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Scaffold S4 — Fixation probability

Five rounds. You are given a starting allele frequency p0 and an effective population size Ne. You predict the fraction of 500 simulated populations that will eventually fix the allele. The truth is then drawn. Repeat with a different p0 and Ne.

Locked — answer the pretest above first.

Running tally

What you just did has a name

You have now predicted the fixation fraction five times across different combinations of p0 and Ne. Look at your tally above — your guesses and the observed fractions both sit near the line guess = p0.

Under pure drift, the probability that an allele eventually fixes equals its current frequency: P(fix) = p0. Ne does not appear. Bigger Ne means it takes longer to resolve — not that the answer is different.

Why: every allele copy in the current pool is equally likely to be the ancestor of the eventually fixed population. There are p · 2N copies of the focal allele out of 2N total. The fraction is p. This is Bayes' theorem applied to neutral drift. You verified it by simulation before anyone wrote the formula.