tree structure has been found to be a good working approximation.
There are some key differences between grammars/lexicons and genotypes.
For one thing, linguistic traits can be acquired throughout one's life
from many different sources, although intitial acquisition and (to a lesser
extent) adolescence seem to be crucial stages. Acquired (linguistic) traits
can also be passed on to others. One consequence is that linguistic history
need not have the form of a tree, with languages splitting but
never rejoining, whereas genetic evolution is largely constrained to have
a tree-like form (despite the possibility of transfer of genetic material
across species boundaries by viral infection and so on). However, as a
practical matter, the assumption that linguistic history is a sort of
tree structure has been found to be a good working approximation.