"The pattern [top-down] is not in dispute--the reason for its existence is.
The "top down" pattern of taxa appearance _is_ generated by retrospectively
grouping taxa hierarchically from a diversifying tree of life. Test this for
yourself, draw an upward branching tree (use Darwin's own diagram). Now begin
at the top by grouping branches until you reach the bottom. You will find
that the more inclusive groups appear first."
Bob responded:
>Keith's got it wrong. In the top-down perspective, you turn Darwin's
>branching tree upside down. By doing so you get a _downward_ branching tree.
> In fact you have fifty upside-down trees, each started by a Cambrian animal
>that was the founder of the tree, or phylum. You don't have to begin by
>"grouping branches." The trunk of each tree is the "inclusive group," the
>phylum, if you will.
>
>After the Cambrian explosion, the branching proceeded downward, at each
>branching a new, lower-level taxon unfolded. Thus it went, taxon by taxon,
>each taxon diversifying as it went, through taxonomic class, order, etc.,
>until today we have species, but no more higher taxa. The picture is this:
>50 phyla 530 million years ago, a dearth of species: today millions of
>species, no new higher taxa. That's the historical reality.
I think Bob does not understand my point. When I refer to an upward
branching tree I am referring to the splitting of lineages through time.
The top of the tree is the present, and the ends of the branches are living
taxa. Grouping taxa in such a tree beginning from the top (the present
living taxa) results in higher level taxonomic groups reaching their peak
diversity before the lower level taxa contained within them. This is
simply a function of classification procedure. This pattern is in no way
opposed to macroevolutionary theory Darwinian or otherwise.
Keith
Keith B. Miller
Department of Geology
Kansas State University
Manhattan, KS 66506
kbmill@ksu.ksu.edu
http://www.ksu.edu/~kbmill/