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JOURNAL OF PALEONTOLOGY, V. 62, NO. 3.1988 |
small-brained species present at all times. The evolution of enlarged brains,
though generally a route to success and survival of new species, was not universal
even among progressive orders. In his latest work, Jerison adds that periods
of expansion are confined to relatively short segments in the histories of most
clades (1985, p. 124): When there have been advances in grade
in a broad spectrum of species ... the advance occurred over at least several
million years, but when an appropriate grade was achieved there was no further
advance. The prosimians appeared to have reached their present grade of encephalization
by the end of the Oligocene. Steady states such as the Neogene stasis in prosimian
encephalization are the rule. The picture is, therefore, more consistent with
a punctuational than with a gradualist model. In one of
the most important dicoveries of modern paleoecology, several colleagues have
demonstrated a strong and repeated tendency in the origin and spread of marine
invertebrate clades: taxa tend to appear first in shallow waters and spread differentially
to greater depths as new clades originate near shore (Sepkoski and Sheehan, 1983;
Sepkoski and Miller, 1985; Jablonski and Bottjer, 1983). Many commentators have
subtly misinterpreted this pattern as a directional trend mediated by the conventional
Darwinian forces of competition and wedging-as advanced forms "push"
a discrete and coherent entity into deeper waters. In most cases, the pattern
should be read as a consequence of increasing variance, not as the shoving of
an entity anywhere. If groups originate nearshore, and cannot invade the land
by constraints of physiology, then where else can their mean and median position
move as their taxic representation increases? Only one potential direction exists
for expansion-farther offshore. Sepkoski and Sheehan (1983, p. 705-707) report
that their best example of this onshore-to-offshore pattern occurred during a
major episode of increase in the number of taxa: "The Ordovician radiations
effected one of the largest and most rapid increases in taxonomic diversity that
occurred during the Phanerozoic. In the first 50 m.y. of the Ordovician, the number
of animal families in the oceans tripled". We have often focused
on the wrong phenomenon in judging the importance of this discovery. As Sepkoski
and Shechan recognize, the "trend" offshore is a simple, almost trivial,
result of expansion in taxa given the asymmetrical distribution of possibilities
around a starting point in shallow waters. The profound issue is our need to understand
why starting points tend to be nearshore; the later "trend" is a much
less interesting consequence. Cope's rule forms the locus classicus
for this theme of increased variance misinterpreted as anagenetic trend. We all
know the panoply of traditional explanations framed in terms of selective advantage
for larger-bodied individuals--competitive success, greater efficiency in
metabolism, larger foraging range, among many others. Although Stanley's seminal
paper (1973) properly reinterpreted this phenomenon as an expansion away from
small-bodied starting points mediated by increasing numbers of taxa within clades,
purely anagenetic, explanations continue to prosper. In a paper titled "Body
size, ecological dominance and Cope's rule," Brown and Maurer (1986, p.250)write,
for example: Presumably the ecological advantage of monopolizing
resources provides the selective pressure that promotes evolution of greater size.
Individuals of large size are favored by intraspecific natural selection, because
they can dominate resource use and consequently leave more offspring than their
smaller relatives. Brown and Maurer do acknowledge a species-level
effect of greater probability for extinction in small populations of specialized
large-bodied creatures; but they view this differential only as a device to clear
some space so that yet another lineage can begin its anagenetic march towards
the personally advantageous realm of bulk. I do not deny that size
increase (or, rather, change of size in general) has often been documented as
the most common of truly anagenetic trends within unbranched lineages (Bown and
Rose, 1987; Malmgren and Kennett, 1981). But most clade-level trends to increasing
body size are products of species sorting (Vrba and Gould, 1986), not a lockstep
march to greater bulk by most species in parallel. Cope's real insight lay in
his other, often neglected "law of the unspecialized" (see Stanley,
1973, p. 10-11)--the principle that morphologically "simple" creatures
(generalized in form and ecology, not just plesiomorphic per se) act as starting
points to most clades, and that such generalists tend to be small in body size.
If originators usually lie near the lower end of a potential range in size, then
Cope's more famous "law" of size increase follows as an automatic consequence
once we view such trends at the proper level of species sorting. Where else can
large numbers of successful species be added? Cope's rule arises from an asymmetry
of possibilities about a starting point. The position of the starting point itself
is the phenomenon demanding explanation; all else follows automatically from an
increase in variance.We owe this elegant reversal of perspective to
Stanley (1973), and should really speak of "Stanley's rule" of increase
by species sorting from small size. Stanley also provided a means for distinguishing
true anagenesis from expansion of variance around asymmetrical starting points.
He recommends that we establish the histograms of size within clades through time,
rather than the geological excursion of central tendencies or extreme values.
If we find a coupling between increased variance and increasing right skew, then
the revised explanation holds. Jablonski (1987) has studied all Late
Cretaceous bivalve and gastropod genera with durations greater than 4 million
years in the Gulf and Atlantic Coastal Plain. He recorded temporal change in range
of variation, rather than excursions of central tendency. Of 58 bivalve genera,
for example, 33 did follow Cope's rule in the "broad" (I would say inappropriate)
sense that the largest late genus exceeds the largest early genus m size. But
this extraction of extremes from complete systems is misleading. Only 11 of these
genera showed a corresponding increase for the smallest species through time--the
lockstep shift of the entire range that anagenetic interpretations require. For
the other 22 genera with increase at the right extreme, size of the smallest species
either remained stable or decreased. Jablonski concludes that "Cope's rule
is driven by an increase in variance rather than a simple directional trend m
body sizes." Using data kindly supplied to me by Richard Norris,
I present a fuller example from a history long interpreted as a canonical example
of Cope's rule--trends in size for planktonic foraminifera. This case possesses
the particular virtue of repetition, for latest Cretaceous and Paleogene extinctions
provide three cycles of evolution from a limited number of ancestral forms.
Raw data for maximum test diameter (mm) of first appearances for 377 species
against time (133 Cretaceous, 103 Paleogene, 141 Neogene) are shown in Figure
3. The basis for invoking Cope's rule is clear enough; each of the three phases
shows a steady increase through time for its largest member (the Paleogene curve
peaks before the end and then declines, but Creta- |