1:9): "The thing that hath been, it is that which shall
be; and that which is done, is that which shall be done; and there is no
new thing under the sun."
Yet we
must study bushes with no prominent directional change if we are to gain
any proper sense of the full range and character of life's history. Even
if we believe (and I will confess to holding this conventional bias myself)
that trends, however rare, are the most interesting of phyletic phenomena--for
they do supply the direction that makes evolution a pageant rather than
a tableau--we still need to know the relative frequency of nonprogressive
evolution, if only to grasp the prevailing substrate from which rare trendiness
builds interesting history. How can we claim to understand evolution if
we only study the percent or two of phenomena that construct life's directional
history and leave the vast field of straight-growing bushes--the story
of most lineages most of the time--in a limbo of conceptual oblivion?
I see
some happy signs of redress, as paleontologists are now beginning to study
this higher order stasis, or nondirectional history of entire bushes.
An excellent and path-breaking case has just been published by Ann F.
Budd and Anthony G. Coates in our leading trade journal, Paleobiology
(vol. 18, 1992, pp. 425-46): "Nonprogressive evolution in a clade
of Cretaceous Montastraea-like corals." Budd and Coates state
their aim in their introduction, and I could not agree more:
Just as the study of stasis within species has facilitated understanding
of morphologic change associated with speciation, we show that study of
nonprogressive evolution offers valuable insight into how the causes of
trends interact and thereby produce complex evolutionary patterns within
clades [evolutionary bushes], regardless of their overall direction.
Montastraea is a genus of massive colonial reef-building corals,
still important in our modern faunas (many readers undoubtedly have a
chunk of Montastraea on their mantle pieces). Budd and Coates studied
the earlier history of the Montastraea bush during the long span
of Cretaceous time--some 80 million years duration, and representing
the last period of dinosaurian domination on land. They found little evidence
of directional change, but rather a story of oscillation within a range
set by minimal and maximal size of corallites (individual coral animals
within the colony). At one end, "large-corallite" species (33-8.0
mm in diameter) are more efficient in removal of sediment and tend to
be more common in regions of turbid water; at the other end, "small-corallite"
species (2.0-3-5 mm in diameter) tend to dominate in clearer waters near
the reef crest. In addition, large-corallite species tend to feed actively
on small planktonic animals, while small-corallite species derive more
nutrition directly from the zooxanthellae (photosynthetic algae) that
live symbiotically within their tissues.
Budd and Coates conjecture that corallite diameters may be held within
these limits by some ecological or developmental constraint at the low
end (implying that still smaller corallites could neither develop nor
function adequately) and by a limit to the number of septa at the high
end. (Septa are the radiating series of plates that form the skeletal
framework for a corallite. The "astraea" in Montastraea
refers to the star-shaped pattern of these radiating septa in cross section.)
The size of corallites might be limited if new septa could not form beyond
a certain number--although this argument is frankly speculative. If
such constraints limit the domain of corallite form, and if each end enjoys
advantages in different environments always available in some parts of
the geographic range, then evolution might just oscillate back and forth,
with no persistent directional component through time. |