BtologtcalJoumal oflhe Linnean Society, 12: 293-504. With 1 figure
December 1979
Macroevolution-myth or reality ?
Presidential Address
P. H. GREENWOOD, D.Sc., F. I . Biol., Pres. L.S.
Department of Zoology,
British Museum (Natural History), London
Address delivered on 24 May 1979
Current concepts of macroevolution-the origin and diversification of higher taxonomic
categories-are reviewed. A reductionist hypothesis, seen in the light of models based on extant
species, seems to be corroborated. There appears to be no reason to think that macroevolution is a
natural phenomenon distinct from a s eciation event giving rise to a new phyletic lineage. Neither is
it necessary to postulate any ‘quantum element in that speciation event.
P
KEY WORDS :- macroevolution- microevolution - punctuated equilibrium- phylogeny- speciation
-adaptive radiation- classification- species selection.
There are several reasons why I have chosen macroevolution- the origin and
diversification of higher taxonomic categories-as the subject of this address.
Foremost among them is my long-standing interest in the origin, diversification
and biological significance of characters which we use to formulate an
hierarchical classification. Then there is my work on the cichlid fishes of Africa,
particularly that on the explosive speciation and evolution of cichlids in Lake
Victoria, which provides a constant reminder of these problems and, I believe,
some insights into their solution as well.
Finally, there is the recent surge of literature centred around the subject of
macroevolution. Here the views of both palaeontologists and neontologists have
been aired, and relevant data from the fields of anatomy, taxonomy and genetics
reexamined (see especially Eldredge & Gould, 1972; Gould & Eldredge, 1977;
Stanley, 1975, and Bock’s (1979) particularly thought-provoking major review
paper). Three text books, all published in 1977, also deal with macroevolution at
some length and from dissimilar vie oints (Dobzhansky, Ayala, Stebbins &
Valentine, 1977; Grant, 1977, and Stans eld, 1977).
I make no claims to introduce new ideas in this address. Rather it is a
commentary on the views of others as seen in the light of my own battles with
similar problems, especially those provided by the cichlid fishes of Lake Victoria.
This young, speciose and trophically multiradiate species Hock would s e e m to
provide an effective tool for testing macroevolutionary hypotheses. In particular
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P. H . GREENWOOD
294
i t would appear to offer a unique glimpse into the early phases of
macroevolutionary processes in sexually reproducing animals. Macroevolution
amongst plants and in asexually reproducing organisms is yet another aspect of
the subject, but it is one I feel incompetent to review and it is ignored for that
reason alone.
As might be expected, there is a wide diversity of opinion on the subject of
macroevolution. Implicit, and often explicit in many discussions is the idea that
the characters involved are both quantitatively and qualitatively different from
those associated with the evolution of species or in the so-called
microevolutionary changes characterizing evolution at the population level. This
in turn implies the existence of macro- and microevolutionary levels of genotypic
change, a much debated topic.
Different background interests and different philosophical attitudes to
roblems (especially taxonomic ones), have all contributed to this
diversity o interpretation, as is clearly seen in the accounts of macroevolution
given in the text books already mentioned. A common feature, however, is a
concern both with the means whereby higher taxonomic categories have evolved,
and with the means whereby the evolving taxa are able to occupy new ‘adaptive’
zones. The latter, more ecologically orientated approach is, of course, linked
closely with the anatomico-morphological one since the occupation of new
adaptive zones is generally associated with the development of particular and
often novel anatomical and functional characteristics.
Put in another way (and I would consider this to be the basic if sometimes
neglected problem), macroevolution is concerned with how certain key
morphological and functional features of a lineage have evolved. The actual
higher categories associated with supposed macroevolutionary events are merely
the evolutionary products of systematists’ activities because the ranking process
will depend largely on the systematists’ philosophical attitudes to classification.
This is not, of course, to deny the real importance of the key characters in life,
nor their role in evolution.
The views expressed in Grant’s (1977: 232) text book neatly summarize
the anatomico- taxonomic problems encountered by many workers:
‘‘. . . . macroevolution involves changes of far greater magnitude than those seen
in microevolution and speciation. Changes of macroevolutionary extent occur
in the development of the characteristics that distinguish major groups such
as genera, families, orders, classes and phyla. Such developments take place
on a scale of geological time”.
Dobzhansky et al. (1977:6), although seemingly not convinced that there are
differences in the modes of what they term subspecific and transspecific
evolution, stress both the ecological and morphological aspects of macroevolution when they write Cop. cit. 2 4 7 ) that “ , . . The taxonomic category to which the
new adaptive type is assigned depends upon the distinctness of form and function
that it attains, and in some measure upon the extent of its subsequent
diversification”.
Both statements beg one question central to the whole issue, namely: how does
one recognize a major group? Much of what is written on macroevolution seems
to be based on the idea that the size of the gap (usually a mo hological one)
separating taxa determines the ranks which should be assigne to the taxa in
question. In other words, characters can be assessed as being of generic, familial,
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ordinal or any other rank according to the degree of their morphlolgical distinctiveness, or their adaptive-functional significance in the life-style of the taxon
bearing them.
A worker’s philosophical approach to classification must influence his concept
of what constitutes macroevolution, and his identification of the process
associated with it. Grant’s views, and particularly those of Dobzhansky et al.,
represent the thinking of the ‘evolutionary’ school (see Mayr, 1974), probably
still the most widely held philosophy amongst taxonomists.
The situation is viewed rather differently when one employs a ranking system
that reflects genealogical relationships based on recency of common ancestry;
that is, a cladistic classification (see Wiley, 1976 and Map, 1974). Recency of
common ancestry in this system is determined by the distribution of various
derived features amongst the taxa involved, and the taxa are ranked according
to the level of universality of their shared derived features.
As compared with the evolutionary one a cladistic classification is gradualistic
and thus allows for the gradual differentiation of ‘new’features and for their incorporation and differentiation as the phylogeny of a group unfolds through
repeated speciation events. Although by no means a conscious cladist, Bock
(1979:23) summarizes the cladistic approach to macroevolution when he
writes “. . . Macroevolution is thus not just a summation of many small changes,
but a sequential summation of many small changes added together in their exact
chronological series”,
An important feature of a cladistic classification in respect to macroevolution
is its dissociation of categorical status from the “size of the morphological gap”,
and thus its unification, on the basis of uniquely shared specializations, of
otherwise superficially diverse taxa within the higher levels of classification.
Even a cladistic approach to phylogeny, with its emphasis on the development
of shared derived character systems, rather than on the importance of large-scale
‘categorical’ characters, still leaves unanswered the question of how the derived
characters originated. In particular, both the cladist and the evolutionary
taxonomist are faced with the problems of whether a change in any feature, or
the origin of an entirely new character, involves evolutionary steps of a kind not
encountered at the microevolutionary level (that is, among the evolving
populations of a species).The central problem as I see it is not “. . .accounting for
the origins of higher taxa” (Dobzhansky et al., 1977:245)-which origins are
entirely the consequences of a taxonomist’s activities-but accounting for the
origin of the differentially shared derived characters which allow one to group taxa
at various hierarchical levels within a phyletic lineage.
There is a growing corpus of opinion amongst zoologists that population
geneticists have not been able to demonstrate the origin of such characters at the
population level. Some other kind of genetical change, it seems, must be involved.
(See discussion in Rosen, 1978, and the views expressed by Waddington (1967) on
“ . . . The whole real guts of evolution-which
is how do you come to have horses
and tigers and things . . .”)
Palaeontologists too have come to doubt the efficacy of gradual phyletic
evolution, as opposed to cladogenic evolution, in accounting for the diversity of
life. In particular their thinking has been influenced by the very slow tempo of
supposed phyletic gradualism, and by the lack of supporting data from population
and molecular genetics (see Gould & Eldredge, 1977).
P. H. GREENWOOD
296
The most detailed criticism of phyletic gradualism as a major element in
macroevolution has stemmed from Gould & Eldredge, who have proposed an
alternative hypothesis, that of punctuated equilibrium (see Eldredge 8c Gould.
1972; Gould & Eldredge, 1977; Stanley, 1975). In essence, the theory of
punctuated equilibrium postulates that evolution is concentrated in very rapid
speciation events followed by the differential success of certain elements from
this random pool of speciation. I t thus follows that “. . . speciation is the
raw material of macroevolution” (Gould 8c Eldredge, 197 7 : 189); gradual
intraspecificor intralineage changes can be discounted.
Gould 8c Eldredge base their theory on the reanalysis of several supposed
examples of phyletic gradualism among fossil faunas. My own research
(Greenwood, 1974, also p. 297 below) on the cichlid fishes of Lake Victoria also
leads me to support the view that speciation, and the genetical reorganization that
accompanies it, is a critical element in the process of macroevolution (or, as I
would prefer to express it, the origin and development of phyletic lineages).
On a theoretical basis it can be argued that since a phyletic lineage is a
monophyletic entity, any new lineage must stem from a single species; thereafter
the lineage dwelo and diversifies through repeated acts of speciation. At least
one key feature o a lineage must have appeared in the stem species or else it’
would not be recognized as such; other diagnostic features originate through the
subsequent acts of speciation. What we eventually recognize as the hierarchical
structuring of the lineage (at any phase of its development) is determined by the
way in which taxonomists bring its constituent taxa together on the basis of their
shared derived features; the more comprehensive the category the greater will be
the totality of its shared derivative features, and hence the greater the apparent
‘size’of the gap separating it from categories lower in the hierarchy.
Expressed somewhat differently, one can postulate that speciation is an
essential, indeed fundamental element in macroevolution and that position in a
taxonomic hierarchy is a cumulative measure of speciation events rather than an
indication of the magnitude of a single morphological change.
What evidence can be found to test the idea that morphological changes
associated with speciation events could lead to new phyletic lineages? I believe
that the cichlid fishes of the African Great Lakes provide just this sort of
evidence, as does Bock’s recent (1979) analysis of certain bird groups (although in
that paper Bock uses the birds to test a different thesis; see below, p. 298).
The cichlids of Lake Victoria comprise a 170 species-strong flock of endemic
taxa currently referred to a single genus Huplochromis (see Greenwood, 1974 and
1978). The lake originated by the backponding and union of several rivers, some
three-quarters of a million years ago. The stem species of its Huplochromis flock
must, therefore, have come from those rivers. Judging by the species inhabiting
present-day east African rivers, the taxa present probably showed a very limited
range of morphological features and feeding habits, and from a phylogenetic
view oint were closely related to one another. There is no evidence to suggest
that t e flock originated through multiple invasions from the rivers, and it must
therefore have developed within what is now the lake basin.
Members of the contemporary flock show a wide range and variety of certain
morphological features, especially those associated with the dentition, the form
and function of the jaws, the morphology of the skull, and the form and size of
the pharyngeal teeth and bones. (The latter are modified gill arches which
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function as a second, and internal, pair of jaws.) Associated with these
anatomical differences there is a wide diversity of feeding habits including
piscivory, mollusc-eating, scraping algal growths from rocks and plants,
browzing on rooted plants, filtering phytoplankton and bottom detritus,
insectivory in various forms, and even feeding on scales scraped from the tail fins
of other fishes.
Within the flock it is possible to arrange species into larger groups on the basis
of group members sharing uniquely derived characters (see Greenwood, 1974).
That is, one can recognize about ten different phyletic lineages.
The morphological characters used for this analysis are mainly those
mentioned above. In themselves the characters might appear to be minor
variants of a basic ‘bauplan’, but species possessing particular features are able to
utilize food sources denied to others not having these characteristics. Thus, the
different groups could be said to have entered different adaptive levels, an
ecological component of the macroevolutionary concept (see Dobzhansky et al.,
1977; Bock, 1979), and one thought to be an important factor in recognizing a
macroevolutionary event (see Stansfield, 197 7 ; Grant, I97 7 : 304).
Of particular relevance to certain questions raised by the ‘size of the
morphological gap’ element in most concepts of macroevolution, is the fact that
in each of these cichlid lineages one can trace a gradual development ot its
characteristic morphological features. The least derived members of any one
lineage show features that are but little removed from the least derived members
ofany other group (see Greenwood, 1974).
In other words, although the most derived taxa in the different lineages are
separated from one another by a distinct morphological gap, the gap is virtually
eliminated when the basal members of different lineages are compared.
Since each step in an intralineage character morphocline is represented by a
species, it would seem reasonable to assume that the development of the
character should also be associated with a speciation event. It would also be
more parsimonious to assume that members of a lineage are related as sister
species (through their origin by successive speciation events) than to postulate
their origin by repeated speciation from a single stem species, with the lineage
characters arising de nova (and in increasingly derived form) at each act of
speciation. (Seediscussion below, p. 299,of Bock’s hypothesis.)
Looked at simply in terms of a gradual change in morphology, and forgetting
for the moment that each point in the morphocline is represented by a
reproductively isolated, contemporaneous and sympatric species, the spectrum
of character development in each lineage bears a strong resemblance to phyletic
gradualism, a concept which underlies so much evolutionary thinking. But, the
gradualism seen in these fishes (and in other groups too, Gould 8c Eldredge,
1977; Bock, 1979) is associated with taxa having distinct specific status. Thus,
because the phenomenon is linked with cladogenesis, it could be referred to as
‘cladogenetic gradualism’.
Not all speciation events contributing to the development of the Lake Victoria
cichlid flock have been markedly anagenetic ones (anagenesis used sensu
Dobzhansky et af., 1977:236).In most lineages one finds several species at a
particular level of derivation, The species in such clusters differ from one another
only slightly in a few morphometric features of the head and body; inevitably,
however, there are clear-cut differences in male breeding coloration, a character
298
P. H GREENWOOD
which is probably of considerable importance in the species' intrinsic isolating
mechanisms (see Greenwood, 1974).
The key lineage features in members of these clusters show little interspecific
variation. Why there should be both stasimorphic and apomorphic types of
speciation is still unclear.
Returning for a moment to the trophic radiation manifest by the Lake Victoria
cichlids, we should consider the nature of the morphological modifications
associated with different feeding habits and which serve to identify the various
phyletic lineages.
Essentially these changes are reducible to differential growth in parts of the
skull and other skeletal elements, their associated musculature, and in the
dentition (both oral and pharyngeal). Seen in isolation, at any level of their
differentiation, the changes would not be considered major ones. Yet
cumulatively within a lineage, and comparatively between lineages, the effect has
been to produce an ecologically, morphologically and functionally diverse
assemblage of species that is truly outstanding. One should also recall that this
diversity was produced from a very few and morpho-ecologically uniform
ancestral species.
Taxonomically, these changes enable one to recognize several distinctive
species aggregates which, unless one is prepared to accept a classification that is
not based on monophyletism, must be recognized as having formal taxonomic as
well as natural genealogical status. In other words, one is faced with a situation
which must be regarded as an early phase of what is generally considered to be
macroevolution.
An analysis of certain insular bird groups (e.g. the Hawaiian honeycreepers
(Drepanididae), the thrashers (Mimidae), or the Galapagos finches (see Bock,
1979, and Lack, 1947))yields similar results to those obtained from the Victoria
cichlids. Here too the modifications are mainly associated with feeding, and
relate to morphological changes in the head skeleton, especially the jaws. And
again they are manifest through the continued existence of different species. The
bird examples feature prominently in Bock's (1979) long and detailed paper
which is also concerned with demonstrating that so-called macroevolutionary
changes can be reduced to changes at a microevolutionary level. But, for Bock,
microevolutionary changes are " , . . those modifications of the level studied by
population geneticists and by animal and plant breeders" (Bock, 1979201, and
" . . , are amenable to experimentation and direct observation (op. cit. 2 1 ) . To
him ". . , macroevolution is viewed as a sequence of microevolutionary events, not
as a sequence of species level changes" (op. cit. : 27). Because a new series arises
from part of a preexisting one, this ultimate reduction of macroevolution to the
population level must be granted, at least in terms of the gene pool sample with
which the new species is endowed at its very inception. The difficulty, as Bock
acknowledges, is to find natural populations distinguished by the sort of
characters which in his bird and my fish examples are apparent at the species
level (butsee later, p .30 1).
To overcome this difficulty, and also the fact that a biological species (sensu
Mayr, 1963: 19) is a non-dimensional one with regard to its unity in time, Bock
develops his reductionist theory of macroevolutionary events on the basis of
evolution within what he terms a phyletic lineage.
A phyletic lineage according to Bock (1979:28) is " . . . the temporal
MACROEVOLUTION-MYTH
OR REALITY ?
299
continuum formed by a species. . . reproducing itself generation by generation
through time”; to quote further (op. c i t . 2 9 “No matter how much phyletic
evolution occurs in a phyletic lineage and no matter how different ancestral and
descendant populations may appear, no species boundaries will be crossed as
one traces a phyletic lineage; hence transspecific evolution has not occurred.
However, cross-sections of the same phyletic lineage at dif€erent points in time
are not different species nor are they the same species. These are simply different
cross-sections of the same phyletic lineage at different times. . .” He proposes
that the larger changes are the temporal summations of smaller changes occurring
in this orthoselectively evolving phyletic continuum.
Bock allows that a phyletic lineage may split (i.e. speciate),but gives no weight
to speciation as a primary process in macroevolution. The emphasis is firmly on
gradual intralineage changes and only secondarily on changes which take place
after speciation has occured (Bock, 1979:36).
I find great difficulty in understanding how Bock’s phyletic lineage model
would work in reality. There are, apparently, two different entities involved (see
Fig. 1A and Bock, 1979: fig. 19A).Fundamentally, there is the persistent phyletic
lineage, a sort of fast breeder of change, and secondarily there are the species,
which embody various of these changes, depending on the time at which they are
thrown off from the parent body Le. the phyletic lineage). The species so formed
can, according to Bock, only be interrelated as a pseudophylogeny, a term and
concept I fail to understand since the species are all derived from a common
ancestor, albeit one which is constantly changing (one recalls (see above) that
Bock’s phyletic lineage is by definition a species reproducing itself through time).
I t seems to me that Bock is, in effect, replacing the ‘systemic mutations’ or
‘quantum evolutionary events’ of other authors (see Grant, 197 7) with his equally
untested phyletic lineage hypothesis. I say untested because I find that the test
which Bock applies is equivocal. He argues (Bock, 197933) that by comparing
the differences between related extant species (a pseudophylogeny) one is
observing a series that is equivalent to the sequence of microevolutionary steps
that led to a macroevolutionary change, but which were effected within the
phyletic lineage (semuBock); see Fig. 1A.
But, if the extant species in a pseudophylogeny are connected back to the
phyletic lineage and their points of’contact with the lineage are taken to represent
successive common ancestors, one produces the typical, dichotomously
branched cladogram of the cladistic taxonomist (Fig. IB).
Since each dichotomy represents a single speciation event, Bock’s reasoning
could just as well be used to demonstrate that the successive microevolutionary
events were associated with speciation, and that the ultimate and
‘macroevolutionary’ event (species G in Fig. 1B) is the summation of these same
changes. All Bock has done in his model is to postulate that changes take place
between successive speciation events, although to corroborate this he can
produce no evidence other than actual species.
I do not see, therefore, that his model is any more testably reductionist than
are the models of those who would reduce ‘macroevolutionary’ changes to ones
associated with speciation events (e.g. Gould & Eldredge, 1977). Especially is this
so when one considers the evidence of substantial genetical differentiation
occuring in spatially isolated populations that have acquired partial reproductive
isolation from their congeners (Ayala, Tracey, Hedgecock & Richmond, 1974;
P. H. GREENWOOD
300
Observed differences between species ( horizontal comparison 1
L
Each step is of the magnitude of a microlevel phyletic
change,or of a species difference at most. They
add up to a total difference equivalent to a
macroevolutionary change.
A
b
C
d
e
f
Figure 1. A. Schematic diagram, as used by Bock (1979), to show the conceptual
transformation from a vertical series of changes within a phyletic lineage (sensu Bock;
see p. 298) to an equivalent pseudophylogeny derived from extent species. The
horizontal comparison of these species provides the morphocline which mves to test
the postulated macroevolutionary modification brought about through a series of
small, successive steps at the microevolutionary (i.e.intraspecific)level. (Modified after
Bock, 1979, fig. 19). B. Data from A arranged as a cladogram (see text, page 299).
0 ,Speciation event.
MACROEVOLUTION-MYTH
OR REALITY ?
30 I
Avise, 1976). In contrast there is little evidence indicative of similar genetical
changes amongst interactive populations. Avise ( 1976 : 13) finds that individuals
in such populations generally share up to 99% of the structural genes surveyed,
but that in populations showing incipient reproductive isolation there are
“ , . , major allelic changes at up to 20% or more of structural genes”.
Animal breeders, as Bock (1979:2 1) notes, have produced phenotypes showing
characters that can be regarded as major modifications of the ancestral type. The
breeder, I grant, is dealing with intraspecific populations, albeit artificially
contrived ones, and his work certainly shows the evolutionary potential
available in such populations. In many respects, however, a breeder’s activities
incorporate several important elements of the allopatric speciation model : he is
working with small, isolated populations, he enforces a degree of inbreeding and,
by exercising a differential selection against those genotypes which, as it were, do
not make the grade he isolates those that do from outside contamination and
thereby creates a simulacrum of reproductive isolation.
The nature of the genetical changes which occur during speciation are still
subject to debate and investigation (see White, 19781, but I can find little to
support Bock’s contention (1979:36) that “. . . most evolutionary divergence
between sister species takes place during the sympatric portion of speciation”
(italics mine, and the assumption that there is a sympatric portion of speciation,
Bock’s). I do not see how one can test this hypothesis, despite Bock’s claim to
have done so in birds (Bock, 1979:36). Bock does not specifj what birds he has
chosen for his test (nor his criteria for a sister-species relationship), but it seems
to me that all he can reasonably demonstrate amongst extant forms is that
sympatric sister species do differ from one another, not the stage in their
evolutionary history at which these differences arose. However, Bock’s claim may
stem from his particular view point of speciation as “. . . the mechanisms
permitting the multiplication of species-evolution of intrinsic isolating
mechanisms”. Even if that is so, he still does not show how he determined the
changes to have taken place after and not before or during the evolution of the
isolating mechanisms.
All in all I do not think that the available evidence fits Bock’s model, although
I would agree that proximally the events leading to macroevolutionary change
must be initiated at the population level prior to speciation. The problem is to
find such an event in a wild population. Perhaps the nearest one can get in this
respect is the marked polymorphism found in an isolated population of cichlid
fishes living in the Cuatro Cienegas basin, Mexico (Sage & Selander, 1975).
Within that apparently monospecific population three very distinct
morphotypes can be recognized. One is distinguished by its fine, densely toothed
pharyngeal bones, the second by its more robust and strongly toothed
pharyngeals, and the third by its molariform pharyngeal teeth and greatly
enlarged pharyngeal bones. Correlated with these anatomical features are equally
distinctive feeding habits : detritus eating, piscivory, and mollusc eating for the
morphs respectively.
The morpho- trophic parallels between this intra-po ulational situation and
that seen at the interspecific level amongst the cichlid shes of Lake Victoria is
very striking indeed. If the Cuatro Cienegas phenomenon really is a case of
intraspecific polymorphism, then it is the only natural example of nascent
macroevolution at the intraspecific level that I know of amongst vertebrates.
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P. H. GREENWOOD
What, however, of its future evolutionary potential in terms of further
diversification? That would seem to be dependent on each of the morphs
becoming a true-breeding, reproductively isolated entity, in other words-a
species.
Weighing the data reviewed by the various authors whose books and papers
have been considered so far, there is seemingly no support for the idea that
macroevolution involves genetical or henotypical changes of a greater
magnitude than those associated with the nal product of speciation events.
Evidence derived from the species flocks of cichlid fishes (Greenwood, 1974),
from the drepanidid and mimid birds (Bock, 19791, from several
palaeontological examples covering different animal groups (Could & Eldredge,
1977), and from genetics (Ayala et al., 1974; Avise, 1976) shows two things.
Firstly, that macroeveolution h e . the origin of new phyletic lineages), although it
must start at the population level, is effectively initiated through speciation.
Secondly, that the changes involved are, at their inception, not of outstanding
magnitude. These data also suggest that bouts of rapid speciation are probably of‘
great importance in triggering off what we later call macroevolution.
Thus, the questions posed by Grant (1977:303) ‘‘. . , What circumstances
surround the origin of tribes, families, orders, classes? What evolutionary factors
are involved in the origin and develo ment of groups of medium or high
taxonomic rank?. . .” can be answered y: Speciation involving the manifestation, in one sister species, of a derived character (or characters), followed by
further similar speciation events stemming in the first instance from that derived
species.
Clearly not every newly evolved species will become the progenitor of a major
phyletic lineage. Many will only contribute to the diversity of related taxa that are
grouped as genera. The nature of the characters expressed through the
genotypical reorganization involved in speciation, interacting with the biotic and
abiotic environments, will largely determine the ultimate historical role of a
species. In this respect there are likely to be species that could be considered as
‘hopeful monsters’. That is, species which deviate from their sister species in one
or more features that enable them to utilize some element in the environment not
exploited by their congeners, or to gain some enhanced protection from adverse
environmental conditions. Newly evolved species which, as it were, closely
replicate their sisters would appear to be less ‘hopeful’. I t may be recalled (see
p. 297) that both kinds of species occur amongst the cichlid fishes in Lake
Victoria.
Stemming from Eldredge 8c Gould’s (1972) first paper on punctuated
equilibrium, and their ideas on macroevolution via the differential success of
certain speciation events, Stanley ( 1975)put forward a theory of species selection,
analogous to natural selection, to explain the direction of transspecific
evolution. Stanley argues that species selection acts upon the “ , . . variation
provided by the largely random process of speciation and favors species that
speciate at high rates or survive for long periods and therefore tend to leave
many daughter species”.
Conceptually, Stanley’s theory, like its analogue natural selection, is open to
criticism as a tautology (see Bethell, 1976; also Rosen, 1978) since it provides no
criteria of fitness independent of mere survival.
Apart from that criticism there are other aspects of Spencer’s theory which I
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feel need clarification. First, the implication that rate of speciation is necessarily
an intrinsic characteristic of a species; surely, more often than not it is the
environment that initiates speciation through the isolation of small populations?
Secondly, there is the implication that one species may give rise to more than one
daughter species provided it survives for a long enough time. Even putting aside
the argument (Henning, 1966) that a mother species ceases to exist once it has
split into two daughter species, I would again suggest that the environment
probably plays the most important role in determining the tempo of evolution
and hence the number of speciation events. Finally, I do not see how he can
decouple the individual from his unit of selection, the species (see Stanley, 1975:
Table 1 :649). It is the individuals which carry the key characters o f a new phyletic
line and it is the success or failure of individuals which will condition the ultimate
history of a species, that is, its contribution to the origin of new phyletic lineages.
So far I have paid little attention to the adaptational aspects of
macroevolutionary theory (see discussions in Grant, 1977 ; Dobzhansky et al.,
1977; Bock, 1979). Dobzhansky et al. for example, go so far as to say that
‘ I . . . . the origins of higher taxa can be explained as adaptive responses to special
ecological opportunities” Cop. czt : 254).
Questions relating to the relative adaptiveness of the ancestor to a new phyletic
lineage obviously will be related to the magnitude of the changes thought to be
involved in the origin of the ancestor. Certainly the Lake Victoria cichlids do not
indicate any problems in that respect because numerous elements in many
morphoclines of progressively derived characters still survive in the lake
(Greenwood, 19741, and the individual changes are of no great magnitude. The
same may be said for the bird examples considered by Bock (19791, no matter
how their mode of macroevolution be interpreted.
The nature of the changes shown by the ancestor of a lineage will determine
how effectively it can exploit the environment in ways different from those of its
ancestors; that is, the extent to which it will begin to exploit a new adaptive zone.
The invasion and exploitation of new adaptive zones is usually associated, often
diagnostically, with macroevolutionary events (see Simpson, 1953 ; Grant, 19 7 7 ;
Dobzhansky et al., 197 7 ; Bock, 1979). Viewed retrospectively this correlation, in
its totality, would seem to be a valid one. But, like the whole concept of
macroevolutionary change, it can be broken down into a cumulative series of
events which cannot be elevated above the microevolutionary level.
The role of natural selection in macroevolution is greatly emphasized by Bock
(1979:30)who refers to it as “ . . . the design factor in evolution”. Yet, as Bethel1
(1976) points out, the flaw in the selectionist argument is the untestable
assumption that there are criteria of fitness independent of survival.
The products of presumed macroevolutionary events have survived and
possess characteristics that enable them to utilize various ecospaces. We must
therefore assume that their common ancestors, at various phases of the lineage’s
history, also survived and in that sense were also adapted. I t would be impossible
to test the idea that these survivors are, as Dobzhansky et al. suggested (see
above), the products of adaptive responses to special ecological opportunities.
Rather they should be viewed, non-teleologically, as the products of evolutionary
change which could, because of those changes, utilize those special opportunities
in the environment.
304
P. H. GREENWOOD
In conclusion, I must return to the question posed in the title of this address,
“Macroevolution-myth or reality?”
None will deny that animals can be classified in a hierarchical manner so as to
express their varying degrees of relationshi . Ideally such a classification
expresses, in Darwin’s words, propinquity o descent, That, in turn implies
tracing descent through repeated speciation events. Evidence from many fields of’
research indicates that the kind of change associated with the evolution of new
species can explain the gradual build up of those features which enable us to
recognize levels of shared common ancestry, and thus to construct our
hierarchical classifications. The term macroevolution is generally used to account
for the ‘origin’ of higher categories, yet these higher categories are in effect only
our attempts to reflect simultaneously both the increasing diversity and the
relationships of organisms within an evolving phyletic lineage. If a lineage is to
be a natural one it must be of monophyletic origin, and that I would argue,
means it stems from a single species. Its further development and diversification
will also depend on speciation events. On that reasoning I would equate
macroevolution with speciation, hence with reality, and would suggest that the
common concept of macroevolution as some kind of megaspeciation event be
treated as the myth. Indeed I wonder ifwe need the word ‘macroevolution ’ at all?
P
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