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Scaffold S6 — Drift, selection, or both?

Five rounds of trajectory classification. You are shown a time-course of allele frequency (or relative fitness) and asked to decide whether it looks like (a) drift only, (b) selection essentially deterministic, or (c) selection with drift. The reveal shows the true generating parameters (for simulations) or a Wright–Fisher + selection fit (for real data), along with the s ≈ 1/Ne boundary.

Locked — answer the pretest above first.

Running tally — classifications

What you just did has a name

The boundary between drift and selection is |s| ≈ 1/Ne. Below that, selection is too weak to overcome the sampling noise of reproduction, and the allele behaves as if neutral. Above it, selection dominates. In between, you see a rising (or falling) trend with substantial scatter — the combined regime.

Round 1 was pure drift in a tiny population: the trajectories fan out with no directional trend. Round 2 was selection in an essentially infinite population: the trajectories are a single deterministic line. Round 3 was the combined regime — the trend is visible but any single replicate could mislead you. Round 4 used Lenski's LTEE Ara+1 fitness record: relative fitness is not an allele frequency, but the statistical question ("is the trend above what random reshuffling would produce?") is identical. Round 5 used a real Florida Scrub-Jay SNP trajectory across 23 years — the observed Δp is consistent with drift at the estimated Ne.

Selection is the correlation; drift is the scatter. Selection shows as a consistent covariance between allele and fitness across many individuals and generations. Drift is the variance around that trend. In small populations or over short timescales, the variance dwarfs the trend — so drift is always the null, and selection must be demonstrated.