Equation 9 · Part 5 · The Peacock Problem: Darwin's Second Theory and Its Hard Tests

Symbol B

\begin{aligned} \Delta \bar{t} &= h_t^{2}\,\beta_t + B\,\beta_p \\ \Delta \bar{p} &= B\,\beta_t + h_p^{2}\,\beta_p \end{aligned}

B

What this part means

the when.

Its job in the formula

B is one of the signed contributions combined to compute the quantity on the left.

Full expressionSymbol BArticle meaning

Where the article explains it

When B is large enough relative to the stabilising pull of $\beta_t$ , the two equations no longer settle to a fixed point; they feed each other, and $\bar t$ and $\bar p$ escalate together until some external limit — a wing that can no longer lift the tail, a display that finally costs more than it wins — brings the pair to a halt.

The passage around this formula

…female preference; write $\beta_t$ and $\beta_p$ for the direct selection gradients acting on each (natural selection pulling $\beta_t$ downward, since the ornament is costly), $h_t^2$ and $h_p^2$ for their heritabilities, and B for the genetic covariance that non-random mating builds between the two traits. The standard Lande-Fisher formalisation of one generation’s change is then a coupled pair: $\begin{aligned} \Delta \bar{t} &= h_t^{2}\,\beta_t + B\,\beta_p \\ \Delta \bar{p} &= B\,\beta_t + h_p^{2}\,\beta_p \end{aligned}$ . Read the first line on its own and the trait looks stable: natural selection ( $\beta_t$ ) pulls the…

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Learn the underlying idea

A variable is a named place for a value. Its letter is a local label: x can mean position in one formula and a data point in another.

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Sources cited in the surrounding passage

[4] Models of speciation by sexual selection on polygenic traits ↗

These citations provide research context; check each source for the exact claim it supports.