transition from Mesohippus to Miohippus68, conforms
to punctuated equilibrium, with stasis in all species of both lines, transition
by rapid branching rather than phyletic transformation, and stratigraphic
overlap of both genera (one set of beds in Wyoming has yielded three species
of Mesohippus and two of Miohippus, all contemporaries).
Prothero and Shubin conclude: "This is contrary to the widely-held
myth about horse species as gradualistically-varying parts of a continuum,
with no real distinctions between species. Throughout the history of horses,
the species are well-marked and static over millions of years. At high
resolution, the gradualistic picture of horse evolution becomes a complex
bush of overlapping, closely related species."
*
Relative frequencies.
Elegant cases
don't make punctuated equilibrium any more
than a swallow makes a summer, but there are a growing number of reports
documenting an overwhelming relative frequency (often an exclusivity) for
punctuated equilibrium in entire groups or faunas. Consider the lifetime
testimonies of taxonomic experts on microfossils69, on brachiopods70,71
and on beetles72. Fortey73 has concluded for trilobites
and graptolites "that the gradualistic mode does occur especially in
pelagic or planktic forms, but accounts for 100% or less of observations
of phyletic change, and is relatively slow".
Other studies access all available lineages in entire faunas and assert
the dominance of punctuated equilibrium. Stanley and Yang26
found no gradualism at all in the classic Tertiary molluscan sequences
of the Gulf and Atlantic Coasts, With the exception of Gryphma,
Hallam23 detected no phyletic change in shape (but only for
body size) in any Jurassic bivalve in Europe. Kelley24,25 documented
the prevalence of punctuation for molluscs in the famous Maryland Miocene
sequence, and Vrba74 has done the same for African bovids.
Even compilations from the literature, so strongly biased by previous
traditions for ignoring stasis as non-data and only documenting putative
gradualism, grant a majority to punctuated equilibrium, as in Barnovsky's75
compendium for Quaternary mammals, with punctuated equilibrium "supported
twice as often as phyletic gradualism . . . the majority of species considered
exhibit most of their morphological change near a speciation event, and
most species seem to be discrete entities". When controlled studies
are done by one team in the field, punctuated equilibrium almost always
seems to predominate. Prothero76 "examined all the mammals
with a reasonably complete record from the Eocene-Oligocene beds of the
Big Badlands of South Dakota and related areas in Wyoming and Nebraska
. . . With one exception (gradual dwarfing in the oreodont Miniochoerus),
we found that all of the Badlands mammals were static through millions
of years, or speciated abruptly (if they changed at all)."
* Inductive patterns.
Even more general
inductive patterns should be explored as
criteria. Stanley38,77 has proposed a series of tests, all carried
out to punctuated equilibrium's advantage. Others suggest that certain environments
and ecologies should be conducive to one preferred mode along the continuum
of possibilities. "Johnson78,79 suggests that punctuated
equilibrium should dominate in the benthic environments that yield most
of our fossil record, while gradualism might prevail in pelagic realms.
Sheldon80 proposes the counter-intuitive but not unreasonable
idea that punctuated equilibrium may prevail in unstable environments, gradualism
in stable regimes.
* Tests from living organisms.
Distinct
evolutionary modes yield disparate patterns as
results; punctuated equilibrium might therefore be tested by studying the
morphological and taxonomic distributions of organisms, including living
faunas. (Several of Stanley's tests38 use modern organisms, and
other criteria from fossils should be explored—especially the biometric
discordance or orthogonality, favourable to punctuated equilibrium and actually
found where investigated25,81, of within and between species
trends.)
Cladistic patterns should provide a good proving ground. Avise82
performed an interesting and much quoted test, favourable to gradualism,
by comparing genetic and morphological differences in two fish clades
of apparently equal age and markedly different speciation frequencies.
But as Mayden83 argued, this test was wrong in its particular
case, and non-optimal as a general procedure; a better method would compare
cladistic sister groups, guaranteed by this status to be equal in age.
Mindel et al.84,85 have now performed such a test on
the reptilian genus Sceloporus and on allozymic data in general,
and have validated punctuated equilibrium's key claim for positive correlation
of evolutionary distance and speciation frequency. Lemen and Freeman's86
interesting proposal for additional cladistic tests cannot be sustained
because they must assume that unbranched arms of their cladograms truly
feature no speciation events along their routes, whereas numerous transient
and extinct species must populate most of these pathways. Wagner87
has developed a way of estimating rapidly branching speciation versus
gradual speciation or transformation from cladograms, and his initial
results favour predominant rapid branching in Palaeozoic gastropods.
Difficulties and prospects
Many semantic and terminological muddles that once impeded resolution
of this debate have been clarified. Opponents now accept that punctuated
equilibrium was never meant as a saltational theory, and that stasis does
not signify rock-hard immobility, but fluctuation of little or no accumulated
consequence, and temporal spread within the range of geographic variability
among contemporary populations—by Stanley's proper criterion, so
strikingly validated in his classic study26. We trust that
everyone now grasps the centrality of relative frequency as a key criterion
(and will allow, we may hope, that enough evidence has now accumulated
to make a case, if not fully prove the point).
Evolutionary biologists have also raised a number of theoretical issues
from their domain of microevolution. Some, like the frequency of sibling
speciation, seem to us either irrelevant or untroublsome as a bias against,
rather than for, our view (as we then underestimate the amount of true
speciation from palaeontologically defined morphospecies, and such an
under-estimate works against punctuated equilibrium). Others, like the
potential lack of correspondence between biospecies and palaeontological
morphospecies, might be worrisome, but available studies, done to assess
the problem in the light of punctuated equilibrium, affirm the identity
of palaeontological taxa with true biospecies (see Jackson and Cheetham88
on bryozoan species, and Michaux27 on palaeontological stasis
in gastropod morphospecies that persist as good genetic biospecies).
But continuing unhappiness, justified this time, focuses upon claims
that speciation causes significant morphological change, for no validation
of such a position has emerged (while the frequency and efficacy of our
original supporting notion. Mayr's genetic revolution" in peripheral
isolates, has been questioned). Moreover, reasonable arguments for potential
change throughout the history of lineages have been advanced6.34,
although the empirics of stasis throws the efficacy of such processes
into doubt. The pattern of punctuated equilibrium exists (at predominant
relative frequency, we would argue) and is robust. Eppur non si muove;
but why then? For the association of morphological change with speciation
remains as a major pattern in the fossil record.
We believe that the solution to this dilemma may be provided in a brilliant
but neglected suggestion of Futuyma89. He holds that morphological
change may accumulate anywhere along the geological trajectory of a species.
But unless that change be "locked up" by acquisition of reproductive
isolation (that is, speciation), it cannot persist or accumulate and must
be washed out during the complexity of interdigitation through time among
varying populations of a species. Thus, species are not special because
their origin permits a unique moment for instigating change, but because
they provide the only mechanism for protecting change. Futuyma writes:
"In the absence of reproductive
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