency can move in opposite directions, so one realized increase does not establish that an allele was favored.

2. Explain the population mechanism

The ideal Wright–Fisher packet draws2Ne allele copies independently with replacement from a pool with A frequency p. If K is the A count, P(K=k)=C(2Ne,k)p^k(1−p)^(2Ne−k). The graph shows these mutually exclusive one-generation outcomes across hypothetical replicates, not frequency through time.

The mean next frequency remains p, while variance is p(1−p)/(2Ne). Immediate loss has probability(1−p)^(2Ne) and immediate fixation p^(2Ne). Effective size here describes sampling behavior; unequal reproduction or demographic structure can make effective size differ from census size. The displayed Ne is stipulated, not estimated.

Expected heterozygosity after this step is2p(1−p)(1−1/(2Ne)) when calculated from the sampled allele frequency. This is an expectation across replicate populations, not a guarantee for every replicate. No mutation or immigration restores an allele after loss in the closed ideal model. Eventual fixation and one-generation fixation are different questions. A founder event samples individuals that establish a new population, whereas a bottleneck sharply reduces an existing population before subsequent reproduction. Either can change allele representation through sampling without an adaptive advantage. If a neutral marker rises from0.2 to0.5 in one small founder group but falls in another independently sampled group, the opposite directions challenge a universal marker advantage; they do not by themselves establish that all survival was random. A bottleneck caused by a hazard can also include selective survival, so distinguish the sampling component from any genotype-dependent effect.

3. Work a complete evidence calculation

With Ne4 and p0.4, eight copies are drawn. Mean next frequency0.4, variance0.24/8=0.03 and immediate loss0.6^8=0.01679616. Immediate fixation0.4^8=0.00065536, not0.4;0.4 describes a different eventual neutral-fixation result under additional continuing-model assumptions.

4. Predict and investigate

Predict first: If the average frequency across many replicates is unchanged, can drift still reduce diversity?

Compare Ne2,4 and10 at p0.4. Record the distribution spread, immediate loss/fixation and expected heterozygosity. Explain why the horizontal axis must be next-generation frequency rather than generation number.

Retain the original population definition, first prediction, controls and full numerical ledger, including conflicting or null results. These are original bounded teaching calculations, not measured populations.

5. Transfer and explain

Apply the mechanism to a changed case

For Ne5,p0.5, find variance, one-generation loss probability and expected heterozygosity.

Retain your calculation, units and assumptions in the lesson notebook before opening this public practice response.

Compare a worked response

Ten copies give variance0.25/10=0.025, immediate loss0.5^10=1/1024, and expected heterozygosity0.5×0.9=0.45. No directional mean change does not mean no random change.

6. Reflect and retain evidence

State a competing mechanism, an observation that would distinguish it, and a limitation of the supplied sampling or life-cycle model. Preserve first reasoning and a criterion-linked human review in the General Biology II workspace. Public practice feedback and simulated evidence do not certify an independently demonstrated comparative argument, field investigation or practical skill.

Supporting scientific reference, checked 30 October 2026. All prose, numerical packets and diagrams are original. No organism collection, culture, breeding, release, personal genetic inference or clinical procedure is requested.