Symbol u

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as verified directly from his derivation [ 7 ] . Let the selective advantage vanish — the allele is neutral, indistinguishable from its alternatives in its effect on survival or reproduction — and the exponentials cancel term by term, leaving $u(p) = p \qquad (s \to 0)$. The probability that a neutral allele eventually takes over the population is exactly its own starting frequency, nothing more. No fitness calculation, no bookkeeping of selection coefficients, is needed to answer the single most consequential question about a new mutation’s fate; the diffusion equation alone answers it.

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