Ernst Mayr and the centrality of species.
Also in 1904, Henry Adams described the most fantastic American spectacle of the year, the greatest illumination ever attempted with the new-fangled invention of incandescent bulbs: "The world has never witnessed so marvelous a phantasm; by night Arabia's crimson sands had never returned a glow half so astonishing, as one wandered among long lines of white palaces, exquisitely lighted by thousands and thousands of electric candles, soft, rich, shadowy, palpable in their sensuous depths." Adams was describing the St. Louis World's Fair, famous for the invention of iced tea, ice cream cones, and the first Olympic Games held in America. Yet, for the illumination provided ever since to our profession of evolutionary biology, Adams might well have been celebrating our cardinal event for that year, the birth on July 5, in Kempten, Germany, of Ernst Mayr.
In the three most important of his many sequential and overlapping careers--ornithologist and systematist, evolutionary theorist, and historian of biology--Ernst Mayr has rooted his intellectual vision in defending a rational and tractable science of organic wholeness against a variety of reductionisms and sterile abstractions. I shall devote this tribute to my dear friend and mentor to a discussion of Mayr's key insight on the centrality of species as evolutionary agents, and to the contribution made by this notion to developing theories of macroevolution--a great advance not achieved as simple realization from the void, but from active struggle against previously dominating views of species as arbitrary segments of evolving lineages, and as units too comprehensive, in any case, to act as fundamental agents in sciences committed to reductionistic perspectives. But I wish to record the larger and
coordinating role of this integrative organismic worldview as a linchpin of Mayr's scientific life, for this philosophical consistency holds a key to the power of his thought and the extent of his influence. This antireductionism inspired him to become a systematist and field biologist in the first place; it underlies his different (and, I believe, correct) view of the Modem Synthesis as a fusion of laboratory-genetic and field-systematist traditions and not (as so often depicted) as a reduction of"descriptive" activities in natural history to "explanatory" models of population genetics; it serves as a conceptual basis for all his major ideas in evolutionary theory, including peripatric speciation, genetic revolutions, founder effects, and unity of the genotype; it informs his fundamental vision of the history of biology as a struggle over centuries between the essentialism of Platonic traditions and the "rightminded" approach that he calls "population thinking."
The centerpiece of Mayr's scientific life, coming between an earlier focus on ornithological field studies and a later dedication to history and philosophy of biological science, rests on his role as a leading architect of one of the half-dozen major scientific achievements in our century: the integration of Mendelian and Darwinian traditions to form for the first time, following factional debate ever since The Origin, a unified theory of evolution called the Modem Synthesis. Mayr was, first, a founder of the synthesis in writing a key volume that integrated systematics within the growing concensus (Mayr 1942); then a practical facilitator in helping to organize the Society that publishes this journal and in serving as the first editor of Evolution (see Smocovitis 1994); then, as a grand codifier of the synthesis in writing its most synoptic compendium (Mayr 1963); and, finally, as its primary historian (Mayr and Provine 1980; Mayr 1982).
If we consider the synthesis as a fusion of three equally robust
disciplines--experimental genetics, population genetics, and studies of natural history expressed primarily by systematics (and not as an imposition of the first two, as reductionist modernisms, on a hidebound, or even moribund, third)--then the role played by Mayr and other field naturalists in building the synthesis is fully constitutive and not only derivative. Mayr (in Mayr and Provine 1980), wearing his historian's hat, has stoutly defended such a view of the synthesis against a strong tradition that sees the population genetics of Fisher, Haldane, and Wright as paramount, and the second phase of the synthesis largely as a whipping of older disciplines into line (Dobzhansky, Mayr, Simpson, and Stebbins). Mayr's motives are, obviously, partisan in large part, but I also think that he is right.
Dobzhansky (1937) became the beacon of the second phase because he came from the one tradition, Russian genetics, that tried to fuse experimental Mendelism with systematics and natural history, not only to impose the first on the second (or to the ignore the second entirely). At Mayr's 1974 conference on the origin and growth of the synthesis (Mayr and Provine 1980), Dobzhansky vividly recalled the impediments to synthesis within American traditions. He had originally left Russia to work with Thomas Hunt Morgan, America's premier experimental geneticist. Dobzhansky recalled Morgan's attitude to natural history:
"Naturalist" was a word almost of contempt with him, the antonym of "scientist." Yet Morgan himself was an excellent naturalist, not only knowing animals and plants but aesthetically enjoying them .... Morgan was profoundly skeptical about species as biological and evolutionary realities. The species problem simply did not interest him .... Biology had to be strictly
reductionistic. Biological phenomena had to be explained in terms of chemistry and physics. Morgan himself knew little chemistry, but the less he knew the more he was fascinated by the powers he believed chemistry to possess. There was no surer way to impress him than to talk about biological phenomena in ostensibly chemical terms (Mayr and Provine 1980).
Dobzhansky also remembered that Morgan "liked to say that genetics can be studied without any reference to evolution." Could the synthesis have taken root in such soil?
Dobzhansky brilliantly set a different problematic for evolutionary theory, one embodied in Darwin's title but not treated as a major theme in his book, one emerging from traditions of systematics and natural history (and scarcely conceivable to someone with Morgan's--and to a large extent Darwin's--views on the unreality of species): How can a theory constructed to describe continuous change in natural populations also explain the discontinuous structure of nature's taxonomic diversity? The central problem of evolution is the origin of discontinuity among species.
This statement sounds commonplace today, but only because Dobzhansky, Mayr, and the synthesis made it so. Morgan and virtually all experimentalists argued that the origin and nature of variation, and its manner of spread through populations, form the key issues in evolutionary theory. Morgan disavowed the species problem as, at best, the hang-up of dull taxonomists and, at worst, a bogus issue, because species are not real in the flow of nature, and we name them only because our poor minds cannot handle continuity. Dobzhansky did not deny the centrality of Morgan's questions, but he argued that evolution works on a series of levels, and that the primary focus of natural history lies not at these lower levels, but at a higher one: the origin of species itself (Darwin's title, after all). Diversity is the primary fact of nature (and the first topic of chapter 1 in Dobzhansky 1937). Diversity arises by the splitting of lineages, by speciation. Speciation leads to discontinuity in nature. How can the continuous process of genetic change yield discontinuity in nature? The origin of discontinuities between species is the key problem of evolutionary theory. Only a naturalist could have reset the stage for synthesis in such a fruitful way.
The origin of hereditary variations is, however, only a part of the mechanism of evolution .... These variations may be compared with building materials, but the presence of an unlimited supply of materials does not in itself give assurance that a building is going to be constructed .... Mutations and chromosomal changes are constantly arising at a finite rate, presumably in all organisms. But in nature we do not find a single greatly variable population of living beings which becomes more and more variable as time goes on; instead, the organic world is segregated into more than a million separate species, each of which possesses its own limited supply of variability which it does not share with the others .... The origin of species ... constitutes a problem which is logically distinct from that of the origin of hereditary variation. (Dobzhansky 1937, p. 119.)
Mayr (1942) then brought this theme to fruition by focusing an entire book on modes of speciation and on realigning taxonomic practice with insights of the developing synthesis. Mayr's first paragraph sets his theme and tone:
The rise of genetics during the first thirty years of this century had a rather unfortunate effect on the prestige of systematics. The spectacular success of experimental work in unraveling the principles of inheritance and the obvious applicability of these results in explaining evolution have tended to push systematics into the background. There was a tendency among laboratory workers to think rather contemptuously of the museum man, who spent his time counting hairs or drawing bristles, and whose final aim seemed to be merely the correct naming of his specimens. A welcome improvement in the mutual understanding between geneticists and systematists has occurred in recent years. (Mayr 1942, p. 3.)
Mayr's later book (1963) is more than twice as long, and ever so much more weighty in its confidence. This work shaped my own evolutionary thinking more than any other, and I am confident that most naturalists of my generation would offer the same testimony. As I read it again in preparing this article--and looked at all my old marginalia, penciled in preparation for my oral exams--I came to appreciate even more the enormous labor and creative thought involved in bringing so much material together. And I finally understand the defining word that once puzzled me in Julian Huxley's review of the book--"magisterial." (The etymology of the word is not "magnificent," though the book is surely that as well, but magister, or teacher. A magister is not a mere schoolroom pedant, but a great preceptor. Magisterial, above all else, means authoritative.)
Although Mayr's 1963 book covers the same general material, and in similar order, as the 1942 version, the works differ profoundly, most of all in the development of Mayr's views on the nature of species and speciation. The necessity of geographic isolation, and the consequent near universality of allopatric speciation, has consistently formed the centerpiece of Mayr's worldview. But, in 1942, pure continuationism reigned. Populations split and the smaller divisions then functioned as microcosms of the ancestral mass, as in the model now called "dumbbell allopatry" and considered (by Mayr at least) both rare and relatively ineffective in producing new species. In other words, Mayr identified no distinctive properties promoting speciation in certain kinds of isolated populations versus others; isolation itself, and the severing of gene flow, made any population ripe for speciation: "The big gaps which we find between species are preceded by the little gaps which we find between subspecies and by the still lesser gaps which we find between populations. Of course, if these populations are distributed as a complete continuum, there are no gaps. But with the least isolation, the first minor gaps will appear." (1942, p. 159.)
But by 1963, Mayr had developed the full apparatus of the distinctive theory that he later called" peripatric" to make a sharper separation from this original, continuationist version of allopatry: small populations, isolated at the periphew of parental ranges, subject to the special maelstrom of influences including greatly enhanced selection and random effects of the founder principle, and often attaining specific status with relative speed by a "genetic revolution." Mayr says (pets. comm. 1994) that he introduced this new apparatus in a paper (1954) that was ignored at the time, but which he considered his most important (Nihil sub sole novurn. He published it in a symposium volume, the greatest repository of unread literature, then and now).
If most of the synthesists eventually grasped Mayr's point about the centrality of speciation as a problem in evolutionary dynamics, almost no one followed him to the next crucial step of recognizing a constitutive role for speciation as the creative and formative force in macroevolution. From my parochial standpoint as a paleontologist, Mayr's unique enlightenment on this issue must stand as his most distinctive contribution to my profession's domain of evolutionary theory.
Most "neontologists" of the modern synthesis (our word for all folks who study modern organisms only) viewed speciation in the fossil record largely as an end-to-end accumulation of adaptive change in the anagenetic mode. Huxley, for example, wrote: "Adaptive radiation is seen to be the result of a number of gradual evolutionary trends, each tending to greater specialization-- in other words to greater adaptive efficiency in various mechanisms subservient to some particular mode of life .... Each single adaptive trend also shows the phenomenon of successional speciation." (1942, p. 487.)
If these writers ever grasped the more important theme of speciation as a primary pump for regulating diversification in geological time, they downgraded such fluctuation in numbers as subservient to anagenesis, never understanding that differential speciation can also power an evolutionary trend.
At most, they allowed that evolution needed such a process of multiplication, lest favorable trends disappear through the extinction of single species bearing their fruits. Speciation therefore became a hedge against death by parceling out, into several iterated lines, a set of adaptations built ariagenetically such that the extinction of one species does not abort the benefit. The trend itself remains anagenetic, for speciation does not contribute to directionality, but only shores up adaptations built in other ways. (Under later views, including punctuated equilibrium, for example, differential speciation is the trend, and anagenetic main trunks do not even exist.)
Even G. G. Simpson, the greatest paleontologist of the synthesis, downgraded speciation in this way: by regarding the process of splitting in a lineage largely as a mechanism for the parceling out of existing variation, and not as a way of building evolutionary novelty:
This sort of differentiation draws mainly on the store of preexisting variability in the population. The group variability is parceled out among subgroups, or a lesser group, pinched off from the main mass, carries with it only a part of the general store of variability .... Speciation, in this sense, is more likely to be a matter of changing proportions of alleles than of absolute genetic distinctions .... The phenotypic differences involved in this mode of evolution are likely to be of a minor sort or degree. They are mostly shifting averages of color patterns and scale counts, small changes in sizes and proportions, and analogous modifications. (Simpson 1944, p. 201.)
Moreover, Simpson added that speciation has limited quantitative importance among the three major modes of evolution (speciation, phyletic evolution, and quantum evolution), for 90% of paleontological data record another process: "nine-tenths of the pertinent data of paleontology fall into patterns in the phyletic mode" (1944, p. 203).
Huxley, in a grand prose flourish, then presented a kind of ultimate trivialization for speciation as a mere, pretty little epiphenomenon upon the grand pattern of evolution--never realizing that the pattern itself might be built by higher-order sorting, operating through the differential success of certain kinds of species!
The formation of many geographically isolated and most genetically isolated species is thus without any bearing upon the main processes of evolution .... Much of the minor systematic diversity to be observed in nature is irrelevant to the main course of evolution, a mere thrill of variety superimposed upon its broad pattern. We may thus say that, while it is inevitable that life should be divided up into species, and that the broad processes of evolution should operate with species as units of organization, the number thus necessitated is far less than the number which actually exist. Species-formation constitutes one aspect of evolution; but a large fraction of it is in a sense an accident, a biological luxury, without bearing upon the major and continuing trends of the evolutionary process. (Huxley 1942, p. 389.)
Amid this relegation to irrelevancy of the primary unit of macroevolution, one prominent voice within the synthesis stood up for the centrality of speciation in constructing large-scale pattern. In a cautious, but prophetic statement, Ernst Mayr (1963, p. 587) wrote: "To state the problems of macroevolution in terms of species and populations as 'units of evolution' reveals previously neglected problems and sometimes leads to an emphasis on different aspects." (Much of later macroevolutionary theory begins with this proposition, and Mayr is therefore its inspiration-ironically in a sense, for much in this developing body of thought has challenged other aspects of the synthesis that Mayr so strongly championed. In its bare-bones mechanism, for example, the theory of punctuated equilibrium is little more than Mayr's peripatric theory of speciation translated into geological time; see Eldredge and Gould 1972; Gould and Eldredge 1993.)
Directly refuting Huxley's charge that speciation is a frill and a luxury in the overall pattern of evolutionary change, Mayr wrote:
I feel that it is the very process of creating so many species which leads to evolutionary progress. Species, in the sense of evolution, are quite comparable to mutations. They also are a necessity for evolutionary progress, even though only one out of many mutations leads to a significant improvement of the genotype. Since each coadapted gene complex has different properties and since these properties are, so to speak, not predictable, it requires the creation of a large number of such gene complexes before one is achieved that will lead to real evolutionary advance. Seen in this light, it appears then that a prodigious multiplication of species is a prerequisite for evolutionary progress .... Without speciation, there would be no diversification of the organic world, no adaptive radiation, and very little evolutionary progress. The species, then, is the keystone of evolution. (196 3, p. 621.)
There is a world of difference between the negative view held by most synthesists--that speciation merely iterates (and therefore buffers) adaptations produced by a different, anagenetic process--and Mayr's recognition that adaptations are pieced together through accumulated events of speciation, each chancy in itself and not directed toward the eventual novel phenotype. In this sense, Mayr's view is the root for a flowering of modern macroevolutionary theory that views speciation as a higher-order analogue of mutation, and interprets trends as differential sorting within this multitude of units, not as organismic selection within anagenetic lineages.
Allow me to make a personal point in closing. I met only a few architects of the first phase of the Synthesis, the founders of population genetics in the 1920s and 1930s (though I greatly value my contacts with Sewall Wright near the close of his life). But it was my greatest privilege to encounter--and to know well, often as dear friends (despite the generational gap)--all the major builders of the second phase, the integration of disciplines from the late 1930s through the 1940s.
As I enter my own scientific middle age, I often find myself trying to understand the source of their great influence and their long, unflagging and continuously productive research-- to the last day of long lives for Dobzhansky and Simpson, and with continuing, undiminished vigor for Mayr and Stebbins. What did these men hold in common that placed them so far above the upper bound of merely good scientific careers? Of course, they are all brilliant--that goes without saying as a component of such intellectual achievement. But what in their hearts and souls made them paragons? For raw intelligence is scarcely enough, and history is littered with brilliance quickly burned out, destroyed, and devoid of lasting accomplishment.
These men are and were so different in basic personality: the ebullient Dobzhansky, the bitter and suspicious Simpson, the methodical and forceful, yet generous Mayr. What did they share that might serve as a lesson for all of us who would learn from the greatest potential mentors of our profession. Only this, I think--but what a precious gift! Every one of them loved his subject with passion that never flagged. They knew that nothing could be as exciting as evolution, nothing more worth a daily struggle for increased understanding, so long as life lasted. They grasped that nothing a man might do could possibly be more noble than dedication to the peculiar recursion that defines the enigma of our being: the possibility that one might use the great gift vouchsafed to us by our quirky and contingent evolution--the human mind--to unravel the source and meaning of its origin. In short, they never lost either wonder or awe. Dear Ernst, as you begin your tenth decade (in an interval that will witness history's millennium), and as the primary representative of your illustrious cohort, we thank you for the girl of your knowledge, but most of all for the blessing of your inspiration.
Dobzhansky, Th. 1937. Genetics and the origin of species. Columbia University Press, New York.
Eldredge, N., and S. J. Gould. 1972. Punctuated equilibria: an alternative to phyletic gradualism. Pp. 82115 in T. J. M. Schopf, ed. Models in paleobiology.
Freeman, Cooper, San Francisco.
Gould, S. J., and N. Eldredge. 1993. Punctuated equilibrium comes of age. Nature 366:223-227.
Huxley, J. S. 1942. Evolution: the modern synthesis.
Allen and Unwin, London.
Mayr, E. 1942. Systematics and the origin of species. Columbia University Press, New York.
____. 1954. Change of genetic environment and evolution. Pp. 157-180 in J. S. Huxley et al., eds.
Evolution as a process. Allen and Unwin, London.
____. 1963. Animal species and evolution. Harvard University Press, Cambridge, Mass.
____.1982. The growth of biological thought. Harvard
University Press, Cambridge, Mass.
Mayr, E., and W. B. Provine, eds. 1980. The evolutionary synthesis. Harvard University Press, Cambridge, Mass.
Simpson, G.G. 1944. Tempo and mode in evolution.
Columbia University Press, New York.
Smocovitis, V. B. 1994. Disciplining evolutionary biology: Ernst Mayr and the founding of the Society for the Study of Evolution and Evolution (19391950). Evolution 48:1-8.
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|Author:||Gould, Stephen Jay|
|Date:||Feb 1, 1994|
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