Genes and the Evolutionary Synthesis

My intention in this and the following chapter is simply to clarify what a few of the better known architects of the synthesis actually had to say about how the entire spectrum of genes through phyla really fits together. I have chosen books rather than a potpourri of articles from the (possibly) more “technical” scientific literature, for precisely this reason: it is in the books that we find the coherent, integrated statements. And each of the four books singled out for particularly close analysis—Genetics and the Origin of Species (Dobzhansky 1937a); Genetics and the Origin of Species second edition (Dobzhansky 1941); Systematics and the Origin of Species (Mayr 1942); and Tempo and Mode in Evolution (Simpson 1944)—is a truly coherent, though not necessarily smoothly linear, argument. In each, some parts seem more vital to the flow of argument than others, but it is clear to the reader from the outset that each is a complete book and not a disjointed melange of unrelated ideas. Then, too, the contents of each author’s papers are to a great extent reflected in the respective books. It is apparent, for example, that Dobzhansky was publishing a variety of papers in the 1930s (many were in his “Genetics of Natural Populations” series republished in a single volume in 1981 by Lewontin et al.) which he liberally drew upon for illustrative material in Genetics and the Origin of Species. The other sorts of papers Dobzhansky was publishing seem to be byproducts of his thinking and research for the various editions of his book. Examples are his papers on species definitions (1935) and isolating mechanisms (1937b) and his theory of the origin of isolating mechanisms by reinforcement (1940). The choice of the four particular books for special treatment in this volume needs further comment. I have omitted such nonconformist works as Robson and Richards (1936), Willis (1940), and Goldschmidt (1940) precisely because they have been almost universally considered as falling outside the limits of the synthesis. The main historical effect of such books (especially Goldschmidt’s) seems to have been as irritants.

Hierarchic Interactions: The Evolutionary Process

Organisms—biology begins with organisms, and indeed a great deal of the history of biology is a trek through progressively finer subdivisions of organisms. When “forefronts” of biology are listed these days, nearly all concern the molecular biology of intracellular (and intraorganelle) physicochemical processes—and quite rightly so. But the ontology of units larger than organisms, while not wholly neglected, is at least as difficult a problem. Organisms are by far the easiest of biological units for us to see, to probe, to conceptualize as “individuals.” But, in the present context, organisms pose a unique problem all their own: they constitute the only class of individuals to be found in both the genealogical and ecological hierarchies. Consider the confusion that permeates even the recent explicitly hierarchical literature: ecology and evolution (as in the quote from Valentine that stands at this chapter’s head) are generally seen as separate areas of inquiry, but the choice of the higher-level individuals to be incorporated into one’s hierarchy very much depends upon one’s point of view. Below the organism level, of course, the distinction between the somatic and germ lines (i.e., in multicellular organisms) once again ensures a clean separation of the elements of the two hierarchies. Hence the conclusion (Eldredge and Salthe 1984) that there must in fact be two independent, yet parallel and interacting, process hierarchies that together combine to yield evolution. Organisms, as members of both hierarchies, threaten to muddy the picture. It is possible, of course, to distinguish between the economic and reproductive functions of organisms, as I have done at length in the preceding chapter. Physiologists, after all, have long been telling their students that reproduction is the one physiological process not essential to the survival of an organism; thus, it is no surprise that it is invariably the first such process to be dispensed with when the organism is stressed. It is easy to distinguish the economic from the reproductive functions of the vast majority of organisms, but in many vertebrates, most especially Homo sapiens, sexuality has clear economic implications, obscuring the distinction between the two hierarchies perhaps even more.

The Evolutionary Hierarchies

Real-world hierarchies are usually treated as patently evident manifestations of nature, a fact that has somehow rendered them trivial. Familiarity breeds contempt. Everyone knows that atoms are the building blocks of molecules, small molecules combine to form the huge molecules of living Systems, organelles and other structures composed of such molecules make up cells, cells link up to form tissues, tissues do likewise to form organs, thence organ Systems, and finally we arrive at the integrated soma of the organism. And organisms are associated to make populations, or demes, or species. The very names of the subdisciplines of biology (especially as conceived fifty years ago) reflect this organisation. Though molecular biology is a recent arrival, physiological genetics, cytology (and cytogenetics), histology, and physiology nicely recognized the components of the somatic or organismic hierarchy. Thus, the somatic hierarchy retains a heuristic value still reflected in general biology texts. The molecules-to-organism hierarchy offers a handy way of organizing biological knowledge, information about living Systems. That nature itself is organized in such a fashion seems to have slipped to secondary significance. It is, though, common to view the sequences of codons that compose functional genes, as well as other organizational features of DNA molecules, as constituting the retention of information. One major biological hierarchy, the genealogical hierarchy, is evidently a hierarchy of information. It is as if the pragmatic, heuristic, epistemological aspects of biological hierarchies, providing us with a handy way of organizing, summarizing, and communicating what we think we know about biological Systems, serve to obscure the significance of hierarchical organization to the very workings of biological nature. All of this does not deny that there is an extensive literature on hierarchies, a multidisciplinary literature that includes a long, if episodic, history within the realm of biology. The analysis I develop here surely does not arise from a vacuum. Yet the current resurgence of hierarchical outlook on evolution reflects more, I think, a return to an alternative way of looking at nature, a way dictated by a pattern of organization of nature that is there for all to see, than it does the thickening of a continuous Intellectual strand that connects us with earlier interests in hierarchy both within biology and without.

Toward Hierarchy: Trends and Tensions in Evolutionary Theory

Is the synthesis a complete and satisfactory evolutionary theory? It would be a trivial and derogatory exercise indeed to depict the synthesis in utterly simplistic terms and then turn around and conclude that it is incomplete and unsatisfactory. Though I have subjected the synthesis to a series of purifying distillations through the course of this book so far, I have in mind by no means solely Mayr’s (1980, p. 1) two-sentence summary of the synthesis as I pose the question of just how complete, workable, and satisfactory a theory of the evolutionary process it is. No serious student of evolutionary theory could ever claim that the modem synthesis is “just population genetics.” Many more phenomena are included than the statics and dynamics of genetic change in populations. What does seem to be true of the synthesis in general is that it focuses its concerns on a certain range of biological entities and attendant processes, and espouses attitudes and ontological positions on others that appear (to me) to exclude the latter from effective integration into the theory per se. Specifically, the synthesis focuses on genes; their replication, recombination, and mutation; and the fate of allelic variation within populations. But species—certainly not the sole province of population genetics—are very much a part of the synthesis, if equivocally so. If it is true that most evolutionary phenomena considered by the synthesis are construed, at least in principle, to be explicable in terms of the dynamics of selection and drift of allelic variation in populations, it is not because other sorts of phenomena, such as macroevolutionary trends, are alleged not to exist. The synthesis takes the (on the whole commendable) attitude of the Missourian who must be shown. We have a highly corroborated theory of the origin, maintenance, and modification of adaptations—through pure, narrowly defined natural selection. The burden of demonstration lies on anyone who would maintain either that some other process builds such (organismic) adaptation or that an additional process (or more than one) is also at work in evolution. Certainly the entire discussion on levels of selection, a discussion to which I return in this chapter, is structured in this general sort of way.

The Structure and Content of the Modern Synthesis

When taken together, the four books of Dobzhansky, Mayr, and Simpson, written as they were so closely together in time (and space—Columbia University and the American Museum of Natural History are within forty blocks of each other in New York City) reveal a relatively minor amount of disarray, a slight lack of cohesion in the early stages of the synthesis. That some of these discrepancies were later removed—most notably through a more universal acceptance of the dominant role of natural selection in effecting adaptive change (Gould 1980b)—is important, if only because it established more of a semblance of agreement and consensus. The acceptance that Mayr (1982, pp. 568-69) reports among nearly all participants at the Princeton conference held in 1947 seems real enough; by the late 1940s the final, polished version of the synthesis apparently had begun to emerge. But we must ask if there were any important additions to evolutionary theory after these four books appeared. Changes in emphasis—for example, on selection, but also in such issues as Mayr’s later (especially 1963) views on the role that species play in evolution—certainly did occur. And, of course, beyond the conceptual lies the straightforward discovery of new phenomena, such as the myriad wonders of the molecular biology of the gene, begun in earnest in the early 1950s and still being announced daily. What concerns me here is more the structure of evolutionary theory than its precise content. Have either new ideas or new data since the publication of these four books materially modified the way we think about evolution? The answer, for the most part, is no; the theory presented in the better recent college textbooks (e.g., Dobzhansky et al. 1977; Futuyma 1979) is substantially the same as the amalgam that arose from the four books analyzed here, with the rough edges sanded and recent discoveries—nearly all concerning the molecular structure of the gene—duly incorporated. But there were some particularly important innovations and shifts of emphasis within the purview of the synthesis.

Systematics, Paleontology, and the Modern Synthesis

Ernst Mayr, a systematist and founding father of the synthetic theory, has recently (Mayr 1980b) assessed the role played by the field of systematics in general in the emergence of the synthesis. Mayr (1980a, 1980b; 1982, chapter 12) actively opposes the conventional supposition that the synthesis is the product of three phases of development: (1) resolution of early difficulties raised in the early days of genetics, largely through the work of Fisher, Haldane, and Wright; (2) the publication of Dobzhansky’s Genetics and the Origin of Species (1937a), which fused concepts of the genetics of populations with the mainstream of Darwinian thought; and (3) the demonstration by systematists (e.g., Mayr 1942), paleontologists (e.g., Simpson 1944), and practitioners of various other biological disciplines that the data of their respective fields are consistent with genetic principles. (See, for example, Shapere 1980, p. 398, for such a view of the historical development of the synthesis.) It is Mayr’s view (e.g., 1980a, 1980b) that these various nongenetics disciplines played a more vital, vigorous and active role than such “me-too-ism” implied by phase 3 above in the conventional view. There is, no doubt, something to be said for this claim, though the historical question per se is not germane to the present inquiry. But Mayr’s (1980b, pp. 127 ff) list of the contributions he feels systematists made directly to the new synthesis is relevant as it suggests a guide to our understanding of Mayr’s own important contribution—Systematics and the Origin of Species (1942, reprinted in 1982). Mayr lists the following contributions of systematics to the emerging synthesis: (1) “population thinking,” (2) “the immense variability of populations,” (3) “the gradualness of evolution,” (4) “the genetic nature of gradual evolution,” (5) “geographic speciation,” (6) “the adaptive nature of observed variation,” (7) “belief in the importance of natural selection,” and (8) the notion (shared with paleontologists) that “macroevolutionary phenomena” are interpretable in terms of “gradual evolution” (i.e., as opposed to saltational models—Mayr 1980b, p. 134). All these topics, and more, are well developed in the pages of Mayr’s Systematics and the Origin of Species.

Approaching Complexity: Thinking About Evolution

Evolution is a complex affair. The very diversity of definitions of biological evolution illustrates the variety of ways in which we think about the subject. Perhaps still the best general description of evolution is Darwin’s “descent with modification,” simply because it simultaneously brings to mind a pattern—a result—while hinting at an understanding of process, an underlying causative mechanism. Other definitions are less broad and thus fare less well as general descriptors of this thing we call evolution. I have characterized evolution as the proposition that all organisms are descended from a single common ancestor (e.g., Eldredge 1982c, based on a suggestion from N. I. Platnick in a personal communication). This is a serviceable (and testable) construct but one that emphasizes a systematist’s concern for pattern. The utterly different and far more popular notion that evolution is “change in gene frequencies within a population” similarly emphasizes process through focusing on a completely different sort of pattern. Yet neither definition is “wrong.” The “modern synthesis” is a body of thought that grapples with the complexities of evolution. As does any good theory, the synthesis attempts to characterize the overall phenomenon and explain it in the simplest terms that seem appropriate and effective. The bewildering array of evolutionary process theories that had accumulated by the 1920s, where each biological discipline seemed bent upon establishing the primacy of its own phenomena and its own insights into processes, amounted to a net disarray for evolutionary biology. As Simpson wrote in 1944 (p. xv): Not long ago paleontologists felt that a geneticist was a person who shut himself in a room, pulled down the shades, watched small flies disporting themselves in milk bottles, and thought that he was studying nature. A pursuit so removed from the realities of life, they said, had no significance for the true biologist. On the other hand, the geneticists said that paleontology had no further contributions to make to biology, that its only point had been the completed demonstration of the truth of evolution, and that it was a subject too purely descriptive to merit the name “science.”