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Re: Olshevsky's Rule, the Critical Response (was Re: "Cope's Rule" Put to the...



In a message dated 97-01-23 03:07:16 EST, znc14@ttacs1.ttu.edu (Jonathan R.
Wagner) writes:

<< In any case, George's restatement of Cope's Rule is still not a
 useful generalization.  I would like to see some sort of evidence,
 preferrably backed up by a theoretical model, for this theory.  I cannot
 accept evidence from phylogenies which accept this rule a priori, what I
 would like to see is a statistical analysis using cladistic techniques. >>

(1) See my response to Ron Orenstein's post.
(2) Read my summary of why Cope's Tendency works, in any printing of
_Mesozoic Meanderings_ #2. In fact, let me excerpt some unedited rewrites
from the forthcoming third edition:

The fundamental principle of phylogeny is that organisms possessing large
enough suites of characters in common do so because they descended from
common ancestors, not coincidentally (i.e., by convergence). This is because
the space of possible morphologies and molecular sequences is so immense that
the chance of the same suite of characters or sequences arising within
unrelated taxa is vanishingly small. Cladistics employs such suites of
characters to discover which of a rather large set of possible phylogenies is
likely to be correct. As such, cladistic analysis is (among other things) a
more rigorous application of Cope's Rule ("organisms tend to evolve from
generalized forms to specialized forms") and Dollo's Law ("anatomical parts
lost through evolution are not restored, only replaced"). Neither of these
"laws" is absolutely true, of course; they are simplified descriptions of two
commonly observed properties of evolution. Their significance lies more in
their breach than their observance, for it is the breaches that are rare and
phyletically useful. ...

Among terrestrial vertebrates, evolution usually follows Cope's Rule
(Stanley, 1973). That is, larger, more specialized forms evolve from smaller,
generalized forms. I will avoid the question of what the terms "generalized"
and "specialized" mean; but the pattern of size increase has a simple and
cogent explanation. All other things being equal, larger individuals tend to
dominate smaller ones of the same species, outcompeting them for resources
and leaving a disproportionate number of offspring. This selection effect
induces the general evolutionary trend toward size increase seen in the
vertebrate fossil record. Counteracting this trend may be such adverse
circumstances as heavy predation, when larger individuals simply become
bigger targets less able to find concealment, and scarcity of resources, when
larger individuals are less able to locate the greater sustenance they
require. The latter circumstance is responsible for island-endemic dwarfism,
for example. Also counteracting this trend are the requirements of
physiological fitness: Giant individuals are seldom as fit as normal-size
individuals, limiting the tolerable size increase within the Bauplan of a
particular species before the Bauplan?and the species?must change. Finally,
small amniotes tend to attain sexual maturity faster and have larger litters
than large amniotes: the well-known r-selection reproductive strategy. This
results in faster genetic turnover among smaller forms (Simpson's
"tachytelic" evolution), which, in the absence of predation, may "radiate"
into numerous lineages. As larger forms evolve from smaller ones, their
evolutionary tempo slows (Simpson's "bradytelic" evolution), and the tendency
to evolve new higher-level Bauplaene decreases. These effects are quite
evident in the evolutionary patterns of the archosaurs. ...

Viewing the evolutionary patterns of archosaurian subclades as smaller
"Hennigian combs" suggests that a single model might account for them all.
Central to this model is the existence of core groups of small, rapidly
evolving (tachytelic), highly diverse archosaurs that comprise evolutionary
engines from which the larger, more slowly evolving (bradytelic) forms arose.
Being small, core forms?such as Compsognathus, or animals directly on the
"mesenosaur-to-robin" lineage?are seldom found as fossils, and when they are,
they tend to be perplexing animals that seldom fit into the better-known
groups. The reason I use the term core group rather than core lineage is that
a core group comprises a bundle of lineages of morphologically similar taxa
that would be extremely difficult to disentangle even if we had a nearly
perfect fossil record. For example, the extant passerine birds form a large
avian core group. If each of its several thousand species were available to
us only as a fossil, we would find it practically impossible to reconstruct
their phylogenies correctly; it is difficult enough even when we have access
to living populations and can chemically analyze their genomes (Sibley &
Ahlquist, 1990).
Because they are small animals, core forms are subject to considerable
predation pressure, often from their own larger descendants, which tends to
keep their evolutionary fecundity in check. But when an extinction event
removes this predation pressure, the lineages within the core groups rapidly
radiate to fill the void. As the new forms become large and widespread, the
likelihood of their preservation as fossils rises, and the fossil record
displays their "abrupt" appearance. The fossil archosaurs we have so far
discovered are thus just the tip of an evolutionary iceberg.
Core groups are practically immune from mass extinctions, though not from
evolutionary change. Large terrestrial vertebrates are unable to find shelter
from mass-extinction agents such as abrupt climatic change, asteroid impact,
and so forth. Burdened with a slow evolutionary tempo, large animals
generally respond to such agents by becoming extinct, and since they are by
virtue of their size prominent in the fossil record, it is their
disappearance that often signals the "mass extinction." New Bauplaene seldom
if ever evolve among large terrestrial vertebrates?although they would, if it
weren't for the meddlesomeness and frequency of mass-extinction agents. As
things stand, the evolution of larger animals consists almost exclusively of
"variations on a theme."
Evolution in a core group occurs at a faster tempo because core-group animals
are smaller, have shorter gestation periods, take less time to attain sexual
maturity, and sometimes (particularly among mammals) have larger litters than
large animals. In the absence of predation, this is the source of radiative
evolution. But it is a wellspring of rapid evolution under most other
circumstances as well. When niches for large forms become occupied,
core-group evolution simply produces new core-group forms. Core groups can
themselves split into other core groups, though not quite as rapidly as
species can speciate. New Bauplaene are thus a comparatively frequent product
of core-group evolution. The primary agent of core-group extinction?if this
process really does occur and is not simply an artifact of core-group
anagenesis?seems to be continual competition for resources with other core
groups, rather than the global mass-extinction events that wipe out the
larger forms every few score million years. One peculiar way for a core group
to become "extinct" is for all its member lineages to evolve into animals of
larger size. This seems to have happened to the ornithischians during the
Jurassic Period. Core-group extinctions (or turnovers) appear to be less
episodic and occur at a more uniform pace than the extinctions of the larger
forms.