\begin{equation}\label{eq:mutation-drift_diffusion2}
\begin{split}
    \frac{\partial\phi(x,t)}{\partial t}&=-\frac{\partial}{\partial x}\bigg(\theta\beta(1-x)\phi(x,t)-\theta\alpha x\phi(x,t)\bigg)+\frac{\partial^2}{\partial x^2} x(1-x)\phi(x,t)\\
    &=-\frac{\partial}{\partial x}\theta(\beta-x)\phi(x,t)+\frac{\partial^2}{\partial x^2} x(1-x)\phi(x,t)\,.
\end{split}
\end{equation}
