\documentclass[beamer]{standalone}
\usepackage{tikz}
\usetikzlibrary{external}
\tikzexternalize % activate! 
\begin{document}
\begin{standaloneframe}

% If overlays do not work, use \only<1-n>{...} where n is the max overlay
  \only<1-7>{
    \begin{tikzpicture}[]
      \draw[color=white] (-5.5,0) -- (4,2);
      \node<2-> (MTT3) at (-1,3) {$\mathrm{MTT}^{n\geq 3}$};
      \node (ACA) at (3,3) {$\mathrm{ACA}_0$};
      \node (RT3) at (1,3) {$\mathrm{RT}^{n\geq 3}$};

      \node<4-> (MTT2) at (-1,2) {$\mathrm{MTT}^{2}$};
      \node (RT2) at (1,1) {$\mathrm{RT}^{2}$};

      \node (WKL) at (3,1) {$\mathrm{WKL}_0$};
      
      \node<3-> (MTT1) at (-1,0) {$\mathrm{MTT}^{1}$};
      \node<3-> (HL) at (-2,1) {$\mathrm{HL}$};
      \node (RT1) at (1,0) {$\mathrm{RT}^1$};
      \node (RCA) at (3,0) {$\mathrm{RCA}_0$};

      \node<6-> (RG1) at (-3,0) {$\mathrm{RG}^{1}$};
      \node<6-> (RG2) at (-3,2) {$\mathrm{RG}^{2}$};
      \node<6-> (RG3) at (-3,3) {$\mathrm{RG}^{3}$};
      \draw<6->[->, double] (RG2) -- (RG1);
      \draw<6->[<->, double] (RG3) -- (MTT3);
      % \draw<6->[<->, double] (RG2) -- (MTT2);
      \draw<6->[<->, double] (RG1) -- (MTT1);
      \draw<6->[->, double] (RG3) -- (RG2);

      \node<7-> (DT1) at (-5,0) {$\mathrm{DT}^{1}$};
      \node<7-> (DT2) at (-5,3) {$\mathrm{DT}^{2}$};
      \draw<7->[->, double] (DT2) -- (DT1);
      \draw<7->[<->, double] (DT2) -- (RG3);
      \draw<7->[<->, double] (DT1) -- (RG1);

      \draw[<->, double] (ACA) -- (RT3);
      \draw<2->[<->, double] (RT3) -- (MTT3);
      \draw<5->[->, double] (MTT3) -- (MTT2);
      \draw<4->[->, double] (MTT2) -- (RT2);
      \draw[->, double] (RT3) -- (RT2);
      \draw[->, double] (RT2) -- (RT1);
      \draw<4->[->, double] (MTT2) -- (MTT1);
      \draw<3->[<->, double] (MTT1) -- (HL);
      \draw[<->, double] (RT1) -- (RCA);
      \draw<3->[<->, double] (MTT1) -- (RT1);

      \draw[<->, dashed, color=red] (RT2) -- (WKL);

      \draw[->, double] (ACA) -- (WKL);
      \draw[->, double] (WKL) -- (RCA);

    \end{tikzpicture}
  }
\end{standaloneframe}
\end{document}