I think you're going to have to explain this question in more detail before jennyp or anyone else can answer it for you. What do you mean by "a model of a species", and what is an "upside down tree shape"?
I've answered a lot of questions about evolution and written a lot of articles about it, and I can't figure out what you're trying to ask here.
I hope my memory serves just a bit. I remember seeing, in a book (possibly by Dawkins, more likely Gould) on evolution some years ago, a diagram of the basic flow of the variety of species.
The diagram more accurately portrayed a tree branch upside down, than one right-side up. By this I mean, and I believe the author was illustrating, that what went on was a large number of species winnowing down to smaller and smaller numbers of species. Something like this:
x
xx
xxxx
xxxxx
xxxxxxxxxx
With recent at top, long ago at bottom.
Instead of a species varying into, and resulting in, more and more variety, there were large numbers of similar-niched species decreasing in number.
Natural selection would explain the winnowing of course, but the mechanism providing the material for winnowing was less clearly understood - by me.
This relates somewhat to the Cambrian explosion question. Or perhaps a punctuated equilibrium. It went agains my common sense understanding of mutation/variation/selection in that it seemed upside down in that manner.
Hope this is clearer and my memory is not hopelessly out of date.
thanks for your reply..