@oristarA,
Tree Space is the total area of a particular Phylogenetic Tree.
Here's a example of one for birds.
There's lots of them, ranging from simple bacterias to highly developed apes like us.
Thank you, by the way, I try to learn something new every day and today I learned what a Phylogenetic tree is and how complex the language used to describe them is. Wow.
Joe(waving from my branch over here.)Nation