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JOURNAL OF PALEONTOLOGY, V. 62, NO. 3. 1988
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FIGURE 1 -- Increase in size of means and extreme values
within a clade as a function of origin near a lower limit in size. From
Stanley (1973).
coelacanths and monoplacophorans are pushed into the deep seas (for we must
have losers as well as winners), while Aysheaia rises from sea to
land to yield the modern Onychophora.
Only one major interpretive (as opposed to methodological) reform has
graced the study of trends in our century: the preference for orthoselection
over various forms of near mysticism and special pleading, and the resulting
emphasis upon opportunism and fluctuations in rate. But the fundamental,
topological conviction had held firm-that trends are anagenetic sequences
expressed as entities exhibiting net movement in a specified direction.
A DIFFERENT TOPOLOGY
If macroevolution has no independent status and must be viewed, in uniformitarian
perspective, as an extrapolation into geological vastness of the apparatus
used to interpret ecological moments in modern populations, then the anagenetic
view must prevail. Shifts in gene frequencies within populations dominate
the ecological moment, and geology surely provides enough scope to make
a trend by extending these effects into deep time. A different view of
trends therefore requires the replacement of this extrapolationist vision-a
model that, among its other doleful effects, implies a subservient status
for paleontology among evolutionary disciplines (as guardians of a descriptive
pageant fully explained by principles from the more prestigious world
of the observable now).
But if we view species as stable entities for most of their geological
existence, not as temporary names for transient states in the great and
continuous flux of life, then we must interpret trends differently. If
species are not generally changing, then macroevolution cannot be an extrapolation
of anagenesis within populations. Macroevolution-including the central
phenomenon of trends -must be conceptualized as the differential success
of species. If microevolution results from the sorting of organisms within
populations, then macroevolution occurs by sorting of species within clades
(Vrba and Gould, 1986). Nature operates an entity making and breaking
machine in the processes of speciation and extinction. If differential
birth and death produce anagenesis in populations, then differential speciation
and extinction forge trends within clades. To understand trends, we must
adopt what Eldredge (1979) called a taxic, rather than a transformational,
view of macroevolution.
But why should a differential production and death of species produce
a trend in anything other than number of entities? Why should the differential
success of species yield a directional movement of traits?
Consider an example of a common situation, first generalized by Stanley
in his seminal article (1973) on reinterpreting Cope's rule. Suppose that
the founding species of a clade originates near one boundary of its potential
range (near the lower limit of potential size, or simply near shore for
a marine Bauplan), and that the number of descendant species within
the clade then increases steadily and substantially. Suppose that the
modal class never changes-that is, the most common size or geographic
position of later species remains at the value of the clade's founding
member. Yet the location of the founder at an edge of the potential range
virtually guarantees that new species will be differentially added in
the direction of greater available space-larger body sizes, or deeper
water in our examples above. Stanley (1973) both recognized this principle
as a generality and empirically documented the right-skewed nature of
histograms for body size within clades. His simulation (Figure 1, from
Stanley, 1973) illustrates my central point both graphically and dramatically.
In such cases, many of our usual measures will yield an apparent trend
anagenetically expressed. If we make a deep epistemological error and
focus on extreme values through time as though they were "things"
in themselves rather than tails of a range in variation, we might interpret
the basic phenomenon-increase in number of species, with asymmetry about
a starting point near one end of a constrained range-as a directional
trend (see Figure 1). If we took a more "sophisticated" view,
and represented the clade by a measure of central tendency, we might still
fall into the same error. Mean values will also increase (though less
dramatically than extremes) towards the open end of the range, simply
because distant excursions from the original value can proceed in only
one direction. If we increase our sophistication and calculate a median
instead of a mean (recognizing the old principle that one Rockefeller
may unfairly balance a thousand paupers in determining a mean income),
we may still note an increase through time because the open end of the
range provides more space for new items in general, not only a larger
vista for distant excursions from the founding value. Thus, we face the
peculiar situation of a clade with an unchanging modal value through time,
but a measured increase anagenetically depicted in extreme values, means,
and medians.
Should we view such a situation as a "trend" towards larger
sizes or deeper waters within the clade? I can imagine certain questions
that would legitimately engender such an interpretation. But surely, the
unchanging modal value makes a powerful case for an overriding stability
of the most common class of species, and this fact should be acknowledged
in any interpretation. More importantly, whatever myopic measure we choose
to employ, the central phenomenon of this clade's history is an increase
in variance mediated by the entity-making machine, not an anagenetic
march anywhere. Our common anagenetic |