"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."
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.
Keith said, "The pattern [top-down] is not in dispute--the reason for its
existence is." Why the top-down pattern? Here is how I answer the question.
The pattern is the result of developmental processes at work in deep time as
well as in local, individual time. I call this the developmental
perspective, or macro-development. It is predicated on the assumption that
principles of development can be applied to large animal groups over geologic
time as well as to individual organisms. This historical dimension is what
has been missing from developmental theory. The developmental perspective
provides it.
I compare features of individual development and patterns of phyletic change
found in the fossil record. There are striking similarities between the two.
1. Both individual organisms and phyletic lineages start out in life with a
general body plan. It is the first anatomical feature of the embryo after
the body axes have been laid down. The body plan is the foremost feature to
appear in Cambrian animals.
2. Both individual animals and phyletic lineages developed very rapidly at
the start. Early development in the embryo is extremely rapid. In human
beings all systems and morphological features are in place in slightly more
than three months after conception. The formation of body plans in the
Cambrian occurred with great rapidity, geologically speaking, in a matter of
5-10 million years.
3. Body plans are extremely stable. Early stages of individual development
are remarkably resistant to evolutionary change, either because they are
resistant to mutations or have extremely efficient repair mechanisms. Only
later appearing, superficial features of the animal, such as coloration, are
subject to adaptive change. Evolution thus does not change the body plan and
other early-appearing features of the embryo. Phyletic body plans have not
changed in more than 500 million years, with the possible exception of some
reformulation of sea urchins' body plans.
4. Development proceeds in characteristic time sequences and directions.
This is the top-down direction of development. According to basic laws of
development formulated by von Baer almost two centuries ago, (a) the general
features, which the embryo shares in common with all members of its phylum,
appear before more specific features; and (b) that specific features emerge
out of general ones, These constitute two of the most fundamental principles
of individual development.
After the Cambrian explosion, phyletic lineages differentiated from the top
down, as I have been saying in these posts-from the few, most general
taxonomic features (such as the body plan) held in common by all members of
the lineage, to the most specific and diverse features distributed among its
multitudinous species. This is the upside-down developmental phyletic tree.
The top-down direction of both individual and phyletic development
represents a realistic link between individual and large scale development,
as noted by K. S. Thomson.
5. The shape of the developmental life span is curvilinear. The entire life
span of individual animals is a manifestation of development. All organisms
start small and morphologically simple at conception; rise rapidly through
the prenatal and juve nile stages; grow large, robust, and complex; reach a
rounded maximum on many vari ables in maturity; decline in old age, and
eventually die. The rise and subsequent decline is an invariant
characteristic of lifelong individual development.
There is also an unvarying succession of changes in ancestral lineages of
higher complex animals, starting with a few, small, insignificant animals,
that eventually increase in size, complexity, population density, and on many
other dimensions; reach a rounded maximum, and then decline to fewer,
smaller, less robust groups. This orderly sequence is isomorphic to life-long
development in individual organisms.
6. Eventual decline. All individual organisms show signs of senescence if
they live long enough. Phyletic lineages of higher, complex animals begin to
show signs of decline and aging after they reach and pass their maximum
growth and development, and will eventually die if they do not first become
extinct.
In short, this is what I'm saying: The overall "shape" of individual
development and the general "shape" of ancestral lineages are remarkably
similar. The only major difference between them is the time
scale--enormously long for the historical phyletic lineages, insignificantly
short for individual organisms. The resemblance between large-scale sequence
of changes covering millions of years and the sequence of changes in early
embryonic and lifelong development of individual organisms is quite
astounding. I argue that it cannot be a meaningless coincidence that so many
phyletic patterns of change in the fossil record are found to resemble
patterns of development found in individual organisms. These similarities
are surely not trivial or merely coincidental. On the contrary, they point
to a deep unity between the overarching historical processes of development,
which I have called macro-development, and small scale individual
development. Patterns in the fossil record are sufficiently similar to
patterns in development of individual organisms that those in the fossil
record can provisionally be consid ered the results of development on a large
historical scale.
One more thing. The ultimate source of individual development is the somatic
genome, and epigenetic phenomena arising in development. The ultimate source
of macro-development is the phyletic germ line and epigenetic phenomena
associated with it. The somatic genome is inherited from the phyletic germ
line and thus derives its characteristics from the germ line.
What I have outlined above will appear in a larger context and greater detail
in a forthcoming article in the September issue of PSCF entitled, "Paradoxes
in Darwinian Theory Resolved by a Theory of Macro-Development."
Shalom,
Bob