How to Retain Leading Spaces?

________________________________
/                                \        ___________________
| Jawed Vertebrates              |       /                   \
|                                |       |   Jawless         |
|    (Bony Fish)                 |       |     Vertebrates   |
|    (Cartilaginous Fish)        |       |                   |
|  ____________________________  |       |                   |
| /                            \ |       |                   |
| | Terrestrial Vertebrates    | |       | (Cyclostomes --   |
| |                            | |       |   e.g. Lampreys)  |
| |  (Amphibia)                | |       \___________________/
| |  ________________________  | |
| | /                        \ | |
| | | Amniota                | | |
| | |                        | | |
| | |   (Reptiles)           | | |
| | |                        | | |
| | |   (Mammals)            | | |
| | |                        | | |
| | \________________________/ | |
| \____________________________/ |
\________________________________/
Not surprisingly, this does indeed yield a phylogenetic tree as follows:
​
Cyclostomes    Bony Fish  Cart. Fish   Amphibia   Reptiles   Mammals
-----------    ---------  ----------   --------   --------   -------
\                \      /              \           \      /
\                \    /                \           \    /
\                \  /                  \           \  /
\                \/                    \           \/
\                \                     \          /
\                \                     \        /   Amniota
\                \                     \      /
\                \                     \    /
\                \                     \  /
\                \                     \/
\                \                    /
Jawless         \                \                  /  Terrestrial Vert.
Vertebrates    \                \________________/
\               /
\             /   Jawed Vertebrates
\           /
\_________/
​
Vertebrates
​
Furthermore, on page 287 Denton applies the same technique to primates:
​
________________________________
/                                \        ___________________
| Gibbon                         |       /                   \
|                                |       |   Monkeys         |
|  ____________________________  |       |                   |
| /  _________    ___________  \ |       |                   |
| | /         \  /           \ | |       \___________________/
| | |         |  |           | | |
| | | Apes    |  |  Man      | | |
| | |         |  |           | | |
| | \_________/  \___________/ | |
| \____________________________/ |
\________________________________/
​
From which we get the following phylogenetic tree:
​
Monkeys    Gibbon     Apes    Man
-------    ------     ----    ---
\         \         \      /
\         \         \    /
\         \         \  /
\         \         \/
\         \        /
\         \      /
\         \    /
\         \  /
\         \/
\        /
\      /
\    /
\  /
\/

I used the face dbcode to change the font to courier within the pre tags, and it seems to work. I have the same problem with the format in the OP, using Win10 and Chrome. I suspect it has something to do with the default font for pre?On further examination, leading white spaces appear to be stripped.

It wasn't working for me previously, but now everything is displaying correctly. It looks fixed on my side.