Huxley and contributors who were primarily British because, as Huxley states in the Foreward, his intention was to facilitate communication between them endorsed the perspective that it was only through discourse across a diversity of disciplines, not through inference from population genetics alone, that questions concerning the nature of species could be addressed intelligibly.
The New Systematics is further distinguished from the volumes that gave us the "synthesis" in that, in J. Huxley's introductory chapter and of course in de Beer's chapter de Beer, , discussion of development and of the multitude of factors internal and external to the organism that influence development encompasses both systematics and evolutionary processes. Although Huxley did not cite Goldschmidt in this publication, he did so on numerous occasions throughout his monograph, Evolution: The Modern Synthesis Huxley, Indeed, in the preface he acknowledged the valuable contributions to his work of "Dobzhansky, Waddington, and Goldschmidt" p.
Equally obvious in retrospect is the fact that in the latter sentiment Huxley was dead wrong, at least with regard to Dobzhansky, Mayr, and Simpson and, consequently, the inheritors of their "synthesis". The difference between Huxley and Dobzhansky and his mimics Mayr and Simpson is that the former scholar was able to dissociate the validity and value of Goldschmidt's experimental work in developmental genetics from his theory of evolution by systemic mutation, whereas the latter individual could not or perhaps chose not to do so.
Might Dobzhansky have become further committed to his conception of species formation because he had not been invited to the conference from which this volume emerged? Perhaps it was because of Huxley's emphasis on the primacy of selection rather than isolation as the provocateur of species formation, which was in direct opposition to Dobzhansky's Dobzhansky, and subsequently Mayr Mayr, and Simpson Simpson, conception of isolation typically geographic followed by selection.
Or might it have been the following statement Huxley, a? It is certainly right to attempt a dynamic, in place of a static, definition by thinking of subspecies and species as stages in a process of evolutionary diversification: but it is impossible to insist on infertility as the sole criterion of this stage. Many groups, especially among plants, universally recognized as species by taxonomists, are capable of fertile intercrossing, and in many others we find sterility between mere strains of obvious species.
Whatever the actual reason, it is clearly the case that the authors of the "American" modern synthesis were anything but synthesizers. Indeed, even though each represented a different biological discipline, Mayr the systematist and Simpson the paleontologist subordinated their fields to Dobzhansky's population genetics.
On the other hand, those scholars, whom these 3 criticized, chastised, pilloried, or just plain ignored and who did not reside in the United States were at least attempting to be synthetic. In this regard, it is perhaps telling that Dobzhansky's Dobzhansky, only discussion of systematics in the second edition of Genetics was to lump it with morphology and then dismiss them both as being "predominantly descriptive and observational disciplines, [which] took precedence among biological sciences during the eighteenth and nineteenth centuries" p.
What an historical irony it is then that those who acknowledge J. Huxley as the author of the phrase, "the modern synthesis", do so as if he had coined it specifically to characterize Dobzhansky's brainchild. In reality, however, Huxley's endeavors represent a real attempt at synthesizing a diversity of biological realms, while Dobzhansky's unidimensional and unilateral melding of Darwinian notions of gradualism and selection with interpretations of experiments in population genetics won out and consequently became the constraint under which a diversity of biological disciplines were intellectually incarcerated and forced to operate.
Thus, and in spite of Dobzhansky's Dobzhansky, seemingly synthetic declaration that "the present aim of the book is to review the genetic information bearing on the problem of organic diversity, and, as far as possible, to correlate it with the pertinent data furnished by taxonomy, ecology, physiology, and other related disciplines" p.
As he informs the reader in the first of many iterations of this theme, "it remains true that genetics has so profound a bearing on the problem of the mechanisms of evolution that any evolutionary theory which disregards the established genetic principles is fault at its source" p. It is therefore with the second edition of Genetics and the Origin of Species that I situate the beginning of the "modern evolutionary synthesis" and its ideological entrenchment in a particular melding of Darwinism and population genetics.
And, to paraphrase de Beer de Beer, , with the solidification of this one particular ideology, the focus in evolutionary biology was shifted intractably to the mechanism of transmission from parent to offspring of the genes that were thought to underlie traits and their variations, as if these were the only elements necessary to understand evolution and the origin of evolutionary novelty.
Lost or at least missing entirely from this perspective was any appreciation of development. To use the terminology of especially Mivart, de Beer, Goldschmidt, and Schindewolf Schindewolf, , and also to acknowledge the intent of Bateson Bateson, , D'Arcy Thompson Thompson, , and Waddington Waddington, , also absent was any interest in or understanding of the "internal factors" that together act and interact to produce novel morphologies, potentially significant evolutionary change, and perhaps then species, which at base remain the centerpiece of systematics.
Consequently, while the practice of systematics had historically been concerned primarily with allocating specimens to named genera and species, erecting new taxa, and sometimes generating statements of evolutionary relationship, Mayr's systematics now focused on "species" through the lens of populational variation as the prelude to change within a species as well as to the splitting of a species into daughter species.
As such, while the conception of the species as an actual biological entity was still discussed, the emphasis was now on a particular process that led smoothly, continuously, and seamlessly from the formation of a species through its persistence, all the while in a state of continual transformation, rather than on how to identify it. Indeed, Mayr's Mayr, biological species definition is linked inextricably to the notion of a process:.
A species consists of a group of populations which replace each other geographically or ecologically and of which the neighboring ones intergrade or interbreed where they are in contact or which are potentially capable of doing so with one or more of the populations in those cases where contact is prevented by geographical or ecological barriers.
Or shorter: Species are groups of actually or potentially interbreeding natural populations, which are reproductively isolated from other such groups. And the process is conceived thusly p.
That speciation is not an abrupt, but a gradual and continuous process is proven by the fact that we find in nature every imaginable level of speciation, ranging from an almost uniform species at one extreme to one in which isolated populations have diverged to such a degree that they can be considered equally well as separate, good species at the other extreme.
I have tried in a recent paper A widespread species is more likely to represent the first stage of speciation than one with a narrowly restricted range. In Mayr p. It was shown by me in a previous publication Mayr, that all this evidence indicates that the normal process of gradual speciation is that of geographic isolation". Dobzhansky Dobzhansky, ; Dobzhansky, had also recognized this as a consequence of embracing a gradual tempo of change.
And in turn, this had led him to argue that even though speciation was a prolonged process of gradual intrapopulational change, insight into the process of species formation could nonetheless be achieved by experimentally studying the genetics of populations from one generation to the next.
Consequently, as J. But to continue the scenario, for Mayr, the emergence of different species is the consequence of a process that is always in motion and continually at play at the subspecific or intrapopulational level. As Mayr saw it, a subspecies is in actuality an "incipient" species poised to become a species in its own right when an econiche into which it could invade became available.
Then, because of its different ecological circumstances, natural selection can mold it into something else. Consequently, as Mayr Mayr, would reiterate:. Speciation is thus an adaptive process toward the most efficient utilization of the environment.
An improvement of the environment will create new niches and will favor speciation; an impoverishment will lead to extinction. Not individuals but populations are the units of evolution and such populations can drift apart decisively through an accumulation of small, often minute, genetic differences.
Such gradual speciation is the norm while instantaneous speciation is the exception. Ergo, echoing Dobzhansky's centralization of population genetics in the evolutionary process, there is only one way in which speciation can and normally does occur.
Nonetheless, Dobzhansky's population thinking greatly influenced the paleontologist Simpson Simpson, in Tempo and Mode in Evolution , which he had begun in and left in the state he had reached when, in December , he went to serve in WWII.
His wife Ann Roe and the paleontologist E. Colbert of the American Museum of Natural History took the manuscript through its publication. In apparent demonstration of his attempt to synthesize paleontology with population genetics, Simpson declares further that, because his interest in this work "is to discuss the 'how' and The "why" for Simpson's decision is clearly to demonstrate that, although paleontology apparently cannot contribute to understanding the process of species formation, it is not only amenable to population thinking but, because its "laboratory" encompasses organisms in the broad dimensions of time and space, it can provide insight into evolutionary questions that laboratory experiments cannot:.
The paleontologist is given only phenotypes, and attempts to relate these to genotypes have so far had little success. But here genetics can provide him with the essential facts. One cannot directly study heredity in fossils, but one can assume that some, if not all, of its mechanisms were the same as those revealed by recent organisms in the laboratory On the other hand, experimental biology in general and genetics in particular have the grave defect that they cannot reproduce the vast and complex horizontal extent of the natural environment and, particularly, the immense span of time in which population changes really occur.
They may reveal what happens to a hundred rats in the course of 10 years under fixed and simple conditions, but not what happened to a billion rats in the course of 10 million years under the fluctuating conditions of earth history.
Obviously, the latter problem is much more important. Although one might infer some bitterness in this statement, Simpson nonetheless attempts a language steeped in population genetics, not just Dobzhansky's, but also Wright's Wright, , , on whose shifting balance theory he bases much of his model of quantum evolution: rapid change between 2 different adaptive peaks.
The irony here is that in his review of Tempo and Mode Wright Wright, took such issue with the paleontologist for misrepresenting his shifting balance theory that Simpson thereafter downplayed discussion of population genetics. Perhaps, as a paleontologist, and thus someone whose work depends solely on preserved morphology, which, in turn informs taxonomic and phylogenetic hypotheses, Simpson should have plumbed his expertise to include a meaningful discussion of systematics which Mayr also failed to do.
But Simpson did not, no doubt because the population thinking demanded by Dobzhansky made species "fuzzy" entities and, because of being in states of constant motion, their identification, especially in the fossil record, artificial at best. It might therefore seem a contradiction that Simpson's next treatise, The Principles of Classification and a Classification of Mammals Simpson, dealt precisely with this topic, namely the identification and naming of species:.
It is impossible to speak of the objects of any study, or to think lucidly about them, unless they are named. It is impossible to examine their relationships to each other their places among the vast, incredibly complex phenomena of the universe, in short to treat them scientifically, without putting them into some sort of formal arrangement Taxonomy is at the same time the most elementary and the most inclusive part of zoology, most elementary because animals cannot be discussed or treated in a scientific way until some taxonomy has been achieved, and most inclusive because taxonomy Emphasis of a particular one of these lines of approach has often obscured the existence of a common goal, and a myopic viewpoint has hidden from many students the final unity of the field of taxonomy In a footnote to the last quote, Simpson Simpson, writes, "The rise of this viewpoint is well demonstrated and exemplified by 'The new systematics,' edited by Julian Huxley, Oxford, " p.
Although Simpson Simpson, listed this compendium in the bibliography of Tempo and Mode , his only direct reference to Huxley was to the latter scholar's discussion in Problems of Relative Growth Huxley, of heterogony of antler size against body size in the deer Cervus elaphus p. Simpson Simpson, then proceeds to explain how modern taxonomy and systematics differed from these pursuits in the olden days, when species were identified by some often unstated morphological rule of thumb: "The aim is not merely to define populations in a better way This Simpson considers the first task of taxonomists, who he characterizes as having only just begun to think in this enlightened manner.
The second task is "the study of phylogeny and the reconstruction of classification in accordance with it," which "extends to all levels of classification, but it is more important and, at present, more fruitful on higher levels" p.
Of further interest is that Simpson had essentially completed the manuscript of Classification by late , just before he went to serve in WWII. Its editing and fact checking under the direction of E. Colbert took until late March , with "only a few minor emendations" by Simpson before its publication on 5 October Simpson, p.
Considering that Simpson was writing Tempo and Mode and Classification in tandem and brought both essentially to final form by late , it is astonishing how disassociated they are in their emphases. It is as if each had been written by a different author. Phylogeny cannot be observed. It is necessarily an inference from observations that bear on it, sometimes rather distantly, and that can usually be interpreted in more than one way The most direct, but unfortunately not the most useful, approach to the phylogeny of recent animals is through their genetics.
The stream of heredity makes phylogeny; in a sense, it is phylogeny. Complete genetic analysis would provide the most priceless data for the mapping of this stream, although it would only exceptionally provide unequivocal and conclusive criteria Aside from such rare cases, genotypic similarities and dissimilarities have to be interpreted phylogenetically in much the same way as phenotypic likenesses and differences Obviously, extinct animals, so much more numerous than recent animals, are excluded [from genetic analysis] Genetical, physiological, embryological, and geographical data have been mentioned first, mainly because the orthodox classifier is likely to think of them last, but morphological data and paleontological data After these assertions, Simpson disappoints in discussing the specifics of reconstructing phylogenetic relationships.
Rather, he merely reiterates the accepted phenetic rule of thumb that animals with more homologous characters in common are more closely related and their common ancestry more recent than animals sharing fewer homologous characters. With the expulsion by the triumvirate of "American" evolutionists of alternative theories of evolutionary change both in tempo and mode and their promoting their melding of Darwinism and population genetics as the only true representation of the process of evolution, the conception was that organisms are in a constant state of flux and change over time.
Thus this constant "dynamic flow" p. In turn, the longer 2 sister "species" shared a common and constantly changing lineage prior to their divergence after which they continued on their separates paths of continual transformation , the more "homologous" features they would have in common. Ergo, it was the consequence of organisms and thus of their lineages being in states of constant Darwinian motion that permits the conclusion that the more similar organisms are, the longer their shared lineage "heritage", and thus the more recent their divergence into separate species.
Hennig, Simpson, and Mayr: a contrast of systematists. With the publication of Hennig's Hennig, , "phylogenetic systematics", the complacency and routine of going about evolutionary business as usual was seriously threatened.
Since there are many nuances and implications of his proposals that go beyond the scope of the discussion here, I will summarize the main relevant points. Consider a hierarchy of relationships or hypotheses of propinquity: those between individuals of the same species tokogenetic relationships , between 2 species, and between the smallest definable monophyletic groups at least 3 species. Expanding on Dobzhansky's and, although not stated, Mayr's species concept: a species is a group of individuals at first spatially and then reproductively isolated from similar isolated groups.
The consequence of these isolating mechanisms leads to distinct morphological gaps between these groups. Thus, in reverse historical order, now separated sister species would merge tokogenetically into one as they approached their last common ancestor or stem species. Although generally assumed, degree of overall similarity is not a reflection of degree of relatedness.
Rather, the sum total of features any organism possesses represents a history of retention of a sequence of character transformation events that likely correspond to the chronological series of hypothetical ancestors of hierarchically nested sets of sister taxa and monophyletic groups.
Characters that are common to a wide range of taxa are for them primitive "plesiomorphous" retentions from a relatively ancient common ancestor.
Characters that are confined to groups of decreasing taxic inclusiveness are sequentially derived "apomorphous" features and reflect a series of increasingly more recent common ancestors. Hypotheses of synapomorphy shared apomorphies , not symplesiomorphy shared pleisomorphies , likely more accurately reflect closeness of relatedness. When a more ancient taxon possesses character state "a" and a more recent taxon "a1", their relative geological age should be the deciding factor between primitive the more ancient and derived the more recent.
Because hypotheses of "primitive" and "derived" are relative within a hierarchy of monophyletic groups, symplesiomorphy at one level in the hierarchy was synapomorphy at a more ancient level. Classification should reflect the hierarchical relationships of monophyletic groups. Taxonomic rank of a monophyletic group should be determined by the age of the older member of that group e. These endeavors constitute "phylogenetic systematics". Simpson Simpson, and subsequently Mayr Mayr, , , had very different reactions to Hennig's Hennig, German publication.
First, Simpson p. And third, he p. Although Simpson also reveals an appreciation of the relative phylogenetic information content of ancestral versus advanced or specialized characters, he does not endorse Hennig's terms "plesiomorph" and "apomorph", which he considers "complex and idiosyncratic".
Simpson's p. That Simpson might have partially embraced this suggestion would, however, seem to be evident in his statement p. But he p. Later in his monograph, Simpson turns to J. Huxley's Huxley, b symposium and resultant volume, The New Systematics , which he p. To make the point he lists the attributes he pp. There are 2 likely contributing factors. Mayr, ; Mayr, Although their titles emphasized taxonomy, their content, especially in the latter work, was clearly a discourse on systematics in general.
Thus it would not be surprising if Mayr was concerned that Simpson might be seen as the real "systematist" of the "synthesis". Whether either of these speculations is correct, it is the case that, in spite of the intellectual camaraderie that Dobzhansky, Mayr, and Simpson presented publicly in coordinating their monographs that formed the foundation of the "modern evolutionary synthesis" and defending their achievement against all heretics, the 3 disliked each other immensely.
It is therefore noteworthy that more than 2 decades later Mayr Mayr, maintains the "front" of civility by quoting Simpson's Simpson, "modern redefinition of the term" systematics p. Although still emphasizing the centrality of populations within species and that the picture of evolution includes branching followed by divergence, Mayr presents himself as the broader minded and thus more informed systematist: "One of the major preoccupations of systematics is to determine, by comparison, what the unique properties of every species and higher taxon are" p.
And it is from this perspective that he criticizes Hennig throughout rest of the monograph. Mayr's objections are that Hennig and his disciples referred to their theoretical position as "the phylogenetic school" p.
Mayr further objects to the "phylogenetic school" identifying itself thusly because, he maintains, the evolutionary process, and therefore phylogeny, consists not only of the branching or splitting of taxa, but also of the subsequent transformation of these new taxa.
Consequently, he proposes the term cladism Rensch, ; Cain and Harisson, to identify Hennig's conception, which he views as being concerned only with branching events, in part because he incorrectly Schwartz, interprets Hennig as asserting that taxa remain static thereafter. As Mayr cautions, "Users of the recent literature are warned to look out for the misleading use of the term phylogeny by the cladists" p. Mayr pp. They show individually very little that is unique to themselves.
They show instead unique combinations of the characters found among other close relatives" p. Yet is this not what one would expect of organisms whose existence as biologically integrated and functioning individuals is the result of a history of primitive retentions from a series of ancestors as well as of other features unique to them. Phylogenetic relationships cannot be predetermined by a systematist's imposed sense of how many features are required to define a species or a clade.
A "unique" combination of primitive and derived features, to use T. But Mayr's dislike of cladism continues, even though his arguments lack clarity of thought. The evolutionary taxonomist [namely, Mayr] agrees with the cladist in assuming that, usually, the more recently their phyletic lines separated, the more similar 2 taxa are. However, the evolutionary taxonomist also gives due weight in his classifications to any unequal divergence of the descendant lines.
By deliberately ignoring these differences, the cladist is often forced to recognize taxa of very unequal value. Did Mayr not begin with a criticism of cladistic classification? Yet he ventures into the realm of phylogenetic reconstruction by stating, incorrectly, that "the evolutionary taxonomist agrees with the cladist in assuming that, usually, the more recently their phyletic lines separated, the more similar 2 taxa are. That is pure phenetics. As I pointed out earlier, by accepting this scenario, one is free to conclude that more recently divergent species will be more similar to each other than either will be to a lineage and its current terminal species that diverged some time in the past.
This is not cladistic thinking, but it is consistent with Mayr's perspective. Thus for him to think that he and cladists are methodologically alike with regard to generating theories of relatedness not only reflects his lack of understanding Hennig's approach to phylogenetic reconstruction, but also demonstrates his belief that whatever he thinks and writes is unassailable and the ultimate word.
In the face of these inconsistencies, and of his subsequent but incorrect comparison of cladism and numerical taxonomy p. Interestingly, in this short discussion of the significance of, and of how to think about, distinguishing relative states of primitiveness and derivedness, Mayr is reminiscent of the British primatologist Le Gros Clark Clark, , who at the beginning of his monograph articulated clearly the importance of making and how to make these distinctions, but thereafter totally disregarded this endeavor and assumed the traditional phenetic approach to reconstructing evolutionary relationships.
Terminating this exploration with Mayr's Principles of Systematic Zoology may seem too abrupt. Nevertheless, it should now be sufficiently clear that there has been a tension rather than an integrated relationship between systematics and the rest of evolutionary biology.
This disconnect between disciplines that should be complementary is, I suggest, due largely to the "modern evolutionary synthesis" not being synthetic but focused exclusively on a particular kind of population thinking as well as to the "synthesizers" concerted efforts to reject, often without justification, alternative evolutionary theories.
Schwartz, ; Schwartz, ; Thorpe et al. The retort that by using the term "selection" one is not being Lamarckian Thorpe et al. This having been said, systematics and evolutionary biology need not remain uneasy bedfellows.
Rather, inroads into developmental biology are increasingly emphasizing that organismal change, which might be synonymized with evolution, is not how it was imagined when the synthesis was enacted see reviews in Schwartz, ; Maresca and Schwartz, ; Schwartz, Further, because of the interrelation between, for example, the physical properties of cells, signaling pathways, epigenetic effects on development, and consequently the origination of form, the false dichotomy that emerged in the s, when morphology was declared unreliable in systematic studies while molecular approaches to systematics were infallible, is no longer sustainable Schwartz, in press.
With the realization that there is an integrated developmental continuum from the molecular through the morphological, morphology can resume a central position in systematic endeavors Schwartz, in press and, as is albeit slowly happening, lead to the generation of alternatives to Darwinism.
But before this can occur, as cladists argued and debated in the s and into the '80s about the assumptions underlying their endeavors, molecular systematists must also face their discipline's underlying assumptions, which, without concern for hypothesis testing, have been elevated to the status of fact.
Huxley Huxley, , b, , Waddington Waddington, , Goldschmidt Goldschmidt, , and Schindewolf Schindewolf, not been so unreservedly dismissed and even denigrated by the architects of the "modern evolutionary synthesis", we might now be witnessing a wider appreciation of the interplay between systematics, development, and evolutionary biology than currently exists amongst those who present themselves as evolutionary biologists and systematists.
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