Hi all,

Allen Aigen wrote,
> The fact that people generally agree that there is a succession of species
implies punctualism.  Otherwise there would be one species that simply
varied in time, like a clinal relation in space.   But if you insist on long
periods of time for stasis, you may not find it always.

Not necessarily. The geologic record is incomplete, "more gap than record"
as Derek Ager called it in his engaging little book, "The Nature of the
Stratigraphical Record". Consider what kind of record we would have if
deposition were continuous in a shallow sea. If sediment accumulated at a
rate of only 1 millimeter per year, in 100 years there would be a layer 100
mm = 10 centimeters thick. In 10,000 years, about the duration of the
Holocene, the layer would be 10,000 mm = 10 meters thick. In 1,000,000
years, the layer would be 1000 meters thick, but the shallow sea would
already be filled with about 3400 feet of sediment! If you allowed the
thought experiment to continue for 65,000,000 years, to reach back to the
time of the last dinosaurs, the layer would be 65,000 meters = 65 kilometers
thick, and even if there were an basin deep enough to accommodate it, any
fossils would be metamorphosed into unrecognizability at the base of such a
monstrously thick deposit. But the Earth is far older than that. Obviously,
sediment does not accumulate indefinitely anywhere.

-- But what a disappointment! That means that we can't have a complete
record of evolution, either. What we actually have is a series of "time
frames" recorded in sediment, some of them deposited gradually and some of
them episodically by storms, floods, etc.

The deep sea has a particularly long, continuous record, and accumulation
rates are much slower than we have been imagining, typically only about 2 to
4 centimeters per thousand years. (Which still adds up to a considerable
thickness in 65 million years; do the math!) In deepsea sediments, the
evolution of microfossils can be tracked with considerable accuracy for tens
of millions of years. Unfortunately, it's not possible to track mollusks
this way, because the deepsea sediments have to be sampled in long, narrow
cores that are only 8 cm wide. Mollusks are too big and too sparsely
distributed in the deep sea, and they dissolve too readily.

There are complications -- this is the real world, after all. Some deepsea
sediments were corroded, dissolved back into seawater. Pteropods, being made
of metastable aragonite, are more susceptible to dissolution than are
foraminifera and other microfossils, so their record is not as satisfactory
as we'd like. Some sedimentary intervals, like the K-T boundary, tend to be
cherty and hard to core properly.

The upshot of all this is that because the geologic record is discontinuous,
the fossil record necessarily looks discontinuous also. David Campbell and
other paleontologists are very well aware of this, and are consequently very
interested when a deposit turns up that has a fauna of intermediate age
between previously known faunas. Also, in some cases, a species shows no
significant change from one deposit to the next for millions of years,
skipping across the gaps, as it were. So we know that some species do evolve
in a punctuated manner. It is harder to establish that other species change
gradually, because of the gaps, but this can be done within especially
suitable "time frames" with continuous deposition over a long period.

Darwin already understood some of this in 1859, by the way, though a lot of
details have been filled in since then. I highly recommend his "Origin of
Species", which includes a chapter on "The Incompleteness of the Geological
Record". The theory of evolution would be intact even if fossils did not
exist and nothing were known about geology: a wonderful fact that
creationists would rather not know, perhaps. We all carry the evidence in
our bodies.

Cheers,
Andy

Andrew K. Rindsberg
Geological Survey of Alabama