My apologies. I had no intention of impugning the motivations of others, and
I regret that I evidently did so.
Let me summarize my top-down thesis and then I will end my participation in
this thread.
1. I define the top-down direction of phyletic development in terms of the
Linnaean system of classification. This is a hierarchical system because it
is built on and conforms to the hierarchical organization of the biologic
world. It does not generate the top-down pattern of taxa appearance; on the
contrary, it is the top-down pattern of taxa appearance itself that generates
the hierarchical classification system. The topmost Linnaean category in the
animal kingdom is the phylum. Phylum Chordata is our phylum because all the
fish, amphibians, reptiles and mammals (including human beings) of the phylum
share the common feature of the spinal column that first appeared as a
notochord in our founder in the Cambrian.
2. Since I use the Linnaean classification system, I also use the metaphor
of a _staircase_, to describe the progression of developmental changes in
phyletic lineages, rather than the branching tree that Keith uses. The top
step of this staircase is the phylum. The lower taxonomic categories are the
lower, descending stairs: classes, orders, families, genera, species.
3. I also define the top-down direction of phyletic development in
historical terms. So I begin at the Cambrian explosion, about 530 million
years ago, at which time 35 or so animals suddenly appeared that were the
founders of all the major groups of animals that are with us today, plus a
dozen or more that died out along the way. These animals are classified at
the level of phyla, because their body plans define the common anatomical
features of group they founded.
4. So the phylum, the top of the staircase, and the top category of the
Linnaean classification system come together conceptually in the historical
moment when the Cambrian animals appeared. The top-down direction of
phyletic development, beginning with the phylum, is thus nicely started as a
historical process.
5. Animals forming the class, the next step down from the phylum, appeared
after the Cambrian explosion. Classes share the basic set of anatomical
features characteristic of the phylum despite substantial dissimilarities of
the anatomical features of the members of the class from each other. Classes
illustrate another feature of the top-down pattern. Each lower step in the
taxonomic progression produces more variable and numerous organisms than the
step above it. Thus the staircase spreads out as it descends. One can enter
the classification system at any point and proceed downward from there.
6. This step-down process continues through all the lower taxonomic levels
of a given group of animals with exceptions and convolutions, of course,
brought about by chance environmental events, until the level of species is
finally reached. At this level there may be thousands and even millions of
species, all descended down the taxonomic staircase from the first founder
animal.
7. Further, Dobzhansky also noted that discontinuity is another fundamental
characteristic of organic diversity, besides its hierarchical ordering. He
said, "Classifiers of animals and plants utilize discontinuities to achieve
a natural and convenient [classification] system. The system is 'natural' in
so far as the discontinuities are observable realities" (Dobzhansky, et. al.,
_Evolution_. Freeman, 1977). The Linnaean system of discrete categories
embodies this discontinuity. He stated further, "The genetic and structural
gaps between species of one genus are on the average smaller than between
genera, smaller between genera than between families, and so forth." Thus
climbing the staircase from the bottom up would become ever more difficult
the higher up the taxonomic scale one moved. This argues against the
bottom-up direction of evolution that Darwin hypothesized.
8. Finally, I hypothesize that the process of development is the process
underlying the early stages of the top-down direction of phyletic change.
Principles of development, in this view, apply to large groups of animals
over geologic time as well as to individual organisms over one life span.
Just as the somatic genome of the individual organism is the primary source
of individual development, so the germ-line of the lineage is the primary
source of phyletic development. Later as development processes run out of
steam, Darwinian mechanisms, especially natural selection, come into play.
This switch-over is suggested by the accelerating number of species
generated in the latter half of the Phanerozoic, especially in the Cenozoic,
but with no new major higher taxonomic categories forthcoming. Since species
are the by-product of natural selection it may be inferred that the Darwinian
natural selection is at work. Its function is to add minor variations to
species to insure, as van Inwagen suggests, "that species possess sufficient
diachronic flexibility that they aren't just automatically wiped out by the
first environmental change that comes along" and thus they enhance the
survival of the lineage. (Darwinism: Science or Philosophy, Buell and
Hearn, 1994).
That's about it. Shalom to all,
Bob