How to draw up a hierarchical tree diagram for taxonomic classification

You can do this sort of tree easily with the tikz-qtree package:

In the code below, the labels are created using a tree with branches that are not actually drawn (using a style blank which sets [draw=none]). This ensures that they line up with the levels of the main tree. By putting two trees as nodes in a matrix we achieve a reasonable separation between the labels and the main tree.

\documentclass{article}
\usepackage{tikz-qtree}
\begin{document}
\tikzset{edge from parent/.style=
{draw, edge from parent path={(\tikzparentnode.south)
-- +(0,-8pt)
-| (\tikzchildnode)}},
blank/.style={draw=none}}
\begin{tikzpicture}
\matrix
{
\node{\Tree
    [.Class  \edge[blank]; 
    [.Order  \edge[blank];
    [.Family \edge[blank]; 
    [.Genus ]]]]};
&
\node{\Tree 
 [.{$\gamma$-proteobacteria} 
    [.Alteromonadales 
        [.Alteromonadaceae  {Glaciecola}  Alteromonas Agarivorans ] ]
    [.Vibrionales [.Vibrionacae Vibrio ]]]};\\
};           
\end{tikzpicture}

\end{document}

run with xelatex

\documentclass{article}
\usepackage{pst-tree}
\def\psedge{\ncangles[angleA=-90,angleB=90]}
\psset{levelsep=14mm,treesep=1.75cm,nodesep=3pt}
\begin{document}

\hspace*{1.3cm}
\pstree{\Tr[name=class]{$\gamma$-proteobacteria}}{%
  \pstree{\Tr[name=order]{Alteromonadales}}{%
    \pstree[thistreesep=3mm]{\Tr[name=family]{Alteromonadaceae}}{%
       \Tr[name=genus]{Glaciecola}
       \Tr{Alteromonas} 
       \Tr{Agarivorans}%
  }}
  \pstree{\Tr{Vibrionales}}{%
    \pstree{\Tr{Vibrionacae}}{%
      \Tr{Vibrio}
  }}
}

\rput[rc](0,0|class){Class}\rput[rc](0,0|order){Order}%
\rput[rc](0,0|family){Family}\rput[rc](0,0|genus){Genus}%
\end{document}