What does this equation mean?

Russell Lande’s 1981 paper gave this verbal logic its standard quantitative-genetic form, modelling the joint evolution of a female preference trait and a male display trait under stabilising natural selection on the display and showed that, “despite stabilizing natural selection on males, various types of mating preferences may create a runaway process in which the outcome of phenotypic evolution depends critically on the genetic variation parameters and initial conditions of a population” [ 4 ] . One compact way to see why sits in the coupling itself. Let $\bar t$ be the population mean of a male display trait and $\bar p$ the population mean of the corresponding 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 ornament back down every generation. But the second term, B\,$\beta_p$ , imports a push from the preference side through the genetic covariance the mating system itself has created. 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. Nothing in the equations names where that limit falls. That absence of an internal stopping rule is the entire content of “runaway”: once the covariance term dominates, the direction is set and the magnitude is set by biomechanics, not by the selective logic that started the process.