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Hope this is clearer and my memory is not hopelessly out of date.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..
most "tree of life" diagrams I have seen have the most recent species at the top branching off (diversifying or speciating) from earlier ancestors.
moreover, though tree of life diagrams are useful tools or illustrations, they are essentially cartoons simplified to the absolute essentials.