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JOURNAL OF PALEONTOLOGY,
V. 62, NO. 3, 1988
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FIGURE 5--Although size of the largest species tends to
increase in each of the three radiations, size of the smallest is either
constant or decreases. Therefore, the main trend is towards an increase
of variation in size, not a general directional movement Cretaceous
in five-mil-lion-year intervals, Paleogene and Neogene in three-million-year
intervals.
mum size at an intermediate time; but
Paleogene correlations for extremes and central tendencies also lie in the same
range). Finally, whatever happens to spread and skewness, the most common
values show no recognizable pattern of change at all. Dividing the total size
range within each phase into 10 arithmetic units, and then designating the unit
with the most species in each time segment as a "modal decade," I obtain
the results of Figure 6 (if two or more decades, have the same maximal number
of species, I designate all with this value). A pattern of
TABLE 1--Correlation coefficients for putative
measures of size trends in planktonic foraminiferal clades vs. time.
See text for explanation.
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| |
Cretaceous
|
Paleogene
|
Neogene
|
|
- Extremes
- Largest
- Smallest
- Central Tendencies
- Mean
- Median
- Modal decade
- Variation
- Total range
- Coefficient of variation
|
0.919
-0.243
0.779
0.752
0.554
0.929
0.783
|
0.440
0.129
0.459
0.290
0.142
0.430
0.407
|
0.879
-0.492
0.767
0.741
0.153
0.867
0.666
|
|
increase
occurs during the first four Cretaceous times. Otherwise, we find no change in
the size of the most common species through time at all. Correlation coefficients
with modal decade (Table 1) affirm this conclusion (I use arithmetic means if
two or more decades have the same maximal number of species at any time). In
conclusion, the resolution of Cope's rule not as anagenesis to larger size, but
as skewed and increasing variance around an asymmetrical starting point, leads
to a range of different questions inconsistent with conventional ideas of directional
selection, acting on organisms and mediated by competition, as the cause of trends.
Why do founders of clades tend to be small? How can we define potential size ranges
for Baupläne? What powers the entity making and breaking machine of
differential species success?DECREASE TRENDS AND THE
ELIMINATION OF EXTREMES In the biases of our preferred anagenetic
mode, we invoke a set of metaphors for the common phenomenon of decreasing variance
misinterpreted as the directional movement of an entity. We speak of "retreat,"
"driving back," "outcompeting," or "hunkering down"
if we view the "trend" as a failure or prelude to extinction; "honing,"
"fine tuning," or "optimization" when we sense that survivors
are the best subset of too much former latitude. And yet, from the more appropriate
standpoint of species as stable individuals within a clade, no entity moves anywhere
when a change in extreme values or central tendencies results from a reduction
in number or disparity of species. I shall consider several kinds of cases for
trends properly treated as decreasing variance but often misinterpreted as anagenetic
"movement" of entities.Asymmetrical trends.--This
category represents the inverse of increase trends discussed in the previous section.
Principles and explanations are similar and I shall not stress this theme again.
The differential shrinkage and removal of one tail from a frequency distribution
will change extreme values (for that tail) most, and means and medians (for the
whole distribution) less. But the causal phenomenon is a removal of entities or
a reduction in disparity among a constant number of species, not a temporal excursion
of one abstracted measure. For example, we often read that coelacanths and monoplacophorans
were pushed out of shallow water into the less inviting oceanic depths. This conjures
up a picture of dispirited losers, marching dejectedly down the continental slope--banished
to geological anonymity in a world of unpreserved sediments from the Devonian
(Neopilina's forebears) or the Cretaceous (Latimeria's ancestors),
until the modern world of dredges and Alvins discovered their hiding places. But
why should we imagine anything moving? Perhaps some species of monoplacophorans
and coelacanths always lived in deep waters. The clades once had species |