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Speciational Evolution or Punctuated Equilibria
by Ernst Mayr
O <http://www.stephenjaygould.org/images/letters/o.gif>
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nly recently have we understood how different are the concepts to
which the term "evolution" has been attached. With wisdom of hindsight, we
can now (250 years after Buffon) distinguish three very different concepts
of evolution: saltational evolution, transformational evolution, and
variational evolution.
Theories postulating saltational evolution are a necessary
consequence of essentialism. If one believes in constant types, only the
sudden production of a new type can lead to evolutionary change. That such
saltations can occur and indeed that their occurrence is a necessity is an
old belief. Almost all of the theories of evolution described by H. F.
Osborn (1894) in his From the Greeks to Darwin were saltational theories,
that is, theories of the sudden origin of new kinds. The Darwinian
revolution (Darwin, 1859) did not end this tradition, which continued to
flourish in the writings of Thomas H. Huxley, William Bateson, Hugo De
Vries, J. C. Willis, Richard Goldschmidt
<http://www.stephenjaygould.org/people/richard_goldschmidt.html> , and Otto
Schindewolf. Traces of this idea can even be found in the writings of some
of the punctuationists.
According to the concept of transformational evolution, first
clearly articulated by Lamarck, evolution consists of the gradual
transformation of organisms from one condition of existence to another.
Almost invariably, transformation theories assume a progression from "lower
to higher" and reflect a belief in cosmic teleology resulting in an
inevitable steady movement toward an ultimate goal, an ultimate perfection.
In biology all so-called orthogenetic theories, from those of K. E. von Baer
to Osborn, L. S. Berg, and Teilhard de Chardin are in this tradition.
As R. C. Lewontin (1983) has correctly pointed out, Darwin
introduced an entirely new concept of evolution: variational evolution. New
gene pools are generated in every generation, and evolution takes place
because the successful individuals produced by these gene pools give rise to
the next generation. Evolution thus is merely contingent on certain
processes articulated by Darwin: variation and selection. No longer is a
fixed object transformed, as in transformational evolution, but an entirely
new start is, so to speak, made in every generation. Evolution is no longer
necessarily progressive; it no longer strives toward perfection or any other
goal. It is opportunistic, hence unpredictable.
What Darwin did not fully realize is that variational evolution
takes place at two hierarchical levels, the level of the deme (population)
and the level of species. Variational evolution at the level of the deme is
what the geneticist deals with. It is effected by individual selection and
leads minimally to the maintenance of fitness of the population through
stabilizing selection.
The second level of variational evolution is that of the species.
Owing to continuing (mostly peripatric) speciation, there is a steady,
highly opportunistic production of new species. Most of them are doomed to
rapid extinction, but a few may make evolutionary inventions, such as
physiological, ecological, or behavioral innovations that give these species
improved competitive potential. In that case they may become the starting
point of successful new phyletic lineages and adaptive radiations. Such
success is nearly always accompanied by the extinction of some competitor.
This process of succession of species is often referred to by the term
"species selection," but to prevent misunderstandings it may be better to
call it "species turnover" (see below).
The transfer from transformational to variational evolution required
a conceptual shift that was only imperfectly carried through by most
Darwinians. As a consequence, geneticists described evolution simply as a
change in gene frequencies in populations, totally ignoring the fact that
evolution consists of the two simultaneous but quite separate phenomena of
adaptation and diversification. The latter results from a process of
multiplication of species, a process almost totally ignored in the writings
of R. A. Fisher <http://www.library.adelaide.edu.au/digitised/fisher/> , J.
B. S. Haldane <http://www.stephenjaygould.org/people/john_haldane.html> ,
Sewall Wright <http://www.stat.wisc.edu/~yandell/www/wright/> , and other
leading evolutionary geneticists.
Transformational thinking likewise continued to dominate
paleontology, expressed in the concept of phyletic gradualism. Since most
paleontologists were typologist (in an almost Platonian sense), they
subconsciously assumed that species were everywhere the same and, thus, at
any given time essentially uniform. Speciation consisted of the gradual
transformation of such species in geological time. Since the gradualness of
such phyletic transformation could be documented in the geological record
only in the rarest cases, it was postulated that the absence of
intermediates was a consequence of the notorious incompleteness of the
fossil record. The so-called evolutionary species definition adopted by most
paleontologists (Simpson, 1961; Wilimann, 1985) reflects the same focus on
the vertical (i.e., time) dimension. If adopted, it leaves only two options:
speciation is explained either by gradual phyletic evolution, with the gaps
between species being due to the deficiency of the fossil record, or by
sympatric saltational speciation. Indeed, most paleontologists adopted both
options. Acceptance of phyletic gradualism does not require the acceptance
of a constant rate of evolutionary change. The rate may accelerate or slow
down, but change leads inexorably to the steady transformation of a lineage.
Even Darwin, for reasons that relate to his struggle against
creationism, stressed the transformational aspect of evolution. He was,
however, fully aware of highly different rates of evolution, from complete
stasis to rates of change so fast that intermediates could not be discovered
in the fossil record (Gingerich, 1984; Rhodes, 1983; and others). Owing to
his adoption of sympatric speciation
<http://www.pbs.org/wgbh/evolution/library/glossary/glossary.html> ,
however, Darwin never needed to consider the geographical component in his
theorizing. When he said that a new species might originate as a local
variety, he did not claim that it was an isolated population. It seems to me
that for Darwin the pulsing of evolutionary rates was a strictly vertical
phenomenon.
The geneticists, with the exception of a few saltationists such as
DeVries and Bateson, usually ignored the problem of speciation altogether.
The only geneticists who showed an interest in the multiplication of species
were those who had been educated as taxonomists, like Theodosius Dobzhansky
and G. L. Stebbins. The problem of relating speciation to macroevolution
occupied primarily three zoologists, Julian Huxley (1942), Mayr (1942,
1954), and Bernhard Rensch (1947), who were neither geneticists nor
paleontologists. Since these three were among the architects of the
evolutionary synthesis <http://www.amazon.com/exec/obidos/ASIN/0674272269/>
, one can state that the problem of the relation between speciation and
macroevolution was not entirely ignored by the evolutionary synthesis.
The widespread neglect of the role of speciation in macroevolution
continued until Niles Eldredge
<http://www.actionbioscience.org/evolution/eldredge.html> and Stephen Jay
Gould (1972) proposed their theory of punctuated equilibria. Whether one
accepts this theory, rejects it, or greatly modifies it, there can be no
doubt that it had a major impact on paleontology and evolutionary biology.
The gist of the theory was that "significant evolutionary change
arises in coincidence with events of branching speciation, and not primarily
through the transformation of lineages" (Gould, 1982a:83, 1983). The
contrast between the previously dominant view of evolutionary change was as
follows. Traditionally, evolution had been seen as a single-phase phenomenon
of gradual change, albeit sometimes more slowly, sometimes more rapidly. Now
evolution was seen as an alternation between speciation events during which
the major evolutionary (particularly morphological) change occurred and
lengthy periods of stasis.
Historical studies have since shown that the term "punctuated
equilibrium" was more novel than the concept. A role for peripheral
populations in speciation was already postulated by L. v. Buch (1825) and
fully substantiated by Darwin for the Galapagos mockingbirds. Unfortunately,
by the time Darwin published the Origin (1859), he had adopted sympatric
speciation (Mayr, 1982a). When he said that a new species might originate as
a local variety, he did not necessarily mean an isolated population. Nor are
the changes in the rate of evolution to which Darwin refers brought in
relation to speciation.
Before going any further in the analysis of the literature, it is
important to call attention to a prevailing confusion between two distinct
evolutionary phenomena, gradualism and uniformity of evolutionary rate.
Darwin emphasized gradualism (Rhodes, 1983), but, as I shall show, even that
term is ambiguous, allowing for two very different interpretations. What
Darwin did not insist upon was a uniformity of rates (Huxley, 1982; Penny,
1983; Rhodes, 1983). The existence of so-called living fossils was known to
paleontologists early in the nineteenth century, and the occurrence of
different rates of evolution in different phyletic lines was paleontological
dogma already in Darwin's lifetime. George Gaylord Simpson
<http://www.stephenjaygould.org/people/george_simpson.html> (1953) analyzed
this phenomenon in great detail and even introduced a special terminology to
characterize lineages with average, very rapid, and extremely slow
evolutionary rates. Eldredge and Gould never claimed to have discovered this
difference in rates, and the part of the ensuing polemic stressing these
differences is therefore irrelevant for the evaluation of the punctuation
theory.
It is not always easy to interpret Darwin's statement (Rhodes, 1983)
because isolation (at least during the process of speciation) had become
unimportant for him owing to his adoption of sympatric speciation. I have
been unable to discover in Darwin's writings any connection between
allopatric speciation and change of evolutionary rate. Gould and Eldredge
correctly state that Simpson likewise failed to make such a connection. His
quantum evolution was a vertical (temporal) phenomenon, as it had to be
considering his evolutionary species definition (Simpson, 1944:207-217).
Paleontologists knew that new species may originate in a very
circumscribed area and turn up in the fossil record only after having spread
more widely (Bernard, 1895). This insight was made use of, however, only in
stratigraphic research and not in studies of macroevolution. On the
contrary, the importance of peripatric speciation was minimized after Fisher
(1930) and Wright (1931, 1932) had asserted, although for different reasons,
that evolution was most rapid in populous, widespread species, a conclusion
adopted also by Dobzhansky (1937, 1951) and by most evolutionists before the
1970s.
I believe I was the first author to develop a detailed model of the
connection between speciation, evolutionary rates, and macroevolution (Mayr,
1954). Although long ignored, my new theory of the importance of peripatric
speciation in macroevolution is now widely recognized. "Mayr's hypothesis of
peripheral isolates and genetic revolution must of necessity be a
centerpiece of the punctuated equilibria theory; it is the theory, for all
practical purposes" (Levinton, 1983:113). 1 once more presented my theory in
great detail (Mayr, 1963:527-555). Under these circumstances it is most
curious that the theory was completely ignored by paleontologists until
brought to light by Eldredge and Gould (1972).
The major novelty of my theory was its claim that the most rapid
evolutionary change does not occur in widespread, populous species, as
claimed by Most geneticists, but in small founder populations. This
conclusion was based on empirical observations gathered during my studies of
the speciation of island birds in the New Guinea region and the Pacific. I
had found again and again that the most aberrant population of a
species-often having reached species rank, and occasionally classified even
as a separate genus-occurred at a peripheral location, indeed usually at the
most isolated peripheral location. Living in an entirely different physical
as well as biotic environment, such a population would have unique
opportunities to enter new niches and to select novel adaptive pathways.
As I pointed out elsewhere (Mayr, 1982b), my conclusion was that a
drastic reorganization of the gene pool is far more easily accomplished in a
small founder population than in any other kind of population. Indeed, I was
unable to find any evidence whatsoever of the occurrence of a drastic
evolutionary acceleration and genetic reconstruction in widespread, populous
species.
In view of frequent recent misrepresentations of my 1954 theory I
must emphasize also what I did not claim (see also Mayr, 1982b):
1. I did not claim that every founder population speciates.
In the vast majority, only minor genetic reorganizations occur, and the
majority of such founder populations soon become extinct or merge with the
parental species.
2. I did not claim that every genetic change in a founder
population is a genetic revolution. Evidently it requires a special
constellation for the occurrence of a more drastic genetic reorganization.
All I claimed was that when a drastic change occurs, it occurs in a
relatively small and isolated population.
3. I did not claim that speciation occurs only in founder
populations. Finally, I nowhere claimed that I chose the name "peripatric'
(Mayr, 1982c) because the founders came from the periphery of the parental
range. I chose that name because the founder populations were at peripheral
locations. My interpretation throughout was very pluralistic and was
naturally misunderstood in an age when singular, deterministic solutions
were strongly preferred.
In 1954 I was already fully aware of the macroevolutionary
consequences of my theory, saying that "rapidly evolving peripherally
isolated populations may be the place of origin of many evolutionary
novelties. Their isolation and comparatively small size may explain
phenomena of rapid evolution and lack of documentation in the fossil record,
hitherto puzzling to the palaeontologist" (p. 179).
I later supplemented my theory by pointing out (Mayr, 1982c) that
peripatric speciation may occur not only in founder populations but also in
any population going through a severe bottleneck such as refuge populations
during Pleistocene glaciations (Haffer, 1974).
The first to pick up my theory was Eldredge (1971), who found in his
study of Paleozoic trilobites that the majority of species showed no change
in species-specific characters throughout the interval of their
stratigraphic occurrence, whereas new species appear quite suddenly in the
strata. He therefore proposed that the allopatric model be substituted in
the minds of palaeontologists for phyletic transformation as the dominant
mechanism of the origin of new species in the fossil record. This was
followed in 1972 by the Eldredge and Gould paper, in which the term
"punctuated equilibrium' was proposed. The Eldredge-Gould proposal was
essentially my 1954 theory, except for a far stronger emphasis on stasis,
indeed a belief that no further evolutionary change would occur after the
speciation process was completed.
Questions and Objections
A modest theory of punctuationism is so strongly supported by facts
and fits, on the whole, so well into the conceptual framework of Darwinism,
that one is rather surprised at the hostility with which it was attacked.
The controversy over punctuationism is, by now, more than twenty years
<http://www.skeptic.com/archives09.html> old, and it is possible to
distinguish different classes of objections. There are questions that deal
with the core ideas of the theory: What is stasis? How can one account for
it? Do all species experience stasis? Is all evolutionary change restricted
to bouts of speciation? If so, why? What are the genetic aspects of
speciation? These and other questions will be analyzed in the second part of
this essay.
But not all the objections raised against punctuationism deal with
these core ideas. Others were raised against rather specific claims made by
Eldredge, Gould, or both, or against the way they treated their evidence. It
will be helpful to deal with these objections first. They relate largely to
claims that are not part of a punctuationist theory of evolution. To deal
with them separately and to test them for their validity will clear the
field for a subsequent testing of the core ideas of punctuationism.
Four aspects of the treatment of punctuationism by Gould and
Eldredge were objected to most frequently. First to receive attention was
the seemingly monolithic nature of the claims. Even though Eldredge and
Gould (1972) nowhere stated that a neospecies enters a period of total
stasis, this is what all their graphic presentations suggested (figs. 5-4,
5-8, 5-10). Furthermore, evolutionary trends are explained by a process of
species selection of completely static species (fig. 5-10). Not
surprisingly, their opponents assumed that Eldredge and Gould had postulated
total stasis for all species after they had completed the process of
speciation.
Professor Gould assures me that they had never adopted such an
extreme position, and in their next paper they stated emphatically: "We
never claimed that gradualism could not occur in theory or did not occur in
fact..The fundamental question is not 'whether at all,' but how often"
(Gould and Eldredge, 1977:119). In their abstract, Gould and Eldredge
specify: "Most species, during their geological history, either do not
change in any appreciable way, or else they fluctuate mildly, with no
apparent direction. Phyletic gradualism is very rare" (p. 115).
The second point of contention was the claim of novelty. Nothing
incensed some evolutionists more than the claims made by Gould and
associates that they had been the first to have discovered, or at least to
have for the first time properly emphasized, various evolutionary phenomena
already widely accepted in the evolutionary literature. G. L. Stebbins and
F. J. Ayala (1981), Verne Grant (1982, 1983), and J. S. Levinton (1983) were
fully justified in rejecting these claims of novelty. In particular, they
showed that an insistence on gradualism by Darwin and his followers was a
denial of saltationism but not a denial of different and changing rates of
evolution.
Third, vigorous objection was raised to the claim that
punctuationism would require a revision of Darwin's "evolutionary
synthesis." "I have been reluctant to admit it, but if Mayr's (1963:586)
characterization of the synthetic theory is accurate, then that theory as a
general proposition is effectively dead" (Gould, 1980:120). The gist of my
statement to which Gould refers was that, contrary to Goldschmidt and
Schindewolf, nothing happens in macroevolution that does not happen in
populations. What Gould actually attacks, and rightly so, is the completely
reductionist characterization of evolution by the mathematical population
geneticists. To equate these reductionist views with the theories of the
evolutionary synthesis is unjustified, however, as I pointed out in a
critical review of similar statements published by M. W. Ho and P. T.
Saunders (Mayr, 1984b). A rejection of the axiom of most population
geneticists, "Evolution is a change of gene frequencies," is not a rejection
of the evolutionary synthesis. The theory of the synthesis is much broader
and constitutes in many respects a return to a more genuine Darwinism. The
events that take place during peripatric speciation, no matter how rapid
they may be, are completely consistent with Darwinism.
Curiously, some authors also mistakenly assume that the occurrence
of stasis would refute Darwinism. Teleological thinking requires continuous
evolutionary change, but Darwin rejected teleology (Mayr, 1984a) and
accepted stasis (Rhodes, 1983). An evolutionary lineage may continue to vary
genetically without undergoing any major reconstruction. Alternatively, a
stable lineage may continue to send out founder populations, some of which,
through peripatric speciation, could become more or less distinct daughter
species.
The fourth reason why punctuationism faced so much opposition is
that at one stage
<http://www.stephenjaygould.org/library/gould_structure.html> Gould pleaded
for a revival of Goldschmidt's
<http://www.stephenjaygould.org/people/richard_goldschmidt.html> ideas and
implied that they were akin to punctuationism. This claim clearly indicated
that there was considerable conceptual confusion as to what punctuated
equilibria really means. Before the possibility that Goldschmidt was a
forerunner of punctuationism can be discussed constructively, it is
necessary to discriminate among four interpretations of punctuationism.
1. An evolutionary novelty originates by a systemic mutation:
the individual produced by such a mutation is the representation of a new
species or higher taxon.
2. Evolutionary change is populational, but all substantial
evolutionary changes takes place during bouts of speciation. As soon as the
process of speciation is completed, the new species stagnates ("stasis") and
is unable to change in any significant way. Early statements by Eldredge and
Gould (1972) and Gould and Eldredge (1977) gave the impression that this was
their interpretation.
3. Phyletic lineages ("evolutionary species") can evolve slowly
and gradually into different species and even genera, but the more
pronounced evolutionary changes and adaptive shifts take place during
speciational bouts in isolated populations. This has been all along my own
interpretation (Mayr, 1954, 1982b) and is presumably that of many
evolutionists familiar with geographic speciation.
4. A multiplication of species (the branching of lineages)
occurs but is of no greater evolutionary importance than changes within
lineages. In fact, phyletic gradualism is responsible for most evolutionary
change. It was this view, held by the majority of paleontologists, that
induced Eldredge and Gould (1972) to propose their theory of punctuated
equilibria
Only the first one of these four theories conflicts with Darwinism.
It was Goldschmidt's theory, and because Goldschmidt has often been cited in
connection with punctuationism, it is necessary to discuss his ideas in more
detail.
To strengthen the punctuationism case, Gould cited Goldschmidt's
views on macroevolution, indicating that "during this decade Goldschmidt
will be largely vindicated in the world of evolutionary biology" (Gould,
1980:186). Goldschmidt had claimed that the differences among subspecies,
and more broadly all geographic variation, was caused by minimal genetic
changes, mutations of alleles, mostly being selected merely for climatic
adaptation. Such changes would not permit any transgression of the ancestral
type. Any genuine evolutionary novelty was due to the origin of a "hopeful
monster," caused by a systemic mutation. This thesis followed from
Goldschmidt's rather eccentric conception of nature of chromosomes and the
genotype. According to him, a systemic mutation is a complete change of the
primary pattern or reaction system into a new one and has the capacity to
produce a strikingly different new individual that could serve as the
founding ancestor of a new type of organism. As J. Maynard Smith (1983:276)
pointed out, hopeful monsters, by contrasts, are drastically altered
phenotypes. They are possible, at least in theory, and it should be possible
to discover empirically how often they occur and how often (if ever) they
are selectively superior.
It entirely misrepresents Goldschmidt's theory to claim that
Goldschmidt "argued that speciation is a rapid event produced by large
genetic changes (systemic mutations) in small populations" (Gould and
Colloway, 1980:394). The whole concept of populations was alien to his
thinking. According to him, a new type is produced by a single systemic
mutation producing a unique individual. Gould (1982) is also wrong in
claiming that Goldschmidt never had the view "that new species arise all at
once, fully formed, by a fortunate macromutation." Actually , this is what
Goldschmidt repeatedly claimed. For instance, he cited with approval
Schindewolf's suggestion that the first bird hatched out of a reptilian egg,
and he was even clearer on this point in a later paper (1952:91-92) than in
his 1940 book.
In refutation of Goldschmidt's claims I demonstrated (Mayr, 1942)
that geographic variation in isolated populations could indeed account for
evolutionary innovations. Such populations have a very different
evolutionary potential than contiguously distributed, clinally varying
populations in a continental species. As I stated (Mayr, 1954), and have
reiterated (Mayr, 1963 1982b), one can defend a moderate form of
punctuationism, based on strictly empirical evidence, without having to
adopt Goldschmidt's theory of systemic mutations.
Some Basic Questions About Punctuationism
The theory of punctuationism, to repeat, consists of two basic
claims: that most or all evolutionary change occurs during speciation
events, and that most species usually enter a phase of total stasis after
the end of the speciation process. The two claims are to some extent two
separate theories.
The controversy that followed the proposal of this theory revealed
that there are considerable conceptual and evidential difficulties in either
substantiating or refuting this theory. First, the nature of the fossil
record makes it exceedingly difficult, if not impossible, to obtain
irrefutable evidence either for stasis or for a very short time span
speciation. Second, throughout the controversy one encounters considerable
terminological vagueness and equivocation, as for instance concerning the
meaning of such words as "gradual," "stasis," "speciation," and "species
selection." A careful analysis of the terms most frequently used in the
punctuationism controversy is therefore indispensable.
Gradualness
Whether evolution is gradual became the focus of a heated
controversy in the punctuationist argument. Darwin (1859), as everyone knew,
had frequently emphasized the gradual nature of evolutionary change (pp. 71,
189, 480), largely because of his opposition to two ideologies dominant in
his time, creationism and essentialism. After these ideologies lost their
power, it was no longer necessary to be so single-mindedly opposed to the
occurrence of discontinuities. Yet the recent controversy concerning the
saltational versus gradual origin of evolutionary novelty revealed an
equivocation.
Most modern authors failed to distinguish between two very different
phenomena: the production of a new taxon, and the production of a new
phenotype. If the production of a new taxon is gradual, it is taxic
gradualism; if it is instantaneous, it is taxic saltation. Likewise, one can
distinguish phenotypic gradualness and phenotypic saltation. What Darwin
mostly argued against was the thesis that evolutionary novelties could
originate through taxic saltation, that is, through the production of a
single individual representing a new type, a new taxon. Instead, he proposed
that all evolutionary innovation is effected through the gradual
transformation of populations.
This distinction became important after Goldschmidt revived the
essentialistic idea that a new higher taxon could be established as the
product of a single systemic mutation. Even though the success of such a
taxic saltation is too improbable to be endorsed by a contemporary
evolutionist, it still leaves the possibility of the occurrence of
phenotypic saltations. If a mutation with a drastic phenotypic change could
be incorporated in a population and become part of a viable phenotypic
polymorphism, it could lead to a seemingly saltational evolutionary change.
Gould (1980:127) indeed envisages a "potential saltational origin for the
essential features of key adaptations. Why may we not imagine that gill arch
bones of an ancestral agnathan moved forward in one step to surround the
mouth and form proto-jaws?" Maynard Smith (1983:276) points out that the
occurrence of "genetic mutations of large phenotypic effect is not
incompatible with Darwinism." Steven M. Stanley (1982) has argued quite
persuasively that gastropod torsion might have originated through a single
mutation. It would have had to pass through a stage of polymorphism until
the new gene had reached fixation. Evidently such a process is feasible, but
its importance in evolution is contradicted by the fact that, among the
millions of existing populations and species, mutations with large
phenotypic effects would have to be exceedingly frequent to permit the
survival of the occasional hopeful monster among the thousands of hopeless
ones. But this in not found. Furthermore, enough mechanisms for the gradual
acquisition of evolutionary novelties are known (Mayr, 1960) to make the
occurrence of drastic mutations dispensable, at least as a normal
evolutionary process.
The argument, thus, is not whether phenotypic saltations are
possible, but rather whether evolution advances through the production of
individuals representing new types or through the rapid transformation of
populations. No matter how rapid, such a populational "saltation" is
nevertheless Darwinian gradualism.
Stasis
Of all the claims made in the punctuationist theory of Eldredge and
Gould, the one that encountered the greatest opposition was that of
"pronounced stasis as the usual fate of most species," after having
completed the phase of origination (Gould, 1982a:86). Yet it was this very
claim which the authors designated as their most important contribution.
The extraordinary longevity of the so-called living fossils had, of
course, been known since the early days of paleontology (Eldredge and
Stanley, 1984; de Ricqles, 1983). But is such stasis the usual fate of most
species? Evidence supporting this claim can be found in Stanley's book
(1979), some review papers (e.g., Levinton, 1983; Gould, 1982b), and recent
volumes of Paleobiology, Systematic Zoology, and other journals. Yet the
literature also reports numerous cases of seeming speciation by phyletic
gradualism (e.g., Van Valen, 1982:99-112). Perhaps most convincing are the
cases of significant evolutionary transformation in continuous phyletic
lineages reported by K. D. Rose and T. M. Brown (1984) for Eocene primates
and by J. Chaline and B. Laurin (1986) for Pliocene rodents. Such phyletic
speciation seems to be more frequent in terrestrial than in marine
organisms.
Two objections have been raised against the seeming cases of
phyletic speciation. First, hiatuses and depositional breaks seem to occur
even in the most complete sequences; second, the so-called species of these
sequences may not be valid species because they usually differ only in minor
characters of size and proportions. Be that as it may, Gould has recently
seemed to concede that speciation by phyletic gradualism does occur.
I agree with Gould that the frequency of stasis in fossil species
revealed by the recent analysis was unexpected by most evolutionary
biologists. Admittedly, stasis is measured in terms of morphological
difference, and the possibility cannot be excluded that biological sibling
species evolved without this being reflected in the morphotype. Let us
tentatively assume that some species enter complete stasis while others
evolve by phyletic gradualism.
The question of what percentage of new species adopts one or the
other of these two options cannot be resolved either by genetic theory or
through the study of living species. It can be decided only though an
analysis of the paleontological evidence, and this poses great
methodological difficulties (Levinton and Simon, 1980; Schopf, 1982). For
instance, in the analysis of the benthic foraminifers, the calculated
average age of 20 million years was based on only 15 percent of the recent
species. For all the others the fossil record was too spotty to permit any
determinations. In other words, the proof of stasis was based on a highly
biased sample, consisting of common widespread species, which one could
expect to have longevity and which comprised a small minority of the entire
fauna. It is conceivable that a considerable fraction of the remaining 85
percent underwent rapid phyletic speciation and thus became unavailable for
analysis. The indications are that the vast majority of the so-called rare
species are short-lived, probably not for reasons of rapid phyletic change
but rather owing to extinction. The best one can do under the circumstances
is to adopt an intermediate position by admitting the occurrence of some
gradual phyletic speciation but pointing also to the unexpectedly large
number of cases in which fossil species showed no morphological change over
many millions of years.
Recent discoveries in molecular biology have raised questions about
the meaning of stasis. The stasis found in morphological characters in such
old genera as Rana, Bufo, Plethodon, or even Drosophila is not at all owing
to the retention of an entirely unchanged genotype. Through the
electrophoresis method, countless changes in quasi-neutral enzyme genes have
been discovered, but numerous other nonmorphological changes have also taken
place in these genera, such as the acquisition of new isolating mechanisms,
as well as of numerous adaptations to changing environments. What has
remained stable, however, is the morphotype, the basic Bauplan. The species
in some lineages that can be inferred to have separated 30 to 60 million
years ago are morphologically still almost indistinguishable except in size,
coloration, and minor differences in skeletal dimensions. [.]
The Contributions of Punctuationism to Evolutionary Theory
Even some of its opponents admit that punctuationism has had an
enormously stimulating effect on evolutionary biology (Rhodes, 1984; Maynard
Smith, 1984b; Gould, 1985). The controversy has brought to light numerous
equivocations and has helped to clarify distinctions between alternatives,
such as between phyletic and allopatric speciation, between phenotypic and
taxic saltations, between various types of group selection, between the
evolutionary potential of small and large populations, between an
uncompromisingly reductionist and a more holistic concept of the genotype,
between various concepts of species selection, and still others. To
eliminate these equivocations it was not only necessary to clarify concepts
but also to show that we needed a broader factual foundation. As Gould has
correctly emphasized, one of the most important contributions of
punctuationism has been its stimulation of fruitful empirical research, much
of it still ongoing.
To be sure, the claims of some punctuationists, such as the
prevalence of total stasis and the impossibility of evolutionary change
without speciation, are clearly invalid. Furthermore, it has been shown that
"speciational evolution" (perhaps a better term than "punctuationism") is
fully consistent with Darwinism; and finally, that seeming evolutionary
saltations, as indicated by the fossil record, can be explained without
invoking systemic mutations or other mechanisms in conflict with molecular
genetics. It is irrelevant for the theory of speciational evolution how
relatively frequent evolutionary stasis is or how frequent the occasional
occurrence of drastic reorganization during peripatric speciation.
Most of all, punctuationism has shown how one-sided has been the
myopic focusing of paleontologists and population geneticists on the
one-dimensional, transformational, upward movement of evolution. It finally
brought general recognition to the insight of those who had come from
taxonomy (E. Poulton, Rensch, Mayr) and had consistently stressed that the
lavish production of diversity is the most important component of evolution.
What had not been realized before is how truly Darwinian
speciational evolution is. It was generally recognized that regular
variational evolution in the Darwinian sense takes place at the level of the
individual and population, but that a similar variational evolution occurs
at the level of species was generally ignored. Transformational evolution of
species (phyletic gradualism) is not nearly as important in evolution as the
production of a rich diversity of species and the establishment of
evolutionary advance by selection among these species. In other words,
speciational evolution is Darwinian evolution at a higher hierarchical
level. The importance of this insight can hardly be exaggerated.
[ Ernst Mayr, "Speciational Evolution or Punctuated Equilibria,"
from Albert Somit and Steven Peterson's The Dynamics of Evolution
<http://www.amazon.com/exec/obidos/ASIN/0801497639/> , New York: Cornell
University Press, 1992, pp. 21-48. ]
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