documentclass{beamer}
usepackage{tikz}
usetikzlibrary{positioning}
tikzset{mybox/.style={draw,minimum width=1.5cm}}
begin{document}
begin{frame}[fragile]{Boxes popping up}
begin{overlayarea}{textwidth}{8cm}
begin{tikzpicture}
node[mybox](A){A};
pause
node[mybox,above right=of A](B){B};
node[mybox,below right=of A](C){C};
draw[-latex] (A.east)--(B.west);
draw[-latex] (A.east)--(C.west);
pause
node[mybox,below right=of B](D){D};
draw[-latex] (B.east)--(D.west);
draw[-latex] (C.east)--(D.west);
pause
node[mybox,right=of D](E){E};
draw[-latex] (D.east)--(E.west);
end{tikzpicture}
end{overlayarea}
end{frame}
end{document}