% --- Hybrid ancestry diagram (for readability)
\begin{figure}[htbp]
\centering
\begin{tikzpicture}[
  node distance=8mm and 14mm,
  every node/.style={font=\small},
  mech/.style={rounded corners, draw=black, very thick, inner sep=4pt, align=center},
  hyb/.style={rounded corners, draw=black!60, dashed, inner sep=3pt, align=center},
  ->, >=Latex
]
% Roots
\node[mech] (L)  {\Lrn};
\node[mech, right=of L] (S) {\Sch};
\node[mech, right=of S] (C) {\Ctl};
\node[mech, below=of L] (E) {\Evo};
\node[mech, right=of E] (Y) {\Sym};
\node[mech, right=of Y] (P) {\Prb};
\node[mech, below=of E] (W) {\Swm};

% Hybrids (examples)
\node[hyb, above=6mm of $(L)!0.5!(S)$] (LS) {\hyb{Lrn+Sch}\\ \footnotesize(AlphaZero-type)};
\node[hyb, above=6mm of $(L)!0.5!(C)$] (LC) {\hyb{Lrn+Ctl}\\ \footnotesize(model-based control)};
\node[hyb, below=6mm of $(L)!0.5!(E)$] (EL) {\hyb{Evo/Lrn}\\ \footnotesize(neuroevolution)};
\node[hyb, below=6mm of $(L)!0.5!(Y)$] (LY) {\hyb{Lrn+Sym}\\ \footnotesize(neuro-symbolic)};
\node[hyb, below=6mm of $(P)!0.5!(C)$] (PC) {\hyb{Prb+Ctl}\\ \footnotesize(Bayesian control)};
\node[hyb, below=6mm of $(E)!0.5!(W)$] (EW) {\hyb{Swm+Evo}\\ \footnotesize(swarm-evolution)};

% Edges
\draw (L) -- (LS); \draw (S) -- (LS);
\draw (L) -- (LC); \draw (C) -- (LC);
\draw (E) -- (EL); \draw (L) -- (EL);
\draw (L) -- (LY); \draw (Y) -- (LY);
\draw (P) -- (PC); \draw (C) -- (PC);
\draw (E) -- (EW); \draw (W) -- (EW);
\end{tikzpicture}
\caption{Hybrid “ancestry” diagram: short-name roots (solid) and exemplar hybrids (dashed).}
\label{fig:opt-hybrid-tree}
\end{figure}